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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 09/11/2014 11:59 AM, Jan Beulich wrote:
> > > > 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.
Linux perf handler takes struct pt_regs as the its sole argument. If we 
pass only few selected registers from hypervisor to the guest then I 
will be passing garbage (partly) to perf.
(I actually do pass that garbage now in my Linux patch but I will be 
fixing this in that series).

-boris


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