linux-kernel - [PATCH v2 0/3] RFC: Precise TSC migration

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Message-Id: <20201203171118.372391-1-mlevitsk@redhat.com>
Date:   Thu,  3 Dec 2020 19:11:15 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     kvm@...r.kernel.org
Cc:     "H. Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Jim Mattson <jmattson@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        linux-kselftest@...r.kernel.org (open list:KERNEL SELFTEST FRAMEWORK),
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        linux-kernel@...r.kernel.org (open list),
        Ingo Molnar <mingo@...hat.com>,
        x86@...nel.org (maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)),
        Joerg Roedel <joro@...tes.org>, Borislav Petkov <bp@...en8.de>,
        Shuah Khan <shuah@...nel.org>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        Andrew Jones <drjones@...hat.com>,
        Oliver Upton <oupton@...gle.com>,
        linux-doc@...r.kernel.org (open list:DOCUMENTATION)
Subject: [PATCH v2 0/3] RFC: Precise TSC migration

Hi!

This is the second version of the work to make TSC migration more accurate,
as was defined by Paulo at:
https://www.spinics.net/lists/kvm/msg225525.html

I omitted most of the semi-offtopic points I raised related to TSC
in the previous RFC where we can continue the discussion.

I do want to raise another thing that I almost forgot.

On AMD systems, the Linux kernel will mark the guest tsc as
unstable unless invtsc is set which is set on recent AMD
hardware.

Take a look at 'unsynchronized_tsc()' to verify this.

This is another thing that IMHO should be fixed at least when
running under KVM.

Note that I forgot to mention that
X86_FEATURE_TSC_RELIABLE also short-circuits this code,
thus giving another reason to enable it under KVM.

Changes from V1:

- added KVM_TSC_STATE_TIMESTAMP_VALID instead of testing ns == 0
- allow diff < 0, because it is still better that capping it to 0
- updated tsc_msr_test unit test to cover this feature
- refactoring

Patches to enable this feature in qemu are in the process of
being sent to qemu-devel mailing list.

Best regards,
    Maxim Levitsky

Maxim Levitsky (3):
  KVM: x86: implement KVM_{GET|SET}_TSC_STATE
  KVM: x86: introduce KVM_X86_QUIRK_TSC_HOST_ACCESS
  kvm/selftests: update tsc_msrs_test to cover
    KVM_X86_QUIRK_TSC_HOST_ACCESS

 Documentation/virt/kvm/api.rst                | 65 +++++++++++++
 arch/x86/include/uapi/asm/kvm.h               |  1 +
 arch/x86/kvm/x86.c                            | 92 ++++++++++++++++++-
 include/uapi/linux/kvm.h                      | 15 +++
 .../selftests/kvm/x86_64/tsc_msrs_test.c      | 79 ++++++++++++++--
 5 files changed, 237 insertions(+), 15 deletions(-)

-- 
2.26.2