> On 30 Jul 2020, at 22:50, Julien Grall <jul...@xen.org> wrote:
>
> Hi Bertrand,
>
> To avoid extra work on your side, I would recommend to wait a bit before
> sending a new version. It would be good to at least settle the conversation
> in v2 regarding the approach taken.
Thanks for the advice.
I thought all discussions where done the first time we started the discussion
on the last thread but there are apparently more point to go through.
>
> On 30/07/2020 11:24, Bertrand Marquis wrote:
>> At the moment on Arm, a Linux guest running with KTPI enabled will
>> cause the following error when a context switch happens in user mode:
>> (XEN) p2m.c:1890: d1v0: Failed to walk page-table va 0xffffff837ebe0cd0
>> The error is caused by the virtual address for the runstate area
>> registered by the guest only being accessible when the guest is running
>> in kernel space when KPTI is enabled.
>> To solve this issue, this patch is doing the translation from virtual
>> address to physical address during the hypercall and mapping the
>> required pages using vmap. This is removing the conversion from virtual
>> to physical address during the context switch which is solving the
>> problem with KPTI.
>
> To echo what Jan said on the previous version, this is a change in a stable
> ABI and therefore may break existing guest. FAOD, I agree in principle with
> the idea. However, we want to explain why breaking the ABI is the *only*
> viable solution.
>
> From my understanding, it is not possible to fix without an ABI breakage
> because the hypervisor doesn't know when the guest will switch back from
> userspace to kernel space. The risk is the information provided by the
> runstate wouldn't contain accurate information and could affect how the guest
> handle stolen time.
>
> Additionally there are a few issues with the current interface:
> 1) It is assuming the virtual address cannot be re-used by the userspace.
> Thanksfully Linux have a split address space. But this may change with KPTI
> in place.
> 2) When update the page-tables, the guest has to go through an invalid
> mapping. So the translation may fail at any point.
>
> IOW, the existing interface can lead to random memory corruption and
> inacurracy of the stolen time.
I agree but i am not sure what you want me to do here.
Should i add more details in the commit message ?
>
>> This is done only on arm architecture, the behaviour on x86 is not
>> modified by this patch and the address conversion is done as before
>> during each context switch.
>> This is introducing several limitations in comparison to the previous
>> behaviour (on arm only):
>> - if the guest is remapping the area at a different physical address Xen
>> will continue to update the area at the previous physical address. As
>> the area is in kernel space and usually defined as a global variable this
>> is something which is believed not to happen. If this is required by a
>> guest, it will have to call the hypercall with the new area (even if it
>> is at the same virtual address).
>> - the area needs to be mapped during the hypercall. For the same reasons
>> as for the previous case, even if the area is registered for a different
>> vcpu. It is believed that registering an area using a virtual address
>> unmapped is not something done.
>
> This is not clear whether the virtual address refer to the current vCPU or
> the vCPU you register the runstate for. From the past discussion, I think you
> refer to the former. It would be good to clarify.
Ok i will try to clarify.
>
> Additionally, all the new restrictions should be documented in the public
> interface. So an OS developper can find the differences between the
> architectures.
>
> To answer Jan's concern, we certainly don't know all the guest OSes existing,
> however we also need to balance the benefit for a large majority of the users.
>
> From previous discussion, the current approach was deemed to be acceptable on
> Arm and, AFAICT, also x86 (see [1]).
>
> TBH, I would rather see the approach to be common. For that, we would an
> agreement from Andrew and Jan in the approach here. Meanwhile, I think this
> is the best approach to address the concern from Arm users.
>From this I get that you want me to document the specific behaviour on Arm on
>the public header describing the hypercall, right ?
Bertrand
>
>> inline functions in headers could not be used as the architecture
>> domain.h is included before the global domain.h making it impossible
>> to use the struct vcpu inside the architecture header.
>> This should not have any performance impact as the hypercall is only
>> called once per vcpu usually.
>> Signed-off-by: Bertrand Marquis <bertrand.marq...@arm.com>
>> ---
>> Changes in v2
>> - use vmap to map the pages during the hypercall.
