Re: [RFC PATCH v1 01/10] pmu.h: add a BUILD_BUG_ON to ensure it fits within one page

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.07.2025 12:22, Edwin Torok wrote:
> On Mon, Jul 28, 2025 at 11:21 AM Andrew Cooper
> <andrew.cooper3@xxxxxxxxxx> wrote:
>>
>> On 25/07/2025 4:06 pm, Edwin Török wrote:
>>> Linux already has a similar BUILD_BUG_ON.
>>> Currently this struct is ~224 bytes on x86-64.
>>>
>>> No functional change.
>>>
>>> Signed-off-by: Edwin Török <edwin.torok@xxxxxxxxx>
>>> ---
>>>  xen/arch/x86/cpu/vpmu.c  | 1 +
>>>  xen/include/public/pmu.h | 3 +++
>>>  2 files changed, 4 insertions(+)
>>>
>>> diff --git a/xen/arch/x86/cpu/vpmu.c b/xen/arch/x86/cpu/vpmu.c
>>> index c28192ea26..7be79c2d00 100644
>>> --- a/xen/arch/x86/cpu/vpmu.c
>>> +++ b/xen/arch/x86/cpu/vpmu.c
>>> @@ -401,6 +401,7 @@ static int vpmu_arch_initialise(struct vcpu *v)
>>>      uint8_t vendor = current_cpu_data.x86_vendor;
>>>      int ret;
>>>
>>> +    BUILD_BUG_ON(sizeof(struct xen_pmu_data) > PAGE_SIZE);
>>>      BUILD_BUG_ON(sizeof(struct xen_pmu_intel_ctxt) > XENPMU_CTXT_PAD_SZ);
>>>      BUILD_BUG_ON(sizeof(struct xen_pmu_amd_ctxt) > XENPMU_CTXT_PAD_SZ);
>>>      BUILD_BUG_ON(sizeof(struct xen_pmu_regs) > XENPMU_REGS_PAD_SZ);
>>
>> This is fine (even if it ought to be elsewhere, but don't worry about that).
>>
>>> diff --git a/xen/include/public/pmu.h b/xen/include/public/pmu.h
>>> index af8b7babdd..15decc024d 100644
>>> --- a/xen/include/public/pmu.h
>>> +++ b/xen/include/public/pmu.h
>>> @@ -93,6 +93,9 @@ DEFINE_XEN_GUEST_HANDLE(xen_pmu_params_t);
>>>   * Architecture-independent fields of xen_pmu_data are WO for the 
>>> hypervisor
>>>   * and RO for the guest but some fields in xen_pmu_arch can be writable
>>>   * by both the hypervisor and the guest (see arch-$arch/pmu.h).
>>> + *
>>> + * PAGE_SIZE bytes of memory are allocated.
>>> + * This struct cannot be larger than PAGE_SIZE.
>>
>> This isn't.  Xen's PAGE_SIZE is not necessarily the same as PAGE_SIZE in
>> the guest consuming this header.
>>
>> This highlights one of the problems that Xen's ABI entrenches.  Being
>> x86-only, it's 4k in practice, but there's no easy solution.
>>
>> I'd just skip this comment.  Anything else is going to get tied up in
>> unrelated bigger problems.
> 
> Thanks, I'll drop this comment in the next version of the series.

As said, I'm happy to ack the change with the comment adjustment dropped.
That is, I could easily carry out what you say above while committing.

Jan