Re: [Xen-devel] [PATCH 1/3] xen/arm: io: Distinguish unhandled IO from aborted one

[Stefano Stabellini]

On Wed, 31 Jan 2018, Julien Grall wrote:
> Hi Stefano,
> 
> On 30/01/18 19:09, Stefano Stabellini wrote:
> > On Tue, 30 Jan 2018, Julien Grall wrote:
> > > > > diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
> > > > > index c8534d6cff..843adf4959 100644
> > > > > --- a/xen/arch/arm/traps.c
> > > > > +++ b/xen/arch/arm/traps.c
> > > > > @@ -1864,10 +1864,10 @@ static inline bool hpfar_is_valid(bool s1ptw,
> > > > > uint8_t fsc)
> > > > >return s1ptw || (fsc == FSC_FLT_TRANS &&
> > > > > !check_workaround_834220());
> > > > >}
> > > > >-static bool try_handle_mmio(struct cpu_user_regs *regs,
> > > > > -const union hsr hsr,
> > > > > -paddr_t gpa)
> > > > > -{
> > > > > +static enum io_state try_handle_mmio(struct cpu_user_regs *regs,
> > > > > + const union hsr hsr,
> > > > > + paddr_t gpa)
> > > > > + {
> > > > >const struct hsr_dabt dabt = hsr.dabt;
> > > > >mmio_info_t info = {
> > > > >.gpa = gpa,
> > > > > @@ -1879,11 +1879,11 @@ static bool try_handle_mmio(struct
> > > > > cpu_user_regs
> > > > > *regs,
> > > > >  /* stage-1 page table should never live in an emulated MMIO
> > > > > region
> > > > > */
> > > > >if ( dabt.s1ptw )
> > > > > -return false;
> > > > > +return IO_UNHANDLED;
> > > > >  /* All the instructions used on emulated MMIO region should
> > > > > be
> > > > > valid */
> > > > >if ( !dabt.valid )
> > > > > -return false;
> > > > > +return IO_UNHANDLED;
> > > > >  /*
> > > > > * Erratum 766422: Thumb store translation fault to Hypervisor
> > > > > may
> > > > > @@ -1896,11 +1896,11 @@ static bool try_handle_mmio(struct
> > > > > cpu_user_regs
> > > > > *regs,
> > > > >if ( rc )
> > > > >{
> > > > >gprintk(XENLOG_DEBUG, "Unable to decode
> > > > > instruction\n");
> > > > > -return false;
> > > > > +return IO_ABORT;
> > > > >}
> > > > >}
> > > > 
> > > > Why do the first two error checks result in IO_UNHANDLED, while the
> > > > third result in IO_ABORT? Specifically in relation to pagetable walk
> > > > failures due to someone else changing stage-2 entry simultaneously (see
> > > > comment toward the end of do_trap_stage2_abort_guest)?
> > > 
> > > Good question. Somehow I considered the first two as part of looking up
> > > the
> > > proper handler and not the device itself.
> > > 
> > > But I guess that at this stage we know that IO was targeting an emulated
> > > region. So we can return IO_ABORT.
> > 
> > That is what I thought as well
> 
> Actually, I have said something completely wrong yesterday. Must have been too
> tired :/.
> 
> At the time you call try_handle_mmio, you still don't know whether the fault
> was because of accessing an emulated MMIO region. You will only be sure when
> find_mmio_handler() has returned a non-NULL pointer (see handle_mmio()).
> 
> So returning IO_UNHANDLED here is correct as you want to try another method to
> handle the fault.
> 
> However, it also means that even bad access to emulated region will result to
> fallback on another method. While this should not be issue, I don't think this
> is future proof (I am mostly worry on the ACPI case where MMIO are mapped
> on-demand).
> 
> So I will send a patch to fold try_handle_mmio() into handle_mmio().