>> - reintroduce initial copy during hypercall.
>> Changes in v3
>> - Fix Coding style
>> - Fix vaddr printing on arm32
>> - use write_atomic to modify state_entry_time update bit (only
>> in guest structure as the bit is not used inside Xen copy)
>> ---
>> xen/arch/arm/domain.c | 161 ++++++++++++++++++++++++++++++-----
>> xen/arch/x86/domain.c | 29 ++++++-
>> xen/arch/x86/x86_64/domain.c | 4 +-
>> xen/common/domain.c | 19 ++---
>> xen/include/asm-arm/domain.h | 9 ++
>> xen/include/asm-x86/domain.h | 16 ++++
>> xen/include/xen/domain.h | 5 ++
>> xen/include/xen/sched.h | 16 +---
>> 8 files changed, 206 insertions(+), 53 deletions(-)
>> diff --git a/xen/arch/arm/domain.c b/xen/arch/arm/domain.c
>> index 31169326b2..8b36946017 100644
>> --- a/xen/arch/arm/domain.c
>> +++ b/xen/arch/arm/domain.c
>> @@ -19,6 +19,7 @@
>> #include <xen/sched.h>
>> #include <xen/softirq.h>
>> #include <xen/wait.h>
>> +#include <xen/vmap.h>
>> #include <asm/alternative.h>
>> #include <asm/cpuerrata.h>
>> @@ -275,36 +276,156 @@ static void ctxt_switch_to(struct vcpu *n)
>> virt_timer_restore(n);
>> }
>> -/* Update per-VCPU guest runstate shared memory area (if registered). */
>> -static void update_runstate_area(struct vcpu *v)
>> +static void cleanup_runstate_vcpu_locked(struct vcpu *v)
>> {
>> - void __user *guest_handle = NULL;
>> + if ( v->arch.runstate_guest )
>> + {
>> + vunmap((void *)((unsigned long)v->arch.runstate_guest & PAGE_MASK));
>> +
>> + put_page(v->arch.runstate_guest_page[0]);
>> +
>> + if ( v->arch.runstate_guest_page[1] )
>> + put_page(v->arch.runstate_guest_page[1]);
>> +
>> + v->arch.runstate_guest = NULL;
>> + }
>> +}
>> +
>> +void arch_vcpu_cleanup_runstate(struct vcpu *v)
>> +{
>> + spin_lock(&v->arch.runstate_guest_lock);
>> +
>> + cleanup_runstate_vcpu_locked(v);
>> +
>> + spin_unlock(&v->arch.runstate_guest_lock);
>> +}
>> +
>> +static int setup_runstate_vcpu_locked(struct vcpu *v, vaddr_t vaddr)
>> +{
>> + unsigned int offset;
>> + mfn_t mfn[2];
>> + struct page_info *page;
>> + unsigned int numpages;
>> struct vcpu_runstate_info runstate;
>> + void *p;
>> - if ( guest_handle_is_null(runstate_guest(v)) )
>> - return;
>> + /* user can pass a NULL address to unregister a previous area */
>> + if ( vaddr == 0 )
>> + return 0;
>> +
>> + offset = vaddr & ~PAGE_MASK;
>> +
>> + /* provided address must be aligned to a 64bit */
>> + if ( offset % alignof(struct vcpu_runstate_info) )
>
> This new restriction wants to be explained in the commit message and public
> header.
>
>> + {
>> + gprintk(XENLOG_WARNING, "Cannot map runstate pointer at 0x%"PRIvaddr
>> + ": Invalid offset\n", vaddr);
>
> We usually enforce 80 character per lines except for format string. So it is
> easier to grep them in the code.
