Re: [PATCH] x86/xen: fix percpu vcpu_info allocation
On Fri, Nov 24, 2023 at 08:48:52AM +0100, Juergen Gross wrote:
> Today the percpu struct vcpu_info is allocated via DEFINE_PER_CPU(),
> meaning that it could cross a page boundary. In this case registering
> it with the hypervisor will fail, resulting in a panic().
>
> This can easily be fixed by using DEFINE_PER_CPU_ALIGNED() instead,
> as struct vcpu_info is guaranteed to have a size of 64 bytes, matching
> the cache line size of x86 64-bit processors (Xen doesn't support
> 32-bit processors).
>
> Fixes: 5ead97c84fa7 ("xen: Core Xen implementation")
> Signed-off-by: Juergen Gross
FWIW, on FreeBSD we also switched to the same approach quite
recently:
https://cgit.freebsd.org/src/commit/sys/xen/xen_common.c?id=20fc5bf7df1db698f2651eaa04a3bc71290e1636
Should have checked the Linux side, sorry.
Roger.
Re: [PATCH] x86/xen: fix percpu vcpu_info allocation
On 27.11.23 16:00, Jan Beulich wrote:
On 27.11.2023 15:57, Boris Ostrovsky wrote:
On 11/24/23 2:48 AM, Juergen Gross wrote:
Today the percpu struct vcpu_info is allocated via DEFINE_PER_CPU(),
meaning that it could cross a page boundary. In this case registering
it with the hypervisor will fail, resulting in a panic().
This can easily be fixed by using DEFINE_PER_CPU_ALIGNED() instead,
as struct vcpu_info is guaranteed to have a size of 64 bytes, matching
the cache line size of x86 64-bit processors (Xen doesn't support
32-bit processors).
Fixes: 5ead97c84fa7 ("xen: Core Xen implementation")
Signed-off-by: Juergen Gross
Reviewed-by: Boris Ostrovsky
although I am not sure in usefulness of BUILD_BUG_ON --- 64 bytes is part of
ABI and hypervisor already has its own BUILD_BUG_ON for this.
I understood the check to guard against SMP_CACHE_BYTES < 64.
Yes, that was the idea. Better safe than sorry.
Juergen
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Re: [PATCH] x86/xen: fix percpu vcpu_info allocation
On 27.11.2023 15:57, Boris Ostrovsky wrote:
>
>
> On 11/24/23 2:48 AM, Juergen Gross wrote:
>> Today the percpu struct vcpu_info is allocated via DEFINE_PER_CPU(),
>> meaning that it could cross a page boundary. In this case registering
>> it with the hypervisor will fail, resulting in a panic().
>>
>> This can easily be fixed by using DEFINE_PER_CPU_ALIGNED() instead,
>> as struct vcpu_info is guaranteed to have a size of 64 bytes, matching
>> the cache line size of x86 64-bit processors (Xen doesn't support
>> 32-bit processors).
>>
>> Fixes: 5ead97c84fa7 ("xen: Core Xen implementation")
>> Signed-off-by: Juergen Gross
>
> Reviewed-by: Boris Ostrovsky
>
> although I am not sure in usefulness of BUILD_BUG_ON --- 64 bytes is part of
> ABI and hypervisor already has its own BUILD_BUG_ON for this.
I understood the check to guard against SMP_CACHE_BYTES < 64.
Jan
>> @@ -160,6 +163,7 @@ void xen_vcpu_setup(int cpu)
>> int err;
>> struct vcpu_info *vcpup;
>>
>> +BUILD_BUG_ON(sizeof(*vcpup) > SMP_CACHE_BYTES);
>> BUG_ON(HYPERVISOR_shared_info == _dummy_shared_info);
>
Re: [PATCH] x86/xen: fix percpu vcpu_info allocation
On 11/24/23 2:48 AM, Juergen Gross wrote:
Today the percpu struct vcpu_info is allocated via DEFINE_PER_CPU(),
meaning that it could cross a page boundary. In this case registering
it with the hypervisor will fail, resulting in a panic().