All right


> > 
> > 
> > > On a side note, it looks like the check dabt.s1ptw is unnecessary because
> > > a
> > > stage 2 abort on stage 1 translation table lookup will not return a valid
> > > instruction syndrome (see B3-1433 in DDI 0406C.c and D10-2460 in DDI
> > > 0487C.a).
> > 
> > in that case, go ahead and remove it as part of this patch, mention it
> > in the commit message
> 
> I will do that in a patch that fold try_handle_mmio() in handle_mmio().
> 
> Cheers,
> 
> -- 
> Julien Grall
> 

___
Xen-devel mailing list
Xen-devel@lists.xenproject.org
https://lists.xenproject.org/mailman/listinfo/xen-devel

Re: [Xen-devel] [PATCH 1/3] xen/arm: io: Distinguish unhandled IO from aborted one

[Julien Grall]

Hi Stefano,

On 30/01/18 19:09, Stefano Stabellini wrote:

On Tue, 30 Jan 2018, Julien Grall wrote:

diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
index c8534d6cff..843adf4959 100644
--- a/xen/arch/arm/traps.c
+++ b/xen/arch/arm/traps.c
@@ -1864,10 +1864,10 @@ static inline bool hpfar_is_valid(bool s1ptw,
uint8_t fsc)
   return s1ptw || (fsc == FSC_FLT_TRANS &&
!check_workaround_834220());
   }
   -static bool try_handle_mmio(struct cpu_user_regs *regs,
-const union hsr hsr,
-paddr_t gpa)
-{
+static enum io_state try_handle_mmio(struct cpu_user_regs *regs,
+ const union hsr hsr,
+ paddr_t gpa)
+ {
   const struct hsr_dabt dabt = hsr.dabt;
   mmio_info_t info = {
   .gpa = gpa,
@@ -1879,11 +1879,11 @@ static bool try_handle_mmio(struct cpu_user_regs
*regs,
 /* stage-1 page table should never live in an emulated MMIO region
*/
   if ( dabt.s1ptw )
-return false;
+return IO_UNHANDLED;
 /* All the instructions used on emulated MMIO region should be
valid */
   if ( !dabt.valid )
-return false;
+return IO_UNHANDLED;
 /*
* Erratum 766422: Thumb store translation fault to Hypervisor may
@@ -1896,11 +1896,11 @@ static bool try_handle_mmio(struct cpu_user_regs
*regs,
   if ( rc )
   {
   gprintk(XENLOG_DEBUG, "Unable to decode instruction\n");
-return false;
+return IO_ABORT;
   }
   }


Why do the first two error checks result in IO_UNHANDLED, while the
third result in IO_ABORT? Specifically in relation to pagetable walk
failures due to someone else changing stage-2 entry simultaneously (see
comment toward the end of do_trap_stage2_abort_guest)?


Good question. Somehow I considered the first two as part of looking up the
proper handler and not the device itself.

But I guess that at this stage we know that IO was targeting an emulated
region. So we can return IO_ABORT.


That is what I thought as well


Actually, I have said something completely wrong yesterday. Must have 
been too tired :/.


At the time you call try_handle_mmio, you still don't know whether the 
fault was because of accessing an emulated MMIO region. You will only be 
sure when find_mmio_handler() has returned a non-NULL pointer (see 
handle_mmio()).


So returning IO_UNHANDLED here is correct as you want to try another 
method to handle the fault.


However, it also means that even bad access to emulated region will 
result to fallback on another method. While this should not be issue, I 
don't think this is future proof (I am mostly worry on the ACPI case 
where MMIO are mapped on-demand).


So I will send a patch to fold try_handle_mmio() into handle_mmio().





On a side note, it looks like the check dabt.s1ptw is unnecessary because a
stage 2 abort on stage 1 translation table lookup will not return a valid
instruction syndrome (see B3-1433 in DDI 0406C.c and D10-2460 in DDI 0487C.a).


in that case, go ahead and remove it as part of this patch, mention it
in the commit message


I will do that in a patch that fold try_handle_mmio() in handle_mmio().

