Messages in this thread | ![/](/images/icornerl.gif) | | Subject | Re: [PATCH v3 04/17] perf: x86/ds: Handle guest PEBS overflow PMI and inject it to guest | From | "Xu, Like" <> | Date | Fri, 15 Jan 2021 22:30:13 +0800 |
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On 2021/1/15 20:01, Peter Zijlstra wrote: > On Thu, Jan 14, 2021 at 11:39:00AM +0800, Xu, Like wrote: > >>> Why do we need to? Can't we simply always forward the PMI if the guest >>> has bits set in MSR_IA32_PEBS_ENABLE ? Surely we can access the guest >>> MSRs at a reasonable rate.. >>> >>> Sure, it'll send too many PMIs, but is that really a problem? >> More vPMI means more guest irq handler calls and >> more PMI virtualization overhead. > Only if you have both guest and host PEBS. And in that case I really > can't be arsed about some overhead to the guest.
Less overhead makes everyone happier.
Ah, can I assume that you're fine with disabling the co-existence of guest PEBS and host PEBS as the first upstream step ?
> >> In addition, >> the correctness of some workloads (RR?) depends on >> the correct number of PMIs and the PMI trigger times >> and virt may not want to break this assumption. > Are you sure? Spurious NMI/PMIs are known to happen anyway. We have far > too much code to deal with them.
https://lore.kernel.org/lkml/20170628130748.GI5981@leverpostej/T/
In the rr workload, the commit change "the PMI interrupts in skid region should be dropped" is reverted since some users complain that:
> It seems to me that it might be reasonable to ignore the interrupt if > the purpose of the interrupt is to trigger sampling of the CPUs > register state. But if the interrupt will trigger some other > operation, such as a signal on an fd, then there's no reason to drop > it.
I assume that if the PMI drop is unacceptable, either will spurious PMI injection.
I'm pretty open if you insist that we really need to do this for guest PEBS enabling.
> >>>> + * If PEBS interrupt threshold on host is not exceeded in a NMI, there >>>> + * must be a PEBS overflow PMI generated from the guest PEBS counters. >>>> + * There is no ambiguity since the reported event in the PMI is guest >>>> + * only. It gets handled correctly on a case by case base for each event. >>>> + * >>>> + * Note: KVM disables the co-existence of guest PEBS and host PEBS. >>> Where; I need a code reference here. >> How about: >> >> Note: KVM will disable the co-existence of guest PEBS and host PEBS. >> In the intel_guest_get_msrs(), when we have host PEBS ctrl bit(s) enabled, >> KVM will clear the guest PEBS ctrl enable bit(s) before vm-entry. >> The guest PEBS users should be notified of this runtime restriction. > Since you had me look at that function, can clean up that > CONFIG_RETPOLINE crud and replace it with static_call() ?
Sure. Let me try it.
--- thx, likexu
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