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From: Paul Durrant <xadimgnik@gmail.com>
Message-ID: <9255f42a-7549-4b4f-8654-e0bdc6c643b6@xen.org>
Date: Fri, 24 May 2024 14:20:09 +0100
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Subject: Re: [RFC PATCH v3 09/21] KVM: x86: Fix KVM clock precision in
 __get_kvmclock()
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>
 <20240522001817.619072-10-dwmw2@infradead.org>
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On 22/05/2024 01:17, David Woodhouse wrote:
> From: David Woodhouse <dwmw@amazon.co.uk>
> 
> When in 'master clock mode' (i.e. when host and guest TSCs are behaving
> sanely and in sync), the KVM clock is defined in terms of the guest TSC.
> 
> When TSC scaling is used, calculating the KVM clock directly from *host*
> TSC cycles leads to a systemic drift from the values calculated by the
> guest from its TSC.
> 
> Commit 451a707813ae ("KVM: x86/xen: improve accuracy of Xen timers")
> had a simple workaround for the specific case of Xen timers, as it had an
> actual vCPU to hand and could use its scaling information. That commit
> noted that it was broken for the general case of get_kvmclock_ns(), and
> said "I'll come back to that".
> 
> Since __get_kvmclock() is invoked without a specific CPU, it needs to
> be able to find or generate the scaling values required to perform the
> correct calculation.
> 
> Thankfully, TSC scaling can only happen with X86_FEATURE_CONSTANT_TSC,
> so it isn't as complex as it might have been.
> 
> In __kvm_synchronize_tsc(), note the current vCPU's scaling ratio in
> kvm->arch.last_tsc_scaling_ratio. That is only protected by the
> tsc_write_lock, so in pvclock_update_vm_gtod_copy(), copy it into a
> separate kvm->arch.master_tsc_scaling_ratio so that it can be accessed
> using the kvm->arch.pvclock_sc seqcount lock. Also generate the mul and
> shift factors to convert to nanoseconds for the corresponding KVM clock,
> just as kvm_guest_time_update() would.
> 
> In __get_kvmclock(), which runs within a seqcount retry loop, use those
> values to convert host to guest TSC and then to nanoseconds. Only fall
> back to using get_kvmclock_base_ns() when not in master clock mode.
> 
> There was previously a code path in __get_kvmclock() which looked like
> it could set KVM_CLOCK_TSC_STABLE without KVM_CLOCK_REALTIME, perhaps
> even on 32-bit hosts. In practice that could never happen as the
> ka->use_master_clock flag couldn't be set on 32-bit, and even on 64-bit
> hosts it would never be set when the system clock isn't TSC-based. So
> that code path is now removed.
> 
> The kvm_get_wall_clock_epoch() function had the same problem; make it
> just call get_kvmclock() and subtract kvmclock from wallclock, with
> the same fallback as before.
> 
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> ---
>   arch/x86/include/asm/kvm_host.h |   4 +
>   arch/x86/kvm/x86.c              | 151 ++++++++++++++++----------------
>   2 files changed, 79 insertions(+), 76 deletions(-)
> 

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