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From: Paul Durrant <xadimgnik@gmail.com>
Message-ID: <04a1543d-4156-4048-b4ec-99240a43d4c4@xen.org>
Date: Fri, 24 May 2024 14:26:05 +0100
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Subject: Re: [RFC PATCH v3 10/21] KVM: x86: Fix software TSC upscaling in
 kvm_update_guest_time()
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>
 <20240522001817.619072-11-dwmw2@infradead.org>
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On 22/05/2024 01:17, David Woodhouse wrote:
> From: David Woodhouse <dwmw@amazon.co.uk>
> 
> There was some confusion in kvm_update_guest_time() when software needs
> to advance the guest TSC.
> 
> In master clock mode, there are two points of time which need to be taken
> into account. First there is the master clock reference point, stored in
> kvm->arch.master_kernel_ns (and associated host TSC ->master_cycle_now).
> Secondly, there is the time *now*, at the point kvm_update_guest_time()
> is being called.
> 
> With software TSC upscaling, the guest TSC is getting further and further
> ahead of the host TSC as time elapses. So at time "now", the guest TSC
> should be further ahead of the host, than it was at master_kernel_ns.
> 
> The adjustment in kvm_update_guest_time() was not taking that into
> account, and was only advancing the guest TSC by the appropriate amount
> for master_kernel_ns, *not* the current time.
> 
> Fix it to calculate them both correctly.
> 
> Since the KVM clock reference point in master_kernel_ns might actually
> be *earlier* than the reference point used for the guest TSC
> (vcpu->last_tsc_nsec), this might lead to a negative delta. Fix the
> compute_guest_tsc() function to cope with negative numbers, which
> then means there is no need to force a master clock update when the
> guest TSC is written.
> 
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk > ---
>   arch/x86/kvm/x86.c | 73 +++++++++++++++++++++++++++++++++++-----------
>   1 file changed, 56 insertions(+), 17 deletions(-)
> 

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