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

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
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).


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