From: Oliver Upton <oupton@google.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Andy Lutomirski <luto@amacapital.net>,
Maxim Levitsky <mlevitsk@redhat.com>,
Thomas Gleixner <tglx@linutronix.de>,
Marcelo Tosatti <mtosatti@redhat.com>,
kvm list <kvm@vger.kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
Jonathan Corbet <corbet@lwn.net>,
Jim Mattson <jmattson@google.com>,
Wanpeng Li <wanpengli@tencent.com>,
"open list:KERNEL SELFTEST FRAMEWORK"
<linux-kselftest@vger.kernel.org>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
Sean Christopherson <sean.j.christopherson@intel.com>,
open list <linux-kernel@vger.kernel.org>,
Ingo Molnar <mingo@redhat.com>,
"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
<x86@kernel.org>, Joerg Roedel <joro@8bytes.org>,
Borislav Petkov <bp@alien8.de>, Shuah Khan <shuah@kernel.org>,
Andrew Jones <drjones@redhat.com>,
"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>
Subject: Re: [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC_STATE
Date: Thu, 10 Dec 2020 12:13:12 -0600 [thread overview]
Message-ID: <CAOQ_QsggH=RaPbiOTGjDviKUCa0r4YgJjLsP7ghUGbcVtr2YJQ@mail.gmail.com> (raw)
In-Reply-To: <eb0cbfaa-251a-810b-3c12-4ee63d082bc8@redhat.com>
On Thu, Dec 10, 2020 at 12:05 PM Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> 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.
So long as it is guaranteed that guest manipulations of IA32_TSC are
reflected in IA32_TSC_ADJUST even if it isn't in the guest's CPUID,
then this seems OK. I think having clear documentation on this subject
is also necessary, as we're going to rely on the combination of
KVM_{GET,SET}_TSC_STATE, disabling KVM_X86_QUIRK_TSC_HOST_ACCESS, and
userspace reading/writing a possibly hidden MSR to pull this off
right.
--
Thanks,
Oliver
> 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?:-)
>
next prev parent reply other threads:[~2020-12-10 18:16 UTC|newest]
Thread overview: 77+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-03 17:11 [PATCH v2 0/3] RFC: Precise TSC migration Maxim Levitsky
2020-12-03 17:11 ` [PATCH v2 1/3] KVM: x86: implement KVM_{GET|SET}_TSC_STATE Maxim Levitsky
2020-12-06 16:19 ` Thomas Gleixner
2020-12-07 12:16 ` Maxim Levitsky
2020-12-07 13:16 ` Vitaly Kuznetsov
2020-12-07 17:41 ` Thomas Gleixner
2020-12-08 9:48 ` Peter Zijlstra
2020-12-10 11:42 ` Paolo Bonzini
2020-12-10 12:14 ` Peter Zijlstra
2020-12-10 12:22 ` Paolo Bonzini
2020-12-10 13:01 ` Peter Zijlstra
2020-12-10 20:20 ` Thomas Gleixner
2020-12-07 16:38 ` Thomas Gleixner
2020-12-07 16:53 ` Andy Lutomirski
2020-12-07 17:00 ` Maxim Levitsky
2020-12-07 18:04 ` Andy Lutomirski
2020-12-07 23:11 ` Marcelo Tosatti
2020-12-08 17:43 ` Andy Lutomirski
2020-12-08 19:24 ` Thomas Gleixner
2020-12-08 20:32 ` Andy Lutomirski
2020-12-09 0:19 ` Thomas Gleixner
2020-12-09 4:08 ` Andy Lutomirski
2020-12-09 10:14 ` Thomas Gleixner
2020-12-10 23:42 ` Andy Lutomirski
2020-12-08 11:24 ` Maxim Levitsky
2020-12-08 9:35 ` Peter Zijlstra
2020-12-07 23:34 ` Marcelo Tosatti
2020-12-07 17:29 ` Oliver Upton
2020-12-08 11:13 ` Maxim Levitsky
2020-12-08 15:57 ` Oliver Upton
2020-12-08 15:58 ` Oliver Upton
2020-12-08 17:10 ` Maxim Levitsky
2020-12-08 16:40 ` Thomas Gleixner
2020-12-08 17:08 ` Maxim Levitsky
2020-12-10 11:48 ` Paolo Bonzini
2020-12-10 14:25 ` Maxim Levitsky