Cheers,

--
Julien Grall

___
Xen-devel mailing list
Xen-devel@lists.xenproject.org
https://lists.xenproject.org/mailman/listinfo/xen-devel

Re: [Xen-devel] [PATCH 1/3] xen/arm: io: Distinguish unhandled IO from aborted one

[Stefano Stabellini]

On Tue, 30 Jan 2018, Julien Grall wrote:
> Hi Stefano,
> 
> On 30/01/18 18:14, Stefano Stabellini wrote:
> > On Tue, 30 Jan 2018, Julien Grall wrote:
> > > Currently, Xen is considering that an IO could either be handled or
> > > unhandled. When unhandled, the stage-2 abort function will try another
> > > way to resolve the abort.
> > > 
> > > However, the MMIO emulation may return unhandled when the address
> > > belongs to an emulated range but was not correct. In that case, Xen
> > > should avodi to try another way and directly inject a guest data abort.
> >   ^ avoid
> > 
> > 
> > > Introduce a tri-state return to distinguish the following state:
> > >  * IO_ABORT: The IO was handled but resulted to an abort
> >^ in an abort
> > 
> > 
> > >  * IO_HANDLED: The IO was handled
> > >  * IO_UNHANDLED: The IO was unhandled
> > > 
> > > For now, it is considered that an IO belonging to an emulated range
> > > could either be handled or inject an abort. This could be revisit in the
> > > future if overlapped region exist (or we want to try another way to
> > > resolve the abort).
> > > 
> > > Signed-off-by: Julien Grall 
> > > ---
> > >   xen/arch/arm/io.c  | 24 ++--
> > >   xen/arch/arm/traps.c   | 34 +++---
> > >   xen/include/asm-arm/mmio.h |  9 -
> > >   3 files changed, 45 insertions(+), 22 deletions(-)
> > > 
> > > diff --git a/xen/arch/arm/io.c b/xen/arch/arm/io.c
> > > index c748d8f5bf..a74ec21b86 100644
> > > --- a/xen/arch/arm/io.c
> > > +++ b/xen/arch/arm/io.c
> > > @@ -23,8 +23,9 @@
> > >   #include 
> > >   #include 
> > >   -static int handle_read(const struct mmio_handler *handler, struct vcpu
> > > *v,
> > > -   mmio_info_t *info)
> > > +static enum io_state handle_read(const struct mmio_handler *handler,
> > > + struct vcpu *v,
> > > + mmio_info_t *info)
> > >   {
> > >   const struct hsr_dabt dabt = info->dabt;
> > >   struct cpu_user_regs *regs = guest_cpu_user_regs();
> > > @@ -37,7 +38,7 @@ static int handle_read(const struct mmio_handler
> > > *handler, struct vcpu *v,
> > >   uint8_t size = (1 << dabt.size) * 8;
> > > if ( !handler->ops->read(v, info, , handler->priv) )
> > > -return 0;
> > > +return IO_ABORT;
> > > /*
> > >* Sign extend if required.
> > > @@ -57,17 +58,20 @@ static int handle_read(const struct mmio_handler
> > > *handler, struct vcpu *v,
> > > set_user_reg(regs, dabt.reg, r);
> > >   -return 1;
> > > +return IO_HANDLED;
> > >   }
> > >   -static int handle_write(const struct mmio_handler *handler, struct vcpu
> > > *v,
> > > -mmio_info_t *info)
> > > +static enum io_state handle_write(const struct mmio_handler *handler,
> > > +  struct vcpu *v,
> > > +  mmio_info_t *info)
> > >   {
> > >   const struct hsr_dabt dabt = info->dabt;
> > >   struct cpu_user_regs *regs = guest_cpu_user_regs();
> > > +int ret;
> > >   -return handler->ops->write(v, info, get_user_reg(regs, dabt.reg),
> > > -   handler->priv);
> > > +ret = handler->ops->write(v, info, get_user_reg(regs, dabt.reg),
> > > +  handler->priv);
> > > +return ( ret ) ? IO_HANDLED : IO_ABORT;
> > >   }
> > > /* This function assumes that mmio regions are not overlapped */
> > > @@ -100,14 +104,14 @@ static const struct mmio_handler
> > > *find_mmio_handler(struct domain *d,
> > >   return handler;
> > >   }
> > >   -int handle_mmio(mmio_info_t *info)
> > > +enum io_state handle_mmio(mmio_info_t *info)
> > >   {
> > >   struct vcpu *v = current;
> > >   const struct mmio_handler *handler = NULL;
> > > handler = find_mmio_handler(v->domain, info->gpa);
> > >   if ( !handler )
> > > -return 0;
> > > +return IO_UNHANDLED;
> > > if ( info->dabt.write )
> > >   return handle_write(handler, v, info);
> > > diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
> > > index c8534d6cff..843adf4959 100644
> > > --- a/xen/arch/arm/traps.c
> > > +++ b/xen/arch/arm/traps.c
> > > @@ -1864,10 +1864,10 @@ static inline bool hpfar_is_valid(bool s1ptw,
> > > uint8_t fsc)
> > >   return s1ptw || (fsc == FSC_FLT_TRANS &&
> > > !check_workaround_834220());
> > >   }
> > >   -static bool try_handle_mmio(struct cpu_user_regs *regs,
> > > -const union hsr hsr,
> > > -paddr_t gpa)
> > > -{
> > > +static enum io_state try_handle_mmio(struct cpu_user_regs *regs,
> > > + const union hsr hsr,
> > > + paddr_t gpa)
> > > + {
> > >   const struct hsr_dabt dabt = hsr.dabt;
> > >  

