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Date: Sat, 4 May 2024 04:42:47 -0300
From: Marcelo Tosatti <mtosatti@redhat.com>
To: David Woodhouse <dwmw2@infradead.org>
Cc: kvm@vger.kernel.org, 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>, 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>, jalliste@amazon.co.uk,
	sveith@amazon.de, zide.chen@intel.com,
	Dongli Zhang <dongli.zhang@oracle.com>
Subject: Re: [PATCH v2 01/15] KVM: x86/xen: Do not corrupt KVM clock in
 kvm_xen_shared_info_init()
Message-ID: <ZjXm9w/y3/NLCxLQ@tpad>
References: <20240427111929.9600-1-dwmw2@infradead.org>
 <20240427111929.9600-2-dwmw2@infradead.org>
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On Sat, Apr 27, 2024 at 12:04:58PM +0100, David Woodhouse wrote:
> From: David Woodhouse <dwmw@amazon.co.uk>
> 
> The KVM clock is an interesting thing. It is defined as "nanoseconds
> since the guest was created", but in practice it runs at two *different*
> rates — or three different rates, if you count implementation bugs.
> 
> Definition A is that it runs synchronously with the CLOCK_MONOTONIC_RAW
> of the host, with a delta of kvm->arch.kvmclock_offset.
> 
> But that version doesn't actually get used in the common case, where the
> host has a reliable TSC and the guest TSCs are all running at the same
> rate and in sync with each other, and kvm->arch.use_master_clock is set.
> 
> In that common case, definition B is used: There is a reference point in
> time at kvm->arch.master_kernel_ns (again a CLOCK_MONOTONIC_RAW time),
> and a corresponding host TSC value kvm->arch.master_cycle_now. This
> fixed point in time is converted to guest units (the time offset by
> kvmclock_offset and the TSC Value scaled and offset to be a guest TSC
> value) and advertised to the guest in the pvclock structure. While in
> this 'use_master_clock' mode, the fixed point in time never needs to be
> changed, and the clock runs precisely in time with the guest TSC, at the
> rate advertised in the pvclock structure.
> 
> The third definition C is implemented in kvm_get_wall_clock_epoch() and
> __get_kvmclock(), using the master_cycle_now and master_kernel_ns fields
> but converting the *host* TSC cycles directly to a value in nanoseconds
> instead of scaling via the guest TSC.
> 
> One might naïvely think that all three definitions are identical, since
> CLOCK_MONOTONIC_RAW is not skewed by NTP frequency corrections; all
> three are just the result of counting the host TSC at a known frequency,
> or the scaled guest TSC at a known precise fraction of the host's
> frequency. The problem is with arithmetic precision, and the way that
> frequency scaling is done in a division-free way by multiplying by a
> scale factor, then shifting right. In practice, all three ways of
> calculating the KVM clock will suffer a systemic drift from each other.
> 
> Eventually, definition C should just be eliminated. Commit 451a707813ae
> ("KVM: x86/xen: improve accuracy of Xen timers") worked around it for
> the specific case of Xen timers, which are defined in terms of the KVM
> clock and suffered from a continually increasing error in timer expiry
> times. That commit notes that get_kvmclock_ns() is non-trivial to fix
> and says "I'll come back to that", which remains true.
> 
> Definitions A and B do need to coexist, the former to handle the case
> where the host or guest TSC is suboptimally configured. But KVM should
> be more careful about switching between them, and the discontinuity in
> guest time which could result.
> 
> In particular, KVM_REQ_MASTERCLOCK_UPDATE will take a new snapshot of
> time as the reference in master_kernel_ns and master_cycle_now, yanking
> the guest's clock back to match definition A at that moment.

KVM_REQ_MASTERCLOCK_UPDATE stops the vcpus because:

 * To avoid that problem, do not allow visibility of distinct
 * system_timestamp/tsc_timestamp values simultaneously: use a master
 * copy of host monotonic time values. Update that master copy
 * in lockstep.

> When invoked from in 'use_master_clock' mode, kvm_update_masterclock()
> should probably *adjust* kvm->arch.kvmclock_offset to account for the
> drift, instead of yanking the clock back to defintion A.

You are likely correct...

> But in the meantime there are a bunch of places where it just doesn't need to be
> invoked at all.
> 
> To start with: there is no need to do such an update when a Xen guest
> populates the shared_info page. This seems to have been a hangover from
> the very first implementation of shared_info which automatically
> populated the vcpu_info structures at their default locations, but even
> then it should just have raised KVM_REQ_CLOCK_UPDATE on each vCPU
> instead of using KVM_REQ_MASTERCLOCK_UPDATE. And now that userspace is
> expected to explicitly set the vcpu_info even in its default locations,
> there's not even any need for that either.
> 
> Fixes: 629b5348841a1 ("KVM: x86/xen: update wallclock region")
> Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> Reviewed-by: Paul Durrant <paul@xen.org>
> ---
>  arch/x86/kvm/xen.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
> index f65b35a05d91..5a83a8154b79 100644
> --- a/arch/x86/kvm/xen.c
> +++ b/arch/x86/kvm/xen.c
> @@ -98,8 +98,6 @@ static int kvm_xen_shared_info_init(struct kvm *kvm)
>  	wc->version = wc_version + 1;
>  	read_unlock_irq(&gpc->lock);
>  
> -	kvm_make_all_cpus_request(kvm, KVM_REQ_MASTERCLOCK_UPDATE);
> -
>  out:
>  	srcu_read_unlock(&kvm->srcu, idx);
>  	return ret;
> -- 
> 2.44.0

So KVM_REQ_MASTERCLOCK_UPDATE is to avoid the race above.

In what contexes is kvm_xen_shared_info_init called from again?

Not clear to me KVM_REQ_MASTERCLOCK_UPDATE is not needed (or that is
needed, for that matter...).