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

>>> On 11.09.14 at 15:54, <boris.ostrovsky@xxxxxxxxxx> wrote:
> 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).

Of course that can't work - the compat structure simply doesn't
exist for public headers.

>> 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.

Then what do you need the GPRs for here? I don't think they're
any better or worse than, say, XMM ones. I could see you needing/
wanting some basic stuff like CS:RIP and SS:RSP and maybe EFLAGS,
but that's about it.


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