linux-kernel - Re: [PATCH 08/10] KVM: x86: Remove per-vCPU "cache" of its reference pvclock

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Message-ID: <b3c0d243-da02-42db-8885-4290d6859c45@xen.org>
Date: Tue, 21 Jan 2025 17:03:18 +0000
From: Paul Durrant <xadimgnik@...il.com>
To: Sean Christopherson <seanjc@...gle.com>,
 Paolo Bonzini <pbonzini@...hat.com>, David Woodhouse <dwmw2@...radead.org>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 syzbot+352e553a86e0d75f5120@...kaller.appspotmail.com,
 Paul Durrant <pdurrant@...zon.com>, David Woodhouse <dwmw@...zon.co.uk>,
 Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH 08/10] KVM: x86: Remove per-vCPU "cache" of its reference
 pvclock

On 18/01/2025 00:55, Sean Christopherson wrote:
> Remove the per-vCPU "cache" of the reference pvclock and instead cache
> only the TSC shift+multiplier.  All other fields in pvclock are fully
> recomputed by kvm_guest_time_update(), i.e. aren't actually persisted.
> 
> In addition to shaving a few bytes, explicitly tracking the TSC shift/mul
> fields makes it easier to see that those fields are tied to hw_tsc_khz
> (they exist to avoid having to do expensive math in the common case).
> And conversely, not tracking the other fields makes it easier to see that
> things like the version number are pulled from the guest's copy, not from
> KVM's reference.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   arch/x86/include/asm/kvm_host.h |  3 ++-
>   arch/x86/kvm/x86.c              | 27 +++++++++++++++------------
>   arch/x86/kvm/xen.c              |  8 ++++----
>   3 files changed, 21 insertions(+), 17 deletions(-)
> 

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