Re: [Xen-devel] [PATCH 1/3] xen/arm: io: Distinguish unhandled IO from aborted one

[Julien Grall]

Hi Stefano,

On 30/01/18 18:14, Stefano Stabellini wrote:

On Tue, 30 Jan 2018, Julien Grall wrote:

Currently, Xen is considering that an IO could either be handled or
unhandled. When unhandled, the stage-2 abort function will try another
way to resolve the abort.

However, the MMIO emulation may return unhandled when the address
belongs to an emulated range but was not correct. In that case, Xen
should avodi to try another way and directly inject a guest data abort.

  ^ avoid



Introduce a tri-state return to distinguish the following state:
 * IO_ABORT: The IO was handled but resulted to an abort

   ^ in an abort



 * IO_HANDLED: The IO was handled
 * IO_UNHANDLED: The IO was unhandled

For now, it is considered that an IO belonging to an emulated range
could either be handled or inject an abort. This could be revisit in the
future if overlapped region exist (or we want to try another way to
resolve the abort).

Signed-off-by: Julien Grall 
---
  xen/arch/arm/io.c  | 24 ++--
  xen/arch/arm/traps.c   | 34 +++---
  xen/include/asm-arm/mmio.h |  9 -
  3 files changed, 45 insertions(+), 22 deletions(-)

diff --git a/xen/arch/arm/io.c b/xen/arch/arm/io.c
index c748d8f5bf..a74ec21b86 100644
--- a/xen/arch/arm/io.c
+++ b/xen/arch/arm/io.c
@@ -23,8 +23,9 @@
  #include 
  #include 
  
-static int handle_read(const struct mmio_handler *handler, struct vcpu *v,

-   mmio_info_t *info)
+static enum io_state handle_read(const struct mmio_handler *handler,
+ struct vcpu *v,
+ mmio_info_t *info)
  {
  const struct hsr_dabt dabt = info->dabt;
  struct cpu_user_regs *regs = guest_cpu_user_regs();
@@ -37,7 +38,7 @@ static int handle_read(const struct mmio_handler *handler, 
struct vcpu *v,
  uint8_t size = (1 << dabt.size) * 8;
  
  if ( !handler->ops->read(v, info, , handler->priv) )

-return 0;
+return IO_ABORT;
  
  /*

   * Sign extend if required.
@@ -57,17 +58,20 @@ static int handle_read(const struct mmio_handler *handler, 
struct vcpu *v,
  
  set_user_reg(regs, dabt.reg, r);
  
-return 1;

+return IO_HANDLED;
  }
  
-static int handle_write(const struct mmio_handler *handler, struct vcpu *v,

-mmio_info_t *info)
+static enum io_state handle_write(const struct mmio_handler *handler,
+  struct vcpu *v,
+  mmio_info_t *info)
  {
  const struct hsr_dabt dabt = info->dabt;
  struct cpu_user_regs *regs = guest_cpu_user_regs();
+int ret;
  
-return handler->ops->write(v, info, get_user_reg(regs, dabt.reg),

-   handler->priv);
+ret = handler->ops->write(v, info, get_user_reg(regs, dabt.reg),
+  handler->priv);
+return ( ret ) ? IO_HANDLED : IO_ABORT;
  }
  
  /* This function assumes that mmio regions are not overlapped */

@@ -100,14 +104,14 @@ static const struct mmio_handler 
*find_mmio_handler(struct domain *d,
  return handler;
  }
  
