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

[Edwin Torok <edwin.torok@xxxxxxxxx>]

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.

--Edwin