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Message-ID: <19cdf9362d469ac6de8503aa335b864ab5fc2553.camel@infradead.org>
Subject: Re: [PATCH] KVM: x86/xen: improve accuracy of Xen timers
From:   David Woodhouse <dwmw2@infradead.org>
To:     kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        "Griffoul, Fred" <fgriffo@amazon.com>
Cc:     Paul Durrant <paul@xen.org>,
        Sean Christopherson <seanjc@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>
Date:   Tue, 31 Oct 2023 19:45:02 +0000
In-Reply-To: <96da7273adfff2a346de9a4a27ce064f6fe0d0a1.camel@infradead.org>
References: <96da7273adfff2a346de9a4a27ce064f6fe0d0a1.camel@infradead.org>
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On Mon, 2023-10-30 at 15:50 +0000, David Woodhouse wrote:
>=20
> +static int do_monotonic(s64 *t, u64 *tsc_timestamp)
> +{
> +	struct pvclock_gtod_data *gtod =3D &pvclock_gtod_data;
> +	unsigned long seq;
> +	int mode;
> +	u64 ns;
> +
> +	do {
> +		seq =3D read_seqcount_begin(&gtod->seq);
> +		ns =3D gtod->clock.base_cycles;
> +		ns +=3D vgettsc(&gtod->clock, tsc_timestamp, &mode);
> +		ns >>=3D gtod->clock.shift;
> +		ns +=3D ktime_to_ns(ktime_add(gtod->clock.offset,
> gtod->offs_boot));
> +	} while (unlikely(read_seqcount_retry(&gtod->seq, seq)));
> +	*t =3D ns;
> +
> +	return mode;
> +}
> +

Hrm, that's basically cargo-culted from do_monotonic_raw() immediately
above it. Should it be adding gtod->offs_boot?

Empirically the answer would appear to be 'no'. When gtod->offs_boot is
non-zero, I see kvm_get_monotonic_and_clockread() returning values
which are precisely that far in advance of what ktime_get() reports.


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