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Message-ID: <9975969725a64c2ba2b398244dba3437bff5154e.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:     Sean Christopherson <seanjc@google.com>
Cc:     Dongli Zhang <dongli.zhang@oracle.com>,
        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:   Tue, 03 Oct 2023 06:54:56 +0100
In-Reply-To: <ZRtl94_rIif3GRpu@google.com>
References: <20230926230649.67852-1-dongli.zhang@oracle.com>
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         <a461bf3f-c17e-9c3f-56aa-726225e8391d@oracle.com>
         <884aa233ef46d5209b2d1c92ce992f50a76bd656.camel@infradead.org>
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On Mon, 2023-10-02 at 17:53 -0700, Sean Christopherson wrote:
>=20
> The two domains use the same "clock" (constant TSC), but different math t=
o compute
> nanoseconds from a given TSC value.=C2=A0 For decently large TSC values, =
this results
> in CLOCK_MONOTONIC_RAW and kvmclock computing two different times in nano=
seconds.

This is the bit I'm still confused about, and it seems to be the root
of all the other problems.

Both CLOCK_MONOTONIC_RAW and kvmclock have *one* job: to convert a
number of ticks of the TSC running at a constant known frequency, to a
number of nanoseconds.

So how in the name of all that is holy do they manage to get
*different* answers?

I get that the mult/shift thing carries some imprecision, but is that
all it is? Can't we ensure that the kvmclock uses the *same* algorithm,
precisely, as CLOCK_MONOTONIC_RAW?


> E.g. IIUC, your proposed patch to use a single RDTSC[*] eliminates the dr=
ift by
> undoing the CLOCK_MONOTONIC_RAW crap using the same TSC value on both the=
 "add"
> and the "subtract", but the underlying train wreck of mixing time domains=
 is
> still there.

> [*] https://lore.kernel.org/all/ee446c823002dc92c8ea525f21d00a9f5d27de59.=
camel@infradead.org

That one's different; those clock domains are *supposed* to be
divergent and the point in the PV wall clock information is to convey
the delta between the two. I just made it use the delta between the two
at a *given* moment, instead of calculating the difference between
*two* different times.

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