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Re: [Xen-devel] [PATCH v9 09/20] x86/VPMU: Add public xenpmu.h

On 08/11/2014 12:15 PM, Boris Ostrovsky wrote:
On 08/11/2014 10:08 AM, Jan Beulich wrote:
On 08.08.14 at 18:55, <boris.ostrovsky@xxxxxxxxxx> wrote:
  static int amd_vpmu_initialise(struct vcpu *v)
-    struct amd_vpmu_context *ctxt;
+    struct xen_pmu_amd_ctxt *ctxt;
      struct vpmu_struct *vpmu = vcpu_vpmu(v);
      uint8_t family = current_cpu_data.x86;
  @@ -382,7 +386,8 @@ static int amd_vpmu_initialise(struct vcpu *v)
  -    ctxt = xzalloc(struct amd_vpmu_context);
+    ctxt = xzalloc_bytes(sizeof(struct xen_pmu_amd_ctxt) +
+                         2 * sizeof(uint64_t) * AMD_MAX_COUNTERS);
So I guess this is one of said sizeof()-s, and I continue to think this
would benefit from using a proper field. It is my understanding that
the real (non-C89 compliant) declaration of struct xen_pmu_amd_ctxt
would be

struct xen_pmu_amd_ctxt {
/* Offsets to counter and control MSRs (relative to xen_arch_pmu.c.amd) */
     uint32_t counters;
     uint32_t ctrls;
     uint64_t values[];

OK, this should work.

Actually, no, it won't: we decided early on that register banks should be outside the fixed portion of the structure. Keeping values[] inside xen_pmu_amd_ctxt doesn't necessarily preclude this *if* it is never referred to by code and is only used for determining the type. But that would be an unreasonable requirement IMO.

I understand the dislike to sizeof(uint64_t). OTOH, your original concern was that we may forget to update size calculation if value changes type sometime later. But the type cannot be anything but uint64_t since it is used in rd/wrmsr().

I also considered introducing pmu_reg_t (typedef'd to uint64_t) but we'd be back again to rd/wrmsr() -- even though compiler may not flag it this still would be rather unnatural.


Just like already done elsewhere you _can_ add variable-sized
arrays to the public interface as long as you guard them suitably.
And then your statement would become

     ctxt = xzalloc_bytes(sizeof(*ctxt) +
                          2 * sizeof(*ctxt->values) * AMD_MAX_COUNTERS);

making clear what the allocation is doing.

@@ -391,7 +396,11 @@ static int amd_vpmu_initialise(struct vcpu *v)
          return -ENOMEM;
  +    ctxt->counters = sizeof(struct xen_pmu_amd_ctxt);
+ ctxt->ctrls = ctxt->counters + sizeof(uint64_t) * AMD_MAX_COUNTERS;
     ctxt->counters = sizeof(*ctxt);
ctxt->ctrls = ctxt->counters + sizeof(*ctxt->values) * AMD_MAX_COUNTERS;


     ctxt->counters = offsetof(typeof(ctxt), values[0]);
     ctxt->ctrls = offsetof(typeof(ctxt), values[AMD_MAX_COUNTERS]);

@@ -228,6 +229,9 @@ void vpmu_initialise(struct vcpu *v)
      struct vpmu_struct *vpmu = vcpu_vpmu(v);
      uint8_t vendor = current_cpu_data.x86_vendor;
+ BUILD_BUG_ON(sizeof(struct xen_pmu_intel_ctxt) > XENPMU_CTXT_PAD_SZ); + BUILD_BUG_ON(sizeof(struct xen_pmu_amd_ctxt) > XENPMU_CTXT_PAD_SZ);
Don't these need to include the variable size array portions too?

Not in BUILD_BUG_ON but I should check whether I have enough space when computing ctxt->{ctrls | counters}.


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