Re: [PATCH 8/9] KVM, VMX: Add support for guest/host-only profiling

[Avi Kivity <avi@redhat.com>]

On 10/03/2011 05:36 PM, Gleb Natapov wrote:
> On Mon, Oct 03, 2011 at 05:00:25PM +0200, Avi Kivity wrote:
> >  On 10/03/2011 03:49 PM, Gleb Natapov wrote:
> >  >Support guest/host-only profiling by switch perf msrs on
> >  >a guest entry if needed.
> >  >
> >  >@@ -6052,6 +6056,26 @@ static void vmx_cancel_injection(struct kvm_vcpu *vcpu)
> >  >   	vmcs_write32(VM_ENTRY_INTR_INFO_FIELD, 0);
> >  >   }
> >  >
> >  >+static void atomic_switch_perf_msrs(struct vcpu_vmx *vmx)
> >  >+{
> >  >+#ifdef CONFIG_PERF_EVENTS
> >
> >  No need for #ifdef (if you also define perf_guest_get_msrs() when
> >  !CONFIG_PERF_EVENTS).
> >
> Yes, but will compiler be smart enough to remove the code of the
> function completely? It will have to figure that vmx->perf_msrs_cnt is
> always 0 somehow.

It won't, but do we care?

> >  >
> >  >+
> >  >+	perf_guest_get_msrs(vmx->perf_msrs_cnt, vmx->perf_msrs);
> >  >+	for (i = 0; i<   vmx->perf_msrs_cnt; i++) {
> >  >+		struct perf_guest_switch_msr *msr =&vmx->perf_msrs[i];
> >  >+		if (msr->host == msr->guest)
> >  >+			clear_atomic_switch_msr(vmx, msr->msr);
> >  >+		else
> >  >+			add_atomic_switch_msr(vmx, msr->msr, msr->guest,
> >  >+					msr->host);
> >
> >  This generates a lot of VMWRITEs even if nothing changes, just to
> >  re-set bits in the VMCS to their existing values.  Need to add
> >  something like this:
> >
> >     if (loaded_vmcs->msr[i].host == msr->host
> >  &&  loaded_vmcs->msr[i].guest == msr->guest)
> >            continue;
> VMWRITE happens only when number of autoloaded MSRs changes (which is
> rare), not on each call to add_atomic_switch_msr(). I thought about
> optimizing this write too by doing
> vmcs_write32(VM_(ENTRY|EXIT)_MSR_LOAD_COUNT, m->nr) only once by
> checking that  m->nr changed during vmentry. Can be done later.

For EFER and PERF_CTRL, it's done unconditionally, no?

> >
> >  btw, shouldn't the msr autoload list be part of loaded_vmcs as well?
> >
> Why?

Any caching is only relative to the vmcs (unless we invalidate the cache 
on vmcs switch).

> >
> >  Do we really need a private buffer?  Perhaps perf_guest_get_msrs()
> >  can return a perf-internal buffer (but then, we will need to copy it
> >  for the optimization above, but that's a separate issue).
> >
> The buffer will be small, so IMHO private one is not an issue. We can
> make it perf internal per cpu buffer I think.
>

I think the API is nicer with perf returning a read-only internal 
buffer; this way there is no kmalloc involved since perf knows its 
internal limits.

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