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From: Paul Durrant <xadimgnik@gmail.com>
Message-ID: <04ec9d6c-e761-4cfc-a2fe-a2d7d398c334@xen.org>
Date: Fri, 24 May 2024 15:03:35 +0100
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Subject: Re: [RFC PATCH v3 13/21] KVM: x86: Improve synchronization in
 kvm_synchronize_tsc()
To: David Woodhouse <dwmw2@infradead.org>, kvm@vger.kernel.org
Cc: Paolo Bonzini <pbonzini@redhat.com>, Jonathan Corbet <corbet@lwn.net>,
 Sean Christopherson <seanjc@google.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>,
 Peter Zijlstra <peterz@infradead.org>, Juri Lelli <juri.lelli@redhat.com>,
 Vincent Guittot <vincent.guittot@linaro.org>,
 Dietmar Eggemann <dietmar.eggemann@arm.com>,
 Steven Rostedt <rostedt@goodmis.org>, Ben Segall <bsegall@google.com>,
 Mel Gorman <mgorman@suse.de>, Daniel Bristot de Oliveira
 <bristot@redhat.com>, Valentin Schneider <vschneid@redhat.com>,
 Shuah Khan <shuah@kernel.org>, linux-doc@vger.kernel.org,
 linux-kernel@vger.kernel.org, jalliste@amazon.co.uk, sveith@amazon.de,
 zide.chen@intel.com, Dongli Zhang <dongli.zhang@oracle.com>,
 Chenyi Qiang <chenyi.qiang@intel.com>
References: <20240522001817.619072-1-dwmw2@infradead.org>
 <20240522001817.619072-14-dwmw2@infradead.org>
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On 22/05/2024 01:17, David Woodhouse wrote:
> From: David Woodhouse <dwmw@amazon.co.uk>
> 
> When synchronizing to an existing TSC (either by explicitly writing zero,
> or the legacy hack where the TSC is written within one second's worth of
> the previously written TSC), the last_tsc_write and last_tsc_nsec values
> were being misrecorded by __kvm_synchronize_tsc(). The *unsynchronized*
> value of the TSC (perhaps even zero) was bring recorded, along with the
> current time at which kvm_synchronize_tsc() was called. This could cause
> *subsequent* writes to fail to synchronize correctly.
> 
> Fix that by resetting {data, ns} to the previous values before passing
> them to __kvm_synchronize_tsc() when synchronization is detected. Except
> in the case where the TSC is unstable and *has* to be synthesised from
> the host clock, in which case attempt to create a nsec/tsc pair which is
> on the correct line.
> 
> Furthermore, there were *three* different TSC reads used for calculating
> the "current" time, all slightly different from each other. Fix that by
> using kvm_get_time_and_clockread() where possible and using the same
> host_tsc value in all cases.
> 
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> ---
>   arch/x86/kvm/x86.c | 32 ++++++++++++++++++++++++++++----
>   1 file changed, 28 insertions(+), 4 deletions(-)
> 

Reviewed-by: Paul Durrant <paul@xen.org>