Re: [Xen-devel] [PATCH v10 09/20] x86/VPMU: Add public xenpmu.h

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 09/11/2014 02:39 AM, Jan Beulich wrote:
> > > > On 10.09.14 at 19:23, <boris.ostrovsky@xxxxxxxxxx> wrote:
> > On 09/10/2014 10:45 AM, Jan Beulich wrote:
> > > > > > On 04.09.14 at 05:41, <boris.ostrovsky@xxxxxxxxxx> wrote:
> > > > +struct xen_pmu_arch {
> > > > +    union {
> > > > +        struct cpu_user_regs regs;
> > > > +        uint8_t pad[256];
> > > > +    } r;
> > > Can you remind me again what you need the union and padding for
> > > here?
> > This structure is laid out in a shared page with a (possibly 32-bit)
> > guest who need to access fields that follow this union.
> Hmm, okay. But how would such a guest make reasonable use of
> the regs field then?
When hypervisor is preparing this data for 32-bit consumer in 
vpmu_do_interrupts() it translates registers to 32-bit version:
        struct compat_cpu_user_regs *cmp;
        gregs = guest_cpu_user_regs();
        cmp = (void *)&vpmu->xenpmu_data->pmu.r.regs;
        XLAT_cpu_user_regs(cmp, gregs);

I remember struggling trying to figure a better way of presenting this 
but ended up with the (void *) cast. IIRC I tried putting 
compat_cpu_user_regs into the union but something didn't quite work 
(with compilation).
> And then - why 256 and not 200? struct
> cpu_user_regs can't change size anyway. Plus, finally, why do
> you expose the GPRs but not any of the other register state?
I wanted to leave some padding in case we decide to add non-GPR 
registers and keep major version of the interface unchanged (only minor 
version will bumped). TBH though, I can't think of any non-GPR registers 
to be ever useful.
-boris

> Yes,
> I realize I should have thought of these earlier, but better now
> than after the new ABI got established.
>
> Jan

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