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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 12.09.14 at 16:21, <boris.ostrovsky@xxxxxxxxxx> wrote:
> On 09/12/2014 02:50 AM, Jan Beulich wrote:
>> Okay, so you're tailoring the hypervisor interface to Linux. That's
>> not what we generally aim for, and hence I continue to think this
>> isn't the right approach.
> 
> But we know that at least one consumer of this interface (Linux/perf) 
> does want to have full GPR set.

You said they use a struct pt_regs somewhere - that's not the same
as struct cpu_user_regs (and the overlap of registers being in both
is not much more than a coincidence), and you didn't clarify yet how
much of this information is _actually_ getting consumed (i.e. this may
well just happen to be a suitable container for conveying a smaller
set).

> So why not provide it? Especially since 
> (I suspect that) doing memcpy may be faster than copying only selected 
> fields.
> 
> If you are categorically against this I can certainly rework this and 
> pass RIP, RSP, CS and EFLAGS only.

My problem with this is that (a) it exposes an arbitrary subset of
registers and (b) in an incompatible way (a 32-bit profiling domain
should very well be told all 64-bit registers of a 64-bit guest it
profiles). Since b would need addressing by a custom structure
anyway, dealing with a at the same time seems reasonable to me.

But of course it would help if other interested parties would voice
their view of this...

Jan


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