>
>> + return -EINVAL;
>> + }
>> +
>> + page = get_page_from_gva(v, vaddr, GV2M_WRITE);
>> + if ( !page )
>> + {
>> + gprintk(XENLOG_WARNING, "Cannot map runstate pointer at 0x%"PRIvaddr
>> + ": Page is not mapped\n", vaddr);
>> + return -EINVAL;
>> + }
>> +
>> + mfn[0] = page_to_mfn(page);
>> + v->arch.runstate_guest_page[0] = page;
>> +
>> + if ( offset > (PAGE_SIZE - sizeof(struct vcpu_runstate_info)) )
>> + {
>> + /* guest area is crossing pages */
>> + page = get_page_from_gva(v, vaddr + PAGE_SIZE, GV2M_WRITE);
>> + if ( !page )
>> + {
>> + put_page(v->arch.runstate_guest_page[0]);
>> + gprintk(XENLOG_WARNING,
>> + "Cannot map runstate pointer at 0x%"PRIvaddr
>> + ": 2nd Page is not mapped\n", vaddr);
>> + return -EINVAL;
>> + }
>> + mfn[1] = page_to_mfn(page);
>> + v->arch.runstate_guest_page[1] = page;
>> + numpages = 2;
>> + }
>> + else
>> + {
>> + v->arch.runstate_guest_page[1] = NULL;
>> + numpages = 1;
>> + }
>> - memcpy(&runstate, &v->runstate, sizeof(runstate));
>> + p = vmap(mfn, numpages);
>> + if ( !p )
>> + {
>> + put_page(v->arch.runstate_guest_page[0]);
>> + if ( numpages == 2 )
>> + put_page(v->arch.runstate_guest_page[1]);
>> - if ( VM_ASSIST(v->domain, runstate_update_flag) )
>> + gprintk(XENLOG_WARNING, "Cannot map runstate pointer at 0x%"PRIvaddr
>> + ": vmap error\n", vaddr);
>> + return -EINVAL;
>> + }
>> +
>> + v->arch.runstate_guest = p + offset;
>> +
>> + if (v == current)
>> + memcpy(v->arch.runstate_guest, &v->runstate, sizeof(v->runstate));
>> + else
>> {
>> - guest_handle = &v->runstate_guest.p->state_entry_time + 1;
>> - guest_handle--;
>> - runstate.state_entry_time |= XEN_RUNSTATE_UPDATE;
>> - __raw_copy_to_guest(guest_handle,
>> - (void *)(&runstate.state_entry_time + 1) - 1,
>> 1);
>> - smp_wmb();
>> + vcpu_runstate_get(v, &runstate);
>> + memcpy(v->arch.runstate_guest, &runstate, sizeof(v->runstate));
>> }
>> - __copy_to_guest(runstate_guest(v), &runstate, 1);
>> + return 0;
>> +}
>> +
>> +int arch_vcpu_setup_runstate(struct vcpu *v,
>> + struct vcpu_register_runstate_memory_area area)
>> +{
>> + int rc;
>> +
>> + spin_lock(&v->arch.runstate_guest_lock);
>> +
>> + /* cleanup if we are recalled */
>> + cleanup_runstate_vcpu_locked(v);
>> +
>> + rc = setup_runstate_vcpu_locked(v, (vaddr_t)area.addr.v);
>> +
>> + spin_unlock(&v->arch.runstate_guest_lock);
>> - if ( guest_handle )
>> + return rc;
>> +}
>> +
>> +
>> +/* Update per-VCPU guest runstate shared memory area (if registered). */
>> +static void update_runstate_area(struct vcpu *v)
>> +{
>> + spin_lock(&v->arch.runstate_guest_lock);
>> +
>> + if ( v->arch.runstate_guest )
>> {
>> - runstate.state_entry_time &= ~XEN_RUNSTATE_UPDATE;
>> - smp_wmb();
>> - __raw_copy_to_guest(guest_handle,
>> - (void *)(&runstate.state_entry_time + 1) - 1,
>> 1);
>> + if ( VM_ASSIST(v->domain, runstate_update_flag) )
>> + {
>> + v->runstate.state_entry_time |= XEN_RUNSTATE_UPDATE;
>> + write_atomic(&(v->arch.runstate_guest->state_entry_time),
>> + v->runstate.state_entry_time);
>
> NIT: You want to indent v-> at the same level as the argument from the first
> line.
