Re: [PATCH] KVM: x86/xen: Do not corrupt KVM clock in kvm_xen_shared_info_init()

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On 07/04/2024 14:15, David Woodhouse wrote:
From: David Woodhouse <dwmw@xxxxxxxxxxxx>

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.

Definition C should simply 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.

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.

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@xxxxxxxxxxxx>
  arch/x86/kvm/xen.c | 2 --
  1 file changed, 2 deletions(-)

Reviewed-by: Paul Durrant <paul@xxxxxxx>

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