linux-kernel - Re: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC

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Message-ID: <eb0cbfaa-251a-810b-3c12-4ee63d082bc8@redhat.com>
Date:   Thu, 10 Dec 2020 19:05:14 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Oliver Upton <oupton@...gle.com>,
        Andy Lutomirski <luto@...capital.net>
Cc:     Maxim Levitsky <mlevitsk@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Jonathan Corbet <corbet@....net>,
        Jim Mattson <jmattson@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        open list <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Joerg Roedel <joro@...tes.org>, Borislav Petkov <bp@...en8.de>,
        Shuah Khan <shuah@...nel.org>,
        Andrew Jones <drjones@...hat.com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC_STATE

On 10/12/20 18:59, Oliver Upton wrote:
> However, I don't believe we can assume the guest's TSCs to be synchronized,
> even if sane guests will never touch them. In this case, I think a per-vCPU
> ioctl is still warranted, allowing userspace to get at the guest CPU adjust
> component of Thomas' equation below (paraphrased):
> 
>          TSC guest CPU = host tsc base + guest base offset + guest CPU adjust

Right now that would be:

- KVM_GET_TSC_STATE (vm) returns host tsc base + guest base offset (plus 
the associated time)

- KVM_GET_MSR *without* KVM_X86_QUIRK_TSC_HOST_ACCESS for guest CPU adjust

and the corresponding SET ioctls.  What am *I* missing?

> Alternatively, a write from userspace to the guest's IA32_TSC_ADJUST with
> KVM_X86_QUIRK_TSC_HOST_ACCESS could have the same effect, but that seems to be
> problematic for a couple reasons. First, depending on the guest's CPUID the
> TSC_ADJUST MSR may not even be available, meaning that the guest could've used
> IA32_TSC to adjust the TSC (eww).

Indeed, the host should always be able to read/write IA32_TSC and 
IA32_TSC_ADJUST.

Thanks,

Paolo

> Second, userspace replaying writes to IA32_TSC
> (in the case IA32_TSC_ADJUST doesn't exist for the guest) seems_very_
> unlikely to work given all the magic handling that KVM does for
> writes to it.
> 
> Is this roughly where we are or have I entirely missed the mark?:-)