linux-kernel - Re: [PATCH v4 09/25] KVM: VMX: nVMX: Support TSC scaling and PERF_GLOBAL

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Date:   Mon, 01 Aug 2022 10:54:54 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org,
        Anirudh Rayabharam <anrayabh@...ux.microsoft.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 09/25] KVM: VMX: nVMX: Support TSC scaling and
 PERF_GLOBAL_CTRL with enlightened VMCS

Paolo Bonzini <pbonzini@...hat.com> writes:

> On 7/29/22 00:13, Sean Christopherson wrote:
>> The only flaw in this is if KVM gets handed a CPUID model that enumerates support
>> for 2025 (or whenever the next update comes) but not 2022.  Hmm, though if Microsoft
>> defines each new "version" as a full superset, then even that theoretical bug goes
>> away.  I'm happy to be optimistic for once and give this a shot.  I definitely like
>> that it makes it easier to see the deltas between versions.
>
> Okay, I have queued the series but I still haven't gone through all the 
> comments.  So this will _not_ be in the 5.21 pull request.
>
> The first patch also needs a bit more thought to figure out the impact 
> on userspace and whether we can consider syndbg niche enough to not care.

(Sorry for delayed replies here, I'm back from vacation now)

The first patch is not a requirement for the rest of the series, we can
discuss it separately. I, however, think that we can just keep checking
HV_FEATURE_DEBUG_MSRS_AVAILABLE in hv_check_msr_access() to be
compatible with existing QEMUs and make QEMU expose both
HV_FEATURE_DEBUG_MSRS_AVAILABLE and HV_ACCESS_DEBUG_MSRS unconditionally
when syndbg feature is enabled as we know that missing
HV_ACCESS_DEBUG_MSRS is just a bug. I don't think we actually need to
be so picky and support VMMs which want to set 'syndbg without access to
it' and 'access to syndbg without syndbg' use-cases. All-or-nothing is
likely good enough.

-- 
Vitaly