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

From: Oliver Upton <oupton@google.com>
To: Marcelo Tosatti <mtosatti@redhat.com>
Cc: kvm@vger.kernel.org, kvmarm@lists.cs.columbia.edu,
	Paolo Bonzini <pbonzini@redhat.com>,
	Sean Christopherson <seanjc@google.com>,
	Marc Zyngier <maz@kernel.org>, Peter Shier <pshier@google.com>,
	Jim Mattson <jmattson@google.com>,
	David Matlack <dmatlack@google.com>,
	Ricardo Koller <ricarkol@google.com>,
	Jing Zhang <jingzhangos@google.com>,
	Raghavendra Rao Anata <rananta@google.com>,
	James Morse <james.morse@arm.com>,
	Alexandru Elisei <Alexandru.Elisei@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	linux-arm-kernel@lists.infradead.org,
	Andrew Jones <drjones@redhat.com>, Will Deacon <will@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>
Subject: Re: [PATCH v7 6/6] KVM: x86: Expose TSC offset controls to userspace
Date: Thu, 26 Aug 2021 20:27:53 +0000	[thread overview]
Message-ID: <YSf5SV0AZMvfIEib@google.com> (raw)
In-Reply-To: <20210826124836.GA155749@fuller.cnet>

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:
> > Paolo,
> > 
> > On Sun, Aug 15, 2021 at 5:11 PM Oliver Upton <oupton@google.com> wrote:
> > >
> > > 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 <oupton@google.com>
> > 
> > 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