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author | David Woodhouse <dwmw@amazon.co.uk> | 2024-02-27 11:49:15 +0000 |
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committer | Sean Christopherson <seanjc@google.com> | 2024-03-04 16:22:32 -0800 |
commit | 451a707813aee24b4a734f28d1d414be0360862b (patch) | |
tree | 3f6a46a3cb0bd47d18518547e7b1c767555d2ce3 /arch/x86/kvm/x86.h | |
parent | 003d914220c97ef93cabfe3ec4e245e2383e19e9 (diff) | |
download | linux-stable-451a707813aee24b4a734f28d1d414be0360862b.tar.gz linux-stable-451a707813aee24b4a734f28d1d414be0360862b.tar.bz2 linux-stable-451a707813aee24b4a734f28d1d414be0360862b.zip |
KVM: x86/xen: improve accuracy of Xen timers
A test program such as http://david.woodhou.se/timerlat.c confirms user
reports that timers are increasingly inaccurate as the lifetime of a
guest increases. Reporting the actual delay observed when asking for
100µs of sleep, it starts off OK on a newly-launched guest but gets
worse over time, giving incorrect sleep times:
root@ip-10-0-193-21:~# ./timerlat -c -n 5
00000000 latency 103243/100000 (3.2430%)
00000001 latency 103243/100000 (3.2430%)
00000002 latency 103242/100000 (3.2420%)
00000003 latency 103245/100000 (3.2450%)
00000004 latency 103245/100000 (3.2450%)
The biggest problem is that get_kvmclock_ns() returns inaccurate values
when the guest TSC is scaled. The guest sees a TSC value scaled from the
host TSC by a mul/shift conversion (hopefully done in hardware). The
guest then converts that guest TSC value into nanoseconds using the
mul/shift conversion given to it by the KVM pvclock information.
But get_kvmclock_ns() performs only a single conversion directly from
host TSC to nanoseconds, giving a different result. A test program at
http://david.woodhou.se/tsdrift.c demonstrates the cumulative error
over a day.
It's non-trivial to fix get_kvmclock_ns(), although I'll come back to
that. The actual guest hv_clock is per-CPU, and *theoretically* each
vCPU could be running at a *different* frequency. But this patch is
needed anyway because...
The other issue with Xen timers was that the code would snapshot the
host CLOCK_MONOTONIC at some point in time, and then... after a few
interrupts may have occurred, some preemption perhaps... would also read
the guest's kvmclock. Then it would proceed under the false assumption
that those two happened at the *same* time. Any time which *actually*
elapsed between reading the two clocks was introduced as inaccuracies
in the time at which the timer fired.
Fix it to use a variant of kvm_get_time_and_clockread(), which reads the
host TSC just *once*, then use the returned TSC value to calculate the
kvmclock (making sure to do that the way the guest would instead of
making the same mistake get_kvmclock_ns() does).
Sadly, hrtimers based on CLOCK_MONOTONIC_RAW are not supported, so Xen
timers still have to use CLOCK_MONOTONIC. In practice the difference
between the two won't matter over the timescales involved, as the
*absolute* values don't matter; just the delta.
This does mean a new variant of kvm_get_time_and_clockread() is needed;
called kvm_get_monotonic_and_clockread() because that's what it does.
Fixes: 536395260582 ("KVM: x86/xen: handle PV timers oneshot mode")
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>
Link: https://lore.kernel.org/r/20240227115648.3104-2-dwmw2@infradead.org
[sean: massage moved comment, tweak if statement formatting]
Signed-off-by: Sean Christopherson <seanjc@google.com>
Diffstat (limited to 'arch/x86/kvm/x86.h')
-rw-r--r-- | arch/x86/kvm/x86.h | 1 |
1 files changed, 1 insertions, 0 deletions
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h index 2f7e19166658..56b7a78f45bf 100644 --- a/arch/x86/kvm/x86.h +++ b/arch/x86/kvm/x86.h @@ -294,6 +294,7 @@ void kvm_inject_realmode_interrupt(struct kvm_vcpu *vcpu, int irq, int inc_eip); u64 get_kvmclock_ns(struct kvm *kvm); uint64_t kvm_get_wall_clock_epoch(struct kvm *kvm); +bool kvm_get_monotonic_and_clockread(s64 *kernel_ns, u64 *tsc_timestamp); int kvm_read_guest_virt(struct kvm_vcpu *vcpu, gva_t addr, void *val, unsigned int bytes, |