>
> Also, I think you are missing a smp_wmb() here.
>
>> + }
>> +
>> + memcpy(v->arch.runstate_guest, &v->runstate, sizeof(v->runstate));
>> +
>> + if ( VM_ASSIST(v->domain, runstate_update_flag) )
>> + {
>> + /* copy must be done before switching the bit */
>> + smp_wmb();
>> + v->runstate.state_entry_time &= ~XEN_RUNSTATE_UPDATE;
>> + write_atomic(&(v->arch.runstate_guest->state_entry_time),
>> + v->runstate.state_entry_time);
>
> Same remark for the indentation.
>
>> + }
>> }
>> +
>> + spin_unlock(&v->arch.runstate_guest_lock);
>> }
>> static void schedule_tail(struct vcpu *prev)
>> @@ -560,6 +681,8 @@ int arch_vcpu_create(struct vcpu *v)
>> v->arch.saved_context.sp = (register_t)v->arch.cpu_info;
>> v->arch.saved_context.pc = (register_t)continue_new_vcpu;
>> + spin_lock_init(&v->arch.runstate_guest_lock);
>> +
>> /* Idle VCPUs don't need the rest of this setup */
>> if ( is_idle_vcpu(v) )
>> return rc;
>> diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
>> index fee6c3931a..b9b81e94e5 100644
>> --- a/xen/arch/x86/domain.c
>> +++ b/xen/arch/x86/domain.c
>> @@ -1642,6 +1642,29 @@ void paravirt_ctxt_switch_to(struct vcpu *v)
>> wrmsr_tsc_aux(v->arch.msrs->tsc_aux);
>> }
>> +int arch_vcpu_setup_runstate(struct vcpu *v,
>> + struct vcpu_register_runstate_memory_area area)
>> +{
>> + struct vcpu_runstate_info runstate;
>> +
>> + runstate_guest(v) = area.addr.h;
>> +
>> + if ( v == current )
>> + __copy_to_guest(runstate_guest(v), &v->runstate, 1);
>> + else
>> + {
>> + vcpu_runstate_get(v, &runstate);
>> + __copy_to_guest(runstate_guest(v), &runstate, 1);
>> + }
>> +
>> + return 0;
>> +}
>> +
>> +void arch_vcpu_cleanup_runstate(struct vcpu *v)
>> +{
>> + set_xen_guest_handle(runstate_guest(v), NULL);
>> +}
>> +
>> /* Update per-VCPU guest runstate shared memory area (if registered). */
>> bool update_runstate_area(struct vcpu *v)
>> {
>> @@ -1660,8 +1683,8 @@ bool update_runstate_area(struct vcpu *v)
>> if ( VM_ASSIST(v->domain, runstate_update_flag) )
>> {
>> guest_handle = has_32bit_shinfo(v->domain)
>> - ? &v->runstate_guest.compat.p->state_entry_time + 1
>> - : &v->runstate_guest.native.p->state_entry_time + 1;
>> + ? &v->arch.runstate_guest.compat.p->state_entry_time + 1
>> + : &v->arch.runstate_guest.native.p->state_entry_time + 1;
>> guest_handle--;
>> runstate.state_entry_time |= XEN_RUNSTATE_UPDATE;
>> __raw_copy_to_guest(guest_handle,
>> @@ -1674,7 +1697,7 @@ bool update_runstate_area(struct vcpu *v)
>> struct compat_vcpu_runstate_info info;
>> XLAT_vcpu_runstate_info(&info, &runstate);
>> - __copy_to_guest(v->runstate_guest.compat, &info, 1);
>> + __copy_to_guest(v->arch.runstate_guest.compat, &info, 1);
>> rc = true;
>> }
>> else
>> diff --git a/xen/arch/x86/x86_64/domain.c b/xen/arch/x86/x86_64/domain.c
>> index c46dccc25a..b879e8dd2c 100644
