On 13/9/2023 6:51 pm, David Woodhouse wrote:
On Wed, 2023-09-13 at 18:37 +0800, Like Xu wrote:
From: Like Xu <likexu@xxxxxxxxxxx>
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 <alexyonghe@xxxxxxxxxxx>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=217423
Suggested-by: Oliver Upton <oliver.upton@xxxxxxxxx>
Original-by: Oliver Upton <oliver.upton@xxxxxxxxx>
Original-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Co-developed-by: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
Signed-off-by: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
Signed-off-by: Like Xu <likexu@xxxxxxxxxxx>
Tested-by: Yong He <alexyonghe@xxxxxxxxxxx>
Reviewed-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
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*.
Nice rule, sorry and thanks for the guidance.
It's OK to remove the Co-developed-by: too. You did the actual typing
of the code here; I just heckled :)
Thank you for reviewing it.
I'll wait for a cooling off period to see if the maintainers need me to post v7.