-int handle_mmio(mmio_info_t *info)

+enum io_state handle_mmio(mmio_info_t *info)
  {
  struct vcpu *v = current;
  const struct mmio_handler *handler = NULL;
  
  handler = find_mmio_handler(v->domain, info->gpa);

  if ( !handler )
-return 0;
+return IO_UNHANDLED;
  
  if ( info->dabt.write )

  return handle_write(handler, v, info);
diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
index c8534d6cff..843adf4959 100644
--- a/xen/arch/arm/traps.c
+++ b/xen/arch/arm/traps.c
@@ -1864,10 +1864,10 @@ static inline bool hpfar_is_valid(bool s1ptw, uint8_t 
fsc)
  return s1ptw || (fsc == FSC_FLT_TRANS && !check_workaround_834220());
  }
  
-static bool try_handle_mmio(struct cpu_user_regs *regs,

-const union hsr hsr,
-paddr_t gpa)
-{
+static enum io_state try_handle_mmio(struct cpu_user_regs *regs,
+ const union hsr hsr,
+ paddr_t gpa)
+ {
  const struct hsr_dabt dabt = hsr.dabt;
  mmio_info_t info = {
  .gpa = gpa,
@@ -1879,11 +1879,11 @@ static bool try_handle_mmio(struct cpu_user_regs *regs,
  
  /* stage-1 page table should never live in an emulated MMIO region */

  if ( dabt.s1ptw )
-return false;
+return IO_UNHANDLED;
  
  /* All the instructions used on emulated MMIO region should be valid */

  if ( !dabt.valid )
-return false;
+return IO_UNHANDLED;
  
  /*

   * Erratum 766422: Thumb store translation fault to Hypervisor may
@@ -1896,11 +1896,11 @@ static bool try_handle_mmio(struct cpu_user_regs *regs,
  if ( rc )
  {
  

Re: [Xen-devel] [PATCH 1/3] xen/arm: io: Distinguish unhandled IO from aborted one

[Stefano Stabellini]

On Tue, 30 Jan 2018, Julien Grall wrote:
> Currently, Xen is considering that an IO could either be handled or
> unhandled. When unhandled, the stage-2 abort function will try another
> way to resolve the abort.
> 
> However, the MMIO emulation may return unhandled when the address
> belongs to an emulated range but was not correct. In that case, Xen
> should avodi to try another way and directly inject a guest data abort.
 ^ avoid


> Introduce a tri-state return to distinguish the following state:
> * IO_ABORT: The IO was handled but resulted to an abort
  ^ in an abort


