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

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


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