Re: [PATCH v7 6/6] KVM: x86: Expose TSC offset controls to userspace

Marcelo,

On Thu, Aug 26, 2021 at 09:48:36AM -0300, Marcelo Tosatti wrote:

On Mon, Aug 23, 2021 at 01:56:30PM -0700, Oliver Upton wrote:

quoted

Paolo,

On Sun, Aug 15, 2021 at 5:11 PM Oliver Upton [off-list ref] wrote:

quoted

To date, VMM-directed TSC synchronization and migration has been a bit
messy. KVM has some baked-in heuristics around TSC writes to infer if
the VMM is attempting to synchronize. This is problematic, as it depends
on host userspace writing to the guest's TSC within 1 second of the last
write.

A much cleaner approach to configuring the guest's views of the TSC is to
simply migrate the TSC offset for every vCPU. Offsets are idempotent,
and thus not subject to change depending on when the VMM actually
reads/writes values from/to KVM. The VMM can then read the TSC once with
KVM_GET_CLOCK to capture a (realtime, host_tsc) pair at the instant when
the guest is paused.

Cc: David Matlack <dmatlack@google.com>
Cc: Sean Christopherson <seanjc@google.com>
Signed-off-by: Oliver Upton <redacted>

Could you please squash the following into this patch? We need to
advertise KVM_CAP_VCPU_ATTRIBUTES to userspace. Otherwise, happy to
resend.

Thanks,
Oliver

Oliver,

Is there QEMU support for this, or are you using your own
userspace with this?

Apologies for not getting back to you on your first mail. Sadly, I am
using our own userspace for this. That being said, adding support to
QEMU shouldn't be too challenging. I can take a stab at it if it makes
the series more amenable to upstream, with the giant disclaimer that I
haven't done work in QEMU before. Otherwise, happy to review someone
else's implementation.

--
Thanks,
Oliver

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