> * IO_HANDLED: The IO was handled
> * IO_UNHANDLED: The IO was unhandled
> 
> For now, it is considered that an IO belonging to an emulated range
> could either be handled or inject an abort. This could be revisit in the
> future if overlapped region exist (or we want to try another way to
> resolve the abort).
> 
> Signed-off-by: Julien Grall 
> ---
>  xen/arch/arm/io.c  | 24 ++--
>  xen/arch/arm/traps.c   | 34 +++---
>  xen/include/asm-arm/mmio.h |  9 -
>  3 files changed, 45 insertions(+), 22 deletions(-)
> 
> diff --git a/xen/arch/arm/io.c b/xen/arch/arm/io.c
> index c748d8f5bf..a74ec21b86 100644
> --- a/xen/arch/arm/io.c
> +++ b/xen/arch/arm/io.c
> @@ -23,8 +23,9 @@
>  #include 
>  #include 
>  
> -static int handle_read(const struct mmio_handler *handler, struct vcpu *v,
> -   mmio_info_t *info)
> +static enum io_state handle_read(const struct mmio_handler *handler,
> + struct vcpu *v,
> + mmio_info_t *info)
>  {
>  const struct hsr_dabt dabt = info->dabt;
>  struct cpu_user_regs *regs = guest_cpu_user_regs();
> @@ -37,7 +38,7 @@ static int handle_read(const struct mmio_handler *handler, 
> struct vcpu *v,
>  uint8_t size = (1 << dabt.size) * 8;
>  
>  if ( !handler->ops->read(v, info, , handler->priv) )
> -return 0;
> +return IO_ABORT;
>  
>  /*
>   * Sign extend if required.
> @@ -57,17 +58,20 @@ static int handle_read(const struct mmio_handler 
> *handler, struct vcpu *v,
>  
>  set_user_reg(regs, dabt.reg, r);
>  
> -return 1;
> +return IO_HANDLED;
>  }
>  
> -static int handle_write(const struct mmio_handler *handler, struct vcpu *v,
> -mmio_info_t *info)
> +static enum io_state handle_write(const struct mmio_handler *handler,
> +  struct vcpu *v,
> +  mmio_info_t *info)
>  {
>  const struct hsr_dabt dabt = info->dabt;
>  struct cpu_user_regs *regs = guest_cpu_user_regs();
> +int ret;
>  
> -return handler->ops->write(v, info, get_user_reg(regs, dabt.reg),
> -   handler->priv);
> +ret = handler->ops->write(v, info, get_user_reg(regs, dabt.reg),
> +  handler->priv);
> +return ( ret ) ? IO_HANDLED : IO_ABORT;
>  }
>  
>  /* This function assumes that mmio regions are not overlapped */
> @@ -100,14 +104,14 @@ static const struct mmio_handler 
> *find_mmio_handler(struct domain *d,
>  return handler;
>  }
>  
> -int handle_mmio(mmio_info_t *info)
> +enum io_state handle_mmio(mmio_info_t *info)
>  {
>  struct vcpu *v = current;
>  const struct mmio_handler *handler = NULL;
>  
>  handler = find_mmio_handler(v->domain, info->gpa);
>  if ( !handler )
> -return 0;
> +return IO_UNHANDLED;
>  
>  if ( info->dabt.write )
>  return handle_write(handler, v, info);
> diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
> index c8534d6cff..843adf4959 100644
> --- a/xen/arch/arm/traps.c
> +++ b/xen/arch/arm/traps.c
> @@ -1864,10 +1864,10 @@ static inline bool hpfar_is_valid(bool s1ptw, uint8_t 
> fsc)
>  return s1ptw || (fsc == FSC_FLT_TRANS && !check_workaround_834220());
>  }
>  
> -static bool try_handle_mmio(struct cpu_user_regs *regs,
> -const union hsr hsr,
> -paddr_t gpa)
> -{
> +static enum io_state try_handle_mmio(struct cpu_user_regs *regs,
> + const union hsr hsr,
> + paddr_t gpa)
> + {
>  const struct hsr_dabt dabt = hsr.dabt;
>  mmio_info_t info = {
>  .gpa = gpa,
> @@ -1879,11 +1879,11 @@ static bool try_handle_mmio(struct cpu_user_regs 
> *regs,
>  
>  /* stage-1 page table should never live in an emulated MMIO region */
>  if ( dabt.s1ptw )
> -return false;
> +return IO_UNHANDLED;
>  
>  /* All the instructions used on emulated MMIO region should be valid */
>  if ( !dabt.valid )
> -return false;
> +return IO_UNHANDLED;
>  
>  /*
>   * Erratum 766422: Thumb store translation fault to Hypervisor may
> @@ -1896,11 

[Xen-devel] [PATCH 1/3] xen/arm: io: Distinguish unhandled IO from aborted one

[Julien Grall]

Currently, Xen is considering that an IO could either be handled or
unhandled. When unhandled, the stage-2 abort function will try another
way to resolve the abort.

However, the MMIO emulation may return unhandled when the address
belongs to an emulated range but was not correct. In that case, Xen
should avodi to try another way and directly inject a guest data abort.

Introduce a tri-state return to distinguish the following state:
* IO_ABORT: The IO was handled but resulted to an abort
* IO_HANDLED: The IO was handled
* IO_UNHANDLED: The IO was unhandled

For now, it is considered that an IO belonging to an emulated range
could either be handled or inject an abort. This could be revisit in the
future if overlapped region exist (or we want to try another way to
resolve the abort).

Signed-off-by: Julien Grall 
---
 xen/arch/arm/io.c  | 24 ++--
 xen/arch/arm/traps.c   | 34 +++---
 xen/include/asm-arm/mmio.h |  9 -
 3 files changed, 45 insertions(+), 22 deletions(-)

diff --git a/xen/arch/arm/io.c b/xen/arch/arm/io.c
index c748d8f5bf..a74ec21b86 100644
--- a/xen/arch/arm/io.c
+++ b/xen/arch/arm/io.c
@@ -23,8 +23,9 @@
 #include 
 #include 
 
-static int handle_read(const struct mmio_handler *handler, struct vcpu *v,
-   mmio_info_t *info)
+static enum io_state handle_read(const struct mmio_handler *handler,
+ struct vcpu *v,
+ mmio_info_t *info)
 {
 const struct hsr_dabt dabt = info->dabt;
 struct cpu_user_regs *regs = guest_cpu_user_regs();
@@ -37,7 +38,7 @@ static int handle_read(const struct mmio_handler *handler, 
struct vcpu *v,
 uint8_t size = (1 << dabt.size) * 8;
 
 if ( !handler->ops->read(v, info, , handler->priv) )
-return 0;
+return IO_ABORT;
 
 /*
  * Sign extend if required.
@@ -57,17 +58,20 @@ static int handle_read(const struct mmio_handler *handler, 
struct vcpu *v,
 
 set_user_reg(regs, dabt.reg, r);
 
-return 1;
+return IO_HANDLED;
 }
 
-static int handle_write(const struct mmio_handler *handler, struct vcpu *v,
-mmio_info_t *info)
+static enum io_state handle_write(const struct mmio_handler *handler,
+  struct vcpu *v,
+  mmio_info_t *info)
 {
 const struct hsr_dabt dabt = info->dabt;
 struct cpu_user_regs *regs = guest_cpu_user_regs();
+int ret;
 
-return handler->ops->write(v, info, get_user_reg(regs, dabt.reg),
-   handler->priv);
+ret = handler->ops->write(v, info, get_user_reg(regs, dabt.reg),
+  handler->priv);
+return ( ret ) ? IO_HANDLED : IO_ABORT;
 }
 
 /* This function assumes that mmio regions are not overlapped */
@@ -100,14 +104,14 @@ static const struct mmio_handler 
*find_mmio_handler(struct domain *d,
 return handler;
 }
 
-int handle_mmio(mmio_info_t *info)
+enum io_state handle_mmio(mmio_info_t *info)
 {
 struct vcpu *v = current;
 const struct mmio_handler *handler = NULL;
 
 handler = find_mmio_handler(v->domain, info->gpa);
 if ( !handler )
-return 0;
+return IO_UNHANDLED;
 
 if ( info->dabt.write )
 return handle_write(handler, v, info);
diff --git a/xen/arch/arm/traps.c b/xen/arch/arm/traps.c
index c8534d6cff..843adf4959 100644
--- a/xen/arch/arm/traps.c
+++ b/xen/arch/arm/traps.c
@@ -1864,10 +1864,10 @@ static inline bool hpfar_is_valid(bool s1ptw, uint8_t 
fsc)
 return s1ptw || (fsc == FSC_FLT_TRANS && !check_workaround_834220());
 }
 
-static bool try_handle_mmio(struct cpu_user_regs *regs,
-const union hsr hsr,
-paddr_t gpa)
-{
+static enum io_state try_handle_mmio(struct cpu_user_regs *regs,
+ const union hsr hsr,
+ paddr_t gpa)
+ {
 const struct hsr_dabt dabt = hsr.dabt;
 mmio_info_t info = {
 .gpa = gpa,
@@ -1879,11 +1879,11 @@ static bool try_handle_mmio(struct cpu_user_regs *regs,
 
 /* stage-1 page table should never live in an emulated MMIO region */
 if ( dabt.s1ptw )
-return false;
+return IO_UNHANDLED;
 
 /* All the instructions used on emulated MMIO region should be valid */
 if ( !dabt.valid )
-return false;
+return IO_UNHANDLED;
 
 /*
  * Erratum 766422: Thumb store translation fault to Hypervisor may
@@ -1896,11 +1896,11 @@ static bool try_handle_mmio(struct cpu_user_regs *regs,
 if ( rc )
 {
 gprintk(XENLOG_DEBUG, "Unable to decode instruction\n");
-return false;
+return IO_ABORT;
 }
 }
 
-return !!handle_mmio();
+return handle_mmio();
 }
 
 /*
@@ -2005,10 +2005,21 @@ static void do_trap_stage2_abort_guest(struct 
cpu_user_regs