Re: [PATCH 02/10] KVM: x86: Improve accuracy of KVM clock when TSC scaling is in force

From: Paolo Bonzini <pbonzini@redhat.com>
To: David Woodhouse <dwmw2@infradead.org>
Cc: kvm@vger.kernel.org, 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>,
	Paul Durrant <paul@xen.org>, Shuah Khan <shuah@kernel.org>,
	 linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
	 linux-kselftest@vger.kernel.org,
	Oliver Upton <oliver.upton@linux.dev>,
	 Marcelo Tosatti <mtosatti@redhat.com>,
	jalliste@amazon.co.uk, sveith@amazon.de
Subject: Re: [PATCH 02/10] KVM: x86: Improve accuracy of KVM clock when TSC scaling is in force
Date: Mon, 22 Apr 2024 17:54:44 +0200	[thread overview]
Message-ID: <CABgObfbK0aqqmAz7Z2efX4hNf7WRBYpoJ1a07oKMZdFXS2r0+g@mail.gmail.com> (raw)
In-Reply-To: <a35e69e07b9cd297dac9993c886667add144e833.camel@infradead.org>

On Mon, Apr 22, 2024 at 5:39 PM David Woodhouse <dwmw2@infradead.org> wrote:

> > ... especially considering that you did use a 64-bit integer here
> > (though---please use u64 not uint64_t; and BTW if you want to add a
> > patch to change kvm_get_time_scale() to u64, please do.
>
> Meh, I'm used to programming in C. Yes, I *am* old enough to have been
> doing this since the last decade of the 1900s, but it *has* been a long
> time since 1999, and my fingers have learned :)

Oh, I am on the same page (working on both QEMU and Linux, adapting my
muscle memory to the context sucks) but u64/s64 is the preferred
spelling and I have been asked to use them before.

> Heh, looks like it was you who made it uint64_t, in 2016. In a commit
> (3ae13faac) which said "Prepare for improving the precision in the next
> patch"... which never came, AFAICT?

Yes, it was posted as
https://lore.kernel.org/lkml/1454944711-33022-5-git-send-email-pbonzini@redhat.com/
but not committed.

As an aside, we discovered later that the patch you list as "Fixes"
fixed another tricky bug: before, kvmclock could jump if the TSC is
set within the 250 ppm tolerance that does not activate TSC scaling.
This is possible after a first live migration, and then the second
live migration used the guest TSC frequency *that userspace desired*
instead of the *actual* TSC frequency.

Before:

        this_tsc_khz = __this_cpu_read(cpu_tsc_khz);
        if (unlikely(vcpu->hw_tsc_khz != this_tsc_khz)) {
                tgt_tsc_khz = vcpu->virtual_tsc_khz;
                kvm_get_time_scale(NSEC_PER_SEC / 1000, tgt_tsc_khz,
                        &vcpu->hv_clock.tsc_shift,
                        &vcpu->hv_clock.tsc_to_system_mul);
                vcpu->hw_tsc_khz = this_tsc_khz;

After:

        tgt_tsc_khz = __this_cpu_read(cpu_tsc_khz);

        // tgt_tsc_khz unchanged because TSC scaling was not enabled
        tgt_tsc_khz = kvm_scale_tsc(v, tgt_tsc_khz);

        if (unlikely(vcpu->hw_tsc_khz != tgt_tsc_khz)) {
                kvm_get_time_scale(NSEC_PER_SEC / 1000, tgt_tsc_khz,
                        &vcpu->hv_clock.tsc_shift,
                        &vcpu->hv_clock.tsc_to_system_mul);
                vcpu->hw_tsc_khz = tgt_tsc_khz;

So in the first case kvm_get_time_scale uses vcpu->virtual_tsc_khz, in
the second case it uses __this_cpu_read(cpu_tsc_khz).

This then caused a mismatch between the actual guest frequency and
what is used by kvm_guest_time_update, which only becomes visible when
migration resets the clock with KVM_GET/SET_CLOCK. KVM_GET_CLOCK
returns what _should have been_ the same value read by the guest, but
it's wrong.

Paolo