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Re: [Xen-devel] [PATCH v4 13/17] x86/VPMU: Add privileged PMU mode

On 02/04/2014 11:01 AM, Jan Beulich wrote:
On 04.02.14 at 16:53, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx> wrote:
On 02/04/2014 06:31 AM, Jan Beulich wrote:
On 21.01.14 at 20:08, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx> wrote:
+            gregs = &v->arch.vpmu.xenpmu_data->pmu.r.regs;
+            gregs->cs = cs;
And now you store a NUL selector (i.e. just the RPL bits) into the
output field?
-        else if ( !is_control_domain(current->domain) &&
-                 !is_idle_vcpu(current) )
+        else
-            /* PV guest */
+            /* HVM guest */
+            struct segment_register cs;
               gregs = guest_cpu_user_regs();
                      gregs, sizeof(struct cpu_user_regs));
+            hvm_get_segment_register(current, x86_seg_cs, &cs);
+            gregs = &v->arch.vpmu.xenpmu_data->pmu.r.regs;
+            gregs->cs = cs.attr.fields.dpl;
And here too? If that's intended, a code comment is a must.
This is HVM-only path, PVH or PV don't go here so cs should be valid.
Isn't the reply of mine a few lines up in PV code? And why would
the selector being wrong for HVM be okay?

This clause is for privileged profiling: we are in PV clause even though the interrupt is taken by an HVM guest.

The diff is somewhat difficult to follow so here is the flow:

   // in privileged mode 'v' is dom0's CPU.
   if ( !is_hvm_domain(v->domain) || (vpmu_mode & XENPMU_MODE_PRIV) )
        if ( !is_hvm_domain(current->domain) )
             // either PV (including dom0) or Xen is interrupted
             // This is the clause we are discussing. 'current' is HVM
             hvm_get_segment_register(current, x86_seg_cs, &cs);
        send_guest_vcpu_virq(v, VIRQ_XENPMU);
        return 1;

So I think CS should be correct for the guest, no?


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