This can easily be fixed by using DEFINE_PER_CPU_ALIGNED() instead,
as struct vcpu_info is guaranteed to have a size of 64 bytes, matching
the cache line size of x86 64-bit processors (Xen doesn't support
32-bit processors).
Fixes: 5ead97c84fa7 ("xen: Core Xen implementation")
Signed-off-by: Juergen Gross
Reviewed-by: Boris Ostrovsky
although I am not sure in usefulness of BUILD_BUG_ON --- 64 bytes is part of
ABI and hypervisor already has its own BUILD_BUG_ON for this.
-boris
@@ -160,6 +163,7 @@ void xen_vcpu_setup(int cpu)
int err;
struct vcpu_info *vcpup;
+ BUILD_BUG_ON(sizeof(*vcpup) > SMP_CACHE_BYTES);
BUG_ON(HYPERVISOR_shared_info == _dummy_shared_info);
[PATCH] x86/xen: fix percpu vcpu_info allocation
Today the percpu struct vcpu_info is allocated via DEFINE_PER_CPU(),
meaning that it could cross a page boundary. In this case registering
it with the hypervisor will fail, resulting in a panic().
This can easily be fixed by using DEFINE_PER_CPU_ALIGNED() instead,
as struct vcpu_info is guaranteed to have a size of 64 bytes, matching
the cache line size of x86 64-bit processors (Xen doesn't support
32-bit processors).
Fixes: 5ead97c84fa7 ("xen: Core Xen implementation")
Signed-off-by: Juergen Gross
---
arch/x86/xen/enlighten.c | 6 +-
arch/x86/xen/xen-ops.h | 2 +-
2 files changed, 6 insertions(+), 2 deletions(-)
diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index 0337392a3121..3c61bb98c10e 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -33,9 +33,12 @@ EXPORT_SYMBOL_GPL(hypercall_page);
* and xen_vcpu_setup for details. By default it points to
share_info->vcpu_info
* but during boot it is switched to point to xen_vcpu_info.
* The pointer is used in xen_evtchn_do_upcall to acknowledge pending events.
+ * Make sure that xen_vcpu_info doesn't cross a page boundary by making it
+ * cache-line aligned (the struct is guaranteed to have a size of 64 bytes,
+ * which matches the cache line size of 64-bit x86 processors).
*/
DEFINE_PER_CPU(struct vcpu_info *, xen_vcpu);
-DEFINE_PER_CPU(struct vcpu_info, xen_vcpu_info);
+DEFINE_PER_CPU_ALIGNED(struct vcpu_info, xen_vcpu_info);
/* Linux <-> Xen vCPU id mapping */
DEFINE_PER_CPU(uint32_t, xen_vcpu_id);
@@ -160,6 +163,7 @@ void xen_vcpu_setup(int cpu)
int err;
struct vcpu_info *vcpup;
+ BUILD_BUG_ON(sizeof(*vcpup) > SMP_CACHE_BYTES);
BUG_ON(HYPERVISOR_shared_info == _dummy_shared_info);
/*
diff --git a/arch/x86/xen/xen-ops.h b/arch/x86/xen/xen-ops.h
index 408a2aa66c69..a87ab36889e7 100644
--- a/arch/x86/xen/xen-ops.h
+++ b/arch/x86/xen/xen-ops.h
@@ -21,7 +21,7 @@ extern void *xen_initial_gdt;
struct trap_info;
void xen_copy_trap_info(struct trap_info *traps);
-DECLARE_PER_CPU(struct vcpu_info, xen_vcpu_info);
+DECLARE_PER_CPU_ALIGNED(struct vcpu_info, xen_vcpu_info);
DECLARE_PER_CPU(unsigned long, xen_cr3);
DECLARE_PER_CPU(unsigned long, xen_current_cr3);
--
2.35.3
