From: Marcelo Tosatti <mtosatti@redhat.com>
To: Oliver Upton <oupton@google.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
kvm@vger.kernel.org, Will Deacon <will@kernel.org>,
Marc Zyngier <maz@kernel.org>, Peter Shier <pshier@google.com>,
David Matlack <dmatlack@google.com>,
Paolo Bonzini <pbonzini@redhat.com>,
kvmarm@lists.cs.columbia.edu,
linux-arm-kernel@lists.infradead.org,
Jim Mattson <jmattson@google.com>
Subject: Re: [PATCH v8 4/7] KVM: x86: Report host tsc and realtime values in KVM_GET_CLOCK
Date: Tue, 28 Sep 2021 15:53:43 -0300 [thread overview]
Message-ID: <20210928185343.GA97247@fuller.cnet> (raw)
In-Reply-To: <20210916181538.968978-5-oupton@google.com>
On Thu, Sep 16, 2021 at 06:15:35PM +0000, Oliver Upton wrote:
> Handling the migration of TSCs correctly is difficult, in part because
> Linux does not provide userspace with the ability to retrieve a (TSC,
> realtime) clock pair for a single instant in time. In lieu of a more
> convenient facility, KVM can report similar information in the kvm_clock
> structure.
>
> Provide userspace with a host TSC & realtime pair iff the realtime clock
> is based on the TSC. If userspace provides KVM_SET_CLOCK with a valid
> realtime value, advance the KVM clock by the amount of elapsed time. Do
> not step the KVM clock backwards, though, as it is a monotonic
> oscillator.
>
> Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Oliver Upton <oupton@google.com>
> ---
> Documentation/virt/kvm/api.rst | 42 ++++++++++++++++++++++++++-------
> arch/x86/include/asm/kvm_host.h | 3 +++
> arch/x86/kvm/x86.c | 36 +++++++++++++++++++++-------
> include/uapi/linux/kvm.h | 7 +++++-
> 4 files changed, 70 insertions(+), 18 deletions(-)
>
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index a6729c8cf063..d0b9c986cf6c 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -993,20 +993,34 @@ such as migration.
> When KVM_CAP_ADJUST_CLOCK is passed to KVM_CHECK_EXTENSION, it returns the
> set of bits that KVM can return in struct kvm_clock_data's flag member.
>
> -The only flag defined now is KVM_CLOCK_TSC_STABLE. If set, the returned
> -value is the exact kvmclock value seen by all VCPUs at the instant
> -when KVM_GET_CLOCK was called. If clear, the returned value is simply
> -CLOCK_MONOTONIC plus a constant offset; the offset can be modified
> -with KVM_SET_CLOCK. KVM will try to make all VCPUs follow this clock,
> -but the exact value read by each VCPU could differ, because the host
> -TSC is not stable.
> +FLAGS:
> +
> +KVM_CLOCK_TSC_STABLE. If set, the returned value is the exact kvmclock
> +value seen by all VCPUs at the instant when KVM_GET_CLOCK was called.
> +If clear, the returned value is simply CLOCK_MONOTONIC plus a constant
> +offset; the offset can be modified with KVM_SET_CLOCK. KVM will try
> +to make all VCPUs follow this clock, but the exact value read by each
> +VCPU could differ, because the host TSC is not stable.
> +
> +KVM_CLOCK_REALTIME. If set, the `realtime` field in the kvm_clock_data
> +structure is populated with the value of the host's real time
> +clocksource at the instant when KVM_GET_CLOCK was called. If clear,
> +the `realtime` field does not contain a value.
> +
> +KVM_CLOCK_HOST_TSC. If set, the `host_tsc` field in the kvm_clock_data
> +structure is populated with the value of the host's timestamp counter (TSC)
> +at the instant when KVM_GET_CLOCK was called. If clear, the `host_tsc` field
> +does not contain a value.
If the host TSCs are not stable, then KVM_CLOCK_HOST_TSC bit (and
host_tsc field) are ambiguous. Shouldnt exposing them be conditional on
stable TSC for the host ?
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next prev parent reply other threads:[~2021-09-28 18:53 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-16 18:15 [PATCH v8 0/7] KVM: x86: Add idempotent controls for migrating system counter state Oliver Upton
2021-09-16 18:15 ` [PATCH v8 1/7] kvm: x86: abstract locking around pvclock_update_vm_gtod_copy Oliver Upton
2021-09-16 18:15 ` [PATCH v8 2/7] KVM: x86: extract KVM_GET_CLOCK/KVM_SET_CLOCK to separate functions Oliver Upton
2021-09-16 18:15 ` [PATCH v8 3/7] KVM: x86: Fix potential race in KVM_GET_CLOCK Oliver Upton
2021-09-29 13:33 ` Marcelo Tosatti
2021-09-16 18:15 ` [PATCH v8 4/7] KVM: x86: Report host tsc and realtime values " Oliver Upton
2021-09-28 18:53 ` Marcelo Tosatti [this message]
2021-09-29 11:20 ` Paolo Bonzini
2021-09-29 18:56 ` Marcelo Tosatti
2021-09-30 19:21 ` Marcelo Tosatti
2021-09-30 23:02 ` Thomas Gleixner
2021-10-01 12:05 ` Marcelo Tosatti
2021-10-01 12:10 ` Marcelo Tosatti
2021-10-01 19:59 ` Thomas Gleixner
2021-10-01 21:03 ` Oliver Upton
2021-10-01 14:17 ` Paolo Bonzini
2021-10-01 14:39 ` Paolo Bonzini
2021-10-01 14:41 ` Paolo Bonzini
2021-10-01 15:39 ` Oliver Upton
2021-10-01 16:42 ` Paolo Bonzini
2021-09-16 18:15 ` [PATCH v8 5/7] kvm: x86: protect masterclock with a seqcount Oliver Upton
2021-09-24 16:42 ` Paolo Bonzini
2021-09-30 17:51 ` Marcelo Tosatti
2021-10-01 16:48 ` Paolo Bonzini
2021-09-16 18:15 ` [PATCH v8 6/7] KVM: x86: Refactor tsc synchronization code Oliver Upton
2021-09-16 18:15 ` [PATCH v8 7/7] KVM: x86: Expose TSC offset controls to userspace Oliver Upton
2021-09-30 19:14 ` Marcelo Tosatti
2021-10-01 9:17 ` Paolo Bonzini
2021-10-01 10:32 ` Marcelo Tosatti
2021-10-01 15:12 ` Paolo Bonzini
2021-10-01 19:11 ` Marcelo Tosatti
2021-10-01 19:33 ` Oliver Upton
2021-10-04 14:30 ` Marcelo Tosatti
2021-10-04 11:44 ` Paolo Bonzini
2021-10-05 15:22 ` Sean Christopherson
2022-02-23 10:02 ` David Woodhouse
2021-09-24 16:43 ` [PATCH v8 0/7] KVM: x86: Add idempotent controls for migrating system counter state Paolo Bonzini
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