From: Like Xu <[email protected]>
The legacy API for setting the TSC is fundamentally broken, and only
allows userspace to set a TSC "now", without any way to account for
time lost to preemption between the calculation of the value, and the
kernel eventually handling the ioctl.
To work around this we have had a hack which, if a TSC is set with a
value which is within a second's worth of a previous vCPU, assumes that
userspace actually intended them to be in sync and adjusts the newly-
written TSC value accordingly.
Thus, when a VMM restores a guest after suspend or migration using the
legacy API, the TSCs aren't necessarily *right*, but at least they're
in sync.
This trick falls down when restoring a guest which genuinely has been
running for less time than the 1 second of imprecision which we allow
for in the legacy API. On *creation* the first vCPU starts its TSC
counting from zero, and the subsequent vCPUs synchronize to that. But
then when the VMM tries to set the intended TSC value, because that's
within a second of what the last TSC synced to, it just adjusts it to
match that.
The correct answer is for the VMM not to use the legacy API of course.
But we can pile further hacks onto our existing hackish ABI, and
declare that the *first* value written by userspace (on any vCPU)
should not be subject to this 'correction' to make it sync up with
values that only from the kernel's default vCPU creation.
To that end: Add a flag in kvm->arch.user_set_tsc, protected by
kvm->arch.tsc_write_lock, to record that a TSC for at least one vCPU in
this KVM *has* been set by userspace. Make the 1-second slop hack only
trigger if that flag is already set.
Reported-by: Yong He <[email protected]>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217423
Suggested-by: Oliver Upton <[email protected]>
Original-by: Oliver Upton <[email protected]>
Original-by: Sean Christopherson <[email protected]>
Co-developed-by: David Woodhouse <[email protected]>
Signed-off-by: David Woodhouse <[email protected]>
Signed-off-by: Like Xu <[email protected]>
Tested-by: Yong He <[email protected]>
---
V5 -> V6 Changelog:
- Refine commit message and comments to make more sense; (David)
- Set kvm->arch.user_set_tsc in the kvm_arch_tsc_set_attr(); (David)
- Continue to allow usersapce to write zero to force a sync; (David)
V5: https://lore.kernel.org/kvm/[email protected]/
arch/x86/include/asm/kvm_host.h | 1 +
arch/x86/kvm/x86.c | 24 ++++++++++++++++++++----
2 files changed, 21 insertions(+), 4 deletions(-)
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 1a4def36d5bb..427be7ef9702 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1322,6 +1322,7 @@ struct kvm_arch {
u64 cur_tsc_offset;
u64 cur_tsc_generation;
int nr_vcpus_matched_tsc;
+ bool user_set_tsc;
u32 default_tsc_khz;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 6c9c81e82e65..354169fbc9c4 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2735,20 +2735,35 @@ static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 data)
* kvm_clock stable after CPU hotplug
*/
synchronizing = true;
- } else {
+ } else if (kvm->arch.user_set_tsc) {
u64 tsc_exp = kvm->arch.last_tsc_write +
nsec_to_cycles(vcpu, elapsed);
u64 tsc_hz = vcpu->arch.virtual_tsc_khz * 1000LL;
/*
- * Special case: TSC write with a small delta (1 second)
- * of virtual cycle time against real time is
- * interpreted as an attempt to synchronize the CPU.
+ * Here lies UAPI baggage: when a user-initiated TSC write has
+ * a small delta (1 second) of virtual cycle time against the
+ * previously set vCPU, we assume that they were intended to be
+ * in sync and the delta was only due to the racy nature of the
+ * legacy API.
+ *
+ * This trick falls down when restoring a guest which genuinely
+ * has been running for less time than the 1 second of imprecision
+ * which we allow for in the legacy API. In this case, the first
+ * value written by userspace (on any vCPU) should not be subject
+ * to this 'correction' to make it sync up with values that only
+ * from the kernel's default vCPU creation. Make the 1-second slop
+ * hack only trigger if the user_set_tsc flag is already set.
+ *
+ * The correct answer is for the VMM not to use the legacy API.
*/
synchronizing = data < tsc_exp + tsc_hz &&
data + tsc_hz > tsc_exp;
}
}
+ if (data)
+ kvm->arch.user_set_tsc = true;
+
/*
* For a reliable TSC, we can match TSC offsets, and for an unstable
* TSC, we add elapsed time in this computation. We could let the
@@ -5536,6 +5551,7 @@ static int kvm_arch_tsc_set_attr(struct kvm_vcpu *vcpu,
tsc = kvm_scale_tsc(rdtsc(), vcpu->arch.l1_tsc_scaling_ratio) + offset;
ns = get_kvmclock_base_ns();
+ kvm->arch.user_set_tsc = true;
__kvm_synchronize_tsc(vcpu, offset, tsc, ns, matched);
raw_spin_unlock_irqrestore(&kvm->arch.tsc_write_lock, flags);
base-commit: 0bb80ecc33a8fb5a682236443c1e740d5c917d1d
--
2.42.0
On Wed, 2023-09-13 at 18:37 +0800, Like Xu wrote:
> From: Like Xu <[email protected]>
>
> The legacy API for setting the TSC is fundamentally broken, and only
> allows userspace to set a TSC "now", without any way to account for
> time lost to preemption between the calculation of the value, and the
> kernel eventually handling the ioctl.
>
> To work around this we have had a hack which, if a TSC is set with a
> value which is within a second's worth of a previous vCPU, assumes that
> userspace actually intended them to be in sync and adjusts the newly-
> written TSC value accordingly.
>
> Thus, when a VMM restores a guest after suspend or migration using the
> legacy API, the TSCs aren't necessarily *right*, but at least they're
> in sync.
>
> This trick falls down when restoring a guest which genuinely has been
> running for less time than the 1 second of imprecision which we allow
> for in the legacy API. On *creation* the first vCPU starts its TSC
> counting from zero, and the subsequent vCPUs synchronize to that. But
> then when the VMM tries to set the intended TSC value, because that's
> within a second of what the last TSC synced to, it just adjusts it to
> match that.
>
Proofreading my own words here... "it just adjusts it to match" is
using the same pronoun for different things and is probably hard to
follow. Perhaps "KVM just adjusts it to match" is nicer.
> The correct answer is for the VMM not to use the legacy API of course.
>
> But we can pile further hacks onto our existing hackish ABI, and
> declare that the *first* value written by userspace (on any vCPU)
> should not be subject to this 'correction' to make it sync up with
> values that only from the kernel's default vCPU creation.
^^
... that only *come* from the kernel's...
>
> To that end: Add a flag in kvm->arch.user_set_tsc, protected by
> kvm->arch.tsc_write_lock, to record that a TSC for at least one vCPU in
> this KVM *has* been set by userspace. Make the 1-second slop hack only
> trigger if that flag is already set.
>
> Reported-by: Yong He <[email protected]>
> Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217423
> Suggested-by: Oliver Upton <[email protected]>
> Original-by: Oliver Upton <[email protected]>
> Original-by: Sean Christopherson <[email protected]>
> Co-developed-by: David Woodhouse <[email protected]>
> Signed-off-by: David Woodhouse <[email protected]>
> Signed-off-by: Like Xu <[email protected]>
> Tested-by: Yong He <[email protected]>
Reviewed-by: David Woodhouse <[email protected]>
Please remove the 'Signed-off-by' from me. You must never ever *type* a
signed-off-by line for anyone else. You only ever cut and paste those
intact when they have provided them for *themselves*.
It's OK to remove the Co-developed-by: too. You did the actual typing
of the code here; I just heckled :)