>> --- a/xen/arch/x86/x86_64/domain.c
>> +++ b/xen/arch/x86/x86_64/domain.c
>> @@ -36,7 +36,7 @@ arch_compat_vcpu_op(
>> break;
>> rc = 0;
>> - guest_from_compat_handle(v->runstate_guest.compat, area.addr.h);
>> + guest_from_compat_handle(v->arch.runstate_guest.compat,
>> area.addr.h);
>> if ( v == current )
>> {
>> @@ -49,7 +49,7 @@ arch_compat_vcpu_op(
>> vcpu_runstate_get(v, &runstate);
>> XLAT_vcpu_runstate_info(&info, &runstate);
>> }
>> - __copy_to_guest(v->runstate_guest.compat, &info, 1);
>> + __copy_to_guest(v->arch.runstate_guest.compat, &info, 1);
>> break;
>> }
>> diff --git a/xen/common/domain.c b/xen/common/domain.c
>> index f0f9c62feb..739c6b7b62 100644
>> --- a/xen/common/domain.c
>> +++ b/xen/common/domain.c
>> @@ -727,7 +727,10 @@ int domain_kill(struct domain *d)
>> if ( cpupool_move_domain(d, cpupool0) )
>> return -ERESTART;
>> for_each_vcpu ( d, v )
>> + {
>> + arch_vcpu_cleanup_runstate(v);
>> unmap_vcpu_info(v);
>> + }
>> d->is_dying = DOMDYING_dead;
>> /* Mem event cleanup has to go here because the rings
>> * have to be put before we call put_domain. */
>> @@ -1167,7 +1170,7 @@ int domain_soft_reset(struct domain *d)
>> for_each_vcpu ( d, v )
>> {
>> - set_xen_guest_handle(runstate_guest(v), NULL);
>> + arch_vcpu_cleanup_runstate(v);
>> unmap_vcpu_info(v);
>> }
>> @@ -1494,7 +1497,6 @@ long do_vcpu_op(int cmd, unsigned int vcpuid,
>> XEN_GUEST_HANDLE_PARAM(void) arg)
>> case VCPUOP_register_runstate_memory_area:
>> {
>> struct vcpu_register_runstate_memory_area area;
>> - struct vcpu_runstate_info runstate;
>> rc = -EFAULT;
>> if ( copy_from_guest(&area, arg, 1) )
>> @@ -1503,18 +1505,7 @@ long do_vcpu_op(int cmd, unsigned int vcpuid,
>> XEN_GUEST_HANDLE_PARAM(void) arg)
>> if ( !guest_handle_okay(area.addr.h, 1) )
>> break;
>> - rc = 0;
>> - runstate_guest(v) = area.addr.h;
>> -
>> - if ( v == current )
>> - {
>> - __copy_to_guest(runstate_guest(v), &v->runstate, 1);
>> - }
>> - else
>> - {
>> - vcpu_runstate_get(v, &runstate);
>> - __copy_to_guest(runstate_guest(v), &runstate, 1);
>> - }
>> + rc = arch_vcpu_setup_runstate(v, area);
>> break;
>> }
>> diff --git a/xen/include/asm-arm/domain.h b/xen/include/asm-arm/domain.h
>> index 6819a3bf38..2f62c3e8f5 100644
>> --- a/xen/include/asm-arm/domain.h
>> +++ b/xen/include/asm-arm/domain.h
>> @@ -204,6 +204,15 @@ struct arch_vcpu
>> */
>> bool need_flush_to_ram;
>> + /* runstate guest lock */
>> + spinlock_t runstate_guest_lock;
>> +
>> + /* runstate guest info */
>> + struct vcpu_runstate_info *runstate_guest;
>> +
>> + /* runstate pages mapped for runstate_guest */
>> + struct page_info *runstate_guest_page[2];
>> +
>> } __cacheline_aligned;
>> void vcpu_show_execution_state(struct vcpu *);
>> diff --git a/xen/include/asm-x86/domain.h b/xen/include/asm-x86/domain.h
