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Message-ID: <884aa233ef46d5209b2d1c92ce992f50a76bd656.camel@infradead.org>
Subject: Re: [PATCH RFC 1/1] KVM: x86: add param to update master clock
 periodically
From:   David Woodhouse <dwmw2@infradead.org>
To:     Dongli Zhang <dongli.zhang@oracle.com>,
        Sean Christopherson <seanjc@google.com>
Cc:     Joe Jin <joe.jin@oracle.com>, x86@kernel.org, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org, pbonzini@redhat.com,
        tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
        dave.hansen@linux.intel.com
Date:   Mon, 02 Oct 2023 09:33:49 +0100
In-Reply-To: <a461bf3f-c17e-9c3f-56aa-726225e8391d@oracle.com>
References: <20230926230649.67852-1-dongli.zhang@oracle.com>
         <377d9706-cc10-dfb8-5326-96c83c47338d@oracle.com>
         <36f3dbb1-61d7-e90a-02cf-9f151a1a3d35@oracle.com>
         <ZRWnVDMKNezAzr2m@google.com>
         <a461bf3f-c17e-9c3f-56aa-726225e8391d@oracle.com>
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On Fri, 2023-09-29 at 13:15 -0700, Dongli Zhang wrote:
>=20
>=20
> We want more frequent KVM_REQ_MASTERCLOCK_UPDATE.
>=20
> This is because:
>=20
> 1. The vcpu->hv_clock (kvmclock) is based on its own mult/shift/equation.
>=20
> 2. The raw monotonic (tsc_clocksource) uses different mult/shift/equation=
.
>=20
> 3. As a result, given the same rdtsc, kvmclock and raw monotonic may retu=
rn
> different results (this is expected because they have different
> mult/shift/equation).
>=20
> 4. However, the base in=C2=A0 kvmclock calculation (tsc_timestamp and sys=
tem_time)
> are derived from raw monotonic clock (master clock)

That just seems wrong. I don't mean that you're incorrect; it seems
*morally* wrong.

In a system with X86_FEATURE_CONSTANT_TSC, why would KVM choose to use
a *different* mult/shift/equation (your #1) to convert TSC ticks to
nanoseconds than the host CLOCK_MONOTONIC_RAW does (your #2).

I understand that KVM can't track the host's CLOCK_MONOTONIC, as it's
adjusted by NTP. But CLOCK_MONOTONIC_RAW is supposed to be consistent.

Fix that, and the whole problem goes away, doesn't it?

What am I missing here, that means we can't do that?

Alternatively... with X86_FEATURE_CONSTANT_TSC, why do the sync at all?
If KVM wants to decide that the TSC runs at a different frequency to
the frequency that the host uses for CLOCK_MONOTONIC_RAW, why can't KVM
just *stick* to that?


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