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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 11.09.14 at 17:26, <boris.ostrovsky@xxxxxxxxxx> wrote:
> On 09/11/2014 10:55 AM, Jan Beulich wrote:
>>>>> 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.
> 
> I believe some perf sub-tools (tracing-related if I am not mistaken) 
> want to have access to traced function's arguments.

And function arguments on x86-64 can very well live in XMM
registers... Hence no, I still don't see why the registers get
exposed here in an incomplete/inconsistent fashion.

Jan


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