On Tue, Oct 04, 2011 at 11:28:51AM +0200, Avi Kivity wrote:
> 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?
>
Dead code, that likely to be inlined, on a hot path.
> >> >
> >> >+
> >> >+ 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?
For those yes. We do not cache their value currently. Can be added, but
this is independent optimization.
>
> >>
> >> 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).
Ah, you are talking about not yet existent cache. Then I see why it
should be in loaded_vmcs for EFER and PERF_CTRL.
>
> >>
> >> 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.
>
Yeah, I am trying it now it it looks nicer.
--
Gleb.
--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