>> index 635335634d..007ccfbf9f 100644
>> --- a/xen/include/asm-x86/domain.h
>> +++ b/xen/include/asm-x86/domain.h
>> @@ -11,6 +11,11 @@
>> #include <asm/x86_emulate.h>
>> #include <public/vcpu.h>
>> #include <public/hvm/hvm_info_table.h>
>> +#ifdef CONFIG_COMPAT
>> +#include <compat/vcpu.h>
>> +DEFINE_XEN_GUEST_HANDLE(vcpu_runstate_info_compat_t);
>> +#endif
>> +
>> #define has_32bit_shinfo(d) ((d)->arch.has_32bit_shinfo)
>> @@ -638,6 +643,17 @@ struct arch_vcpu
>> struct {
>> bool next_interrupt_enabled;
>> } monitor;
>> +
>> +#ifndef CONFIG_COMPAT
>> +# define runstate_guest(v) ((v)->arch.runstate_guest)
>> + XEN_GUEST_HANDLE(vcpu_runstate_info_t) runstate_guest; /* guest address
>> */
>> +#else
>> +# define runstate_guest(v) ((v)->arch.runstate_guest.native)
>> + union {
>> + XEN_GUEST_HANDLE(vcpu_runstate_info_t) native;
>> + XEN_GUEST_HANDLE(vcpu_runstate_info_compat_t) compat;
>> + } runstate_guest;
>> +#endif
>> };
>> struct guest_memory_policy
>> diff --git a/xen/include/xen/domain.h b/xen/include/xen/domain.h
>> index 7e51d361de..5e8cbba31d 100644
>> --- a/xen/include/xen/domain.h
>> +++ b/xen/include/xen/domain.h
>> @@ -5,6 +5,7 @@
>> #include <xen/types.h>
>> #include <public/xen.h>
>> +#include <public/vcpu.h>
>> #include <asm/domain.h>
>> #include <asm/numa.h>
>> @@ -63,6 +64,10 @@ void arch_vcpu_destroy(struct vcpu *v);
>> int map_vcpu_info(struct vcpu *v, unsigned long gfn, unsigned offset);
>> void unmap_vcpu_info(struct vcpu *v);
>> +int arch_vcpu_setup_runstate(struct vcpu *v,
>> + struct vcpu_register_runstate_memory_area
>> area);
>> +void arch_vcpu_cleanup_runstate(struct vcpu *v);
>> +
>> int arch_domain_create(struct domain *d,
>> struct xen_domctl_createdomain *config);
>> diff --git a/xen/include/xen/sched.h b/xen/include/xen/sched.h
>> index ac53519d7f..fac030fb83 100644
>> --- a/xen/include/xen/sched.h
>> +++ b/xen/include/xen/sched.h
>> @@ -29,11 +29,6 @@
>> #include <public/vcpu.h>
>> #include <public/event_channel.h>
>> -#ifdef CONFIG_COMPAT
>> -#include <compat/vcpu.h>
>> -DEFINE_XEN_GUEST_HANDLE(vcpu_runstate_info_compat_t);
>> -#endif
>> -
>> /*
>> * Stats
>> *
>> @@ -166,16 +161,7 @@ struct vcpu
>> struct sched_unit *sched_unit;
>> struct vcpu_runstate_info runstate;
>> -#ifndef CONFIG_COMPAT
>> -# define runstate_guest(v) ((v)->runstate_guest)
>> - XEN_GUEST_HANDLE(vcpu_runstate_info_t) runstate_guest; /* guest address
>> */
>> -#else
>> -# define runstate_guest(v) ((v)->runstate_guest.native)
>> - union {
>> - XEN_GUEST_HANDLE(vcpu_runstate_info_t) native;
>> - XEN_GUEST_HANDLE(vcpu_runstate_info_compat_t) compat;
>> - } runstate_guest; /* guest address */
>> -#endif
>> +
>> unsigned int new_state;
>> /* Has the FPU been initialised? */
>
> [1] <b6511a29-35a4-a1d0-dd29-7de4103ec...@citrix.com>
>
>
> --
> Julien Grall
