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

From: Avi Kivity
Date: Tue Oct 04 2011 - 05:29:23 EST

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)
> >+{
> No need for #ifdef (if you also define perf_guest_get_msrs() when
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?

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.

error compiling committee.c: too many arguments to function

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