Date: Fri, 24 May 2024 14:20:09 +0100
From: Paul Durrant <xadimgnik@...il.com>
To: David Woodhouse <dwmw2@...radead.org>, kvm@...r.kernel.org
Cc: Paolo Bonzini <pbonzini@...hat.com>, Jonathan Corbet <corbet@....net>,
 Sean Christopherson <seanjc@...gle.com>, Thomas Gleixner
 <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
 Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Dietmar Eggemann <dietmar.eggemann@....com>,
 Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
 Mel Gorman <mgorman@...e.de>, Daniel Bristot de Oliveira
 <bristot@...hat.com>, Valentin Schneider <vschneid@...hat.com>,
 Shuah Khan <shuah@...nel.org>, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, jalliste@...zon.co.uk, sveith@...zon.de,
 zide.chen@...el.com, Dongli Zhang <dongli.zhang@...cle.com>,
 Chenyi Qiang <chenyi.qiang@...el.com>
Subject: Re: [RFC PATCH v3 09/21] KVM: x86: Fix KVM clock precision in
 __get_kvmclock()

On 22/05/2024 01:17, David Woodhouse wrote:
> From: David Woodhouse <dwmw@...zon.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@...zon.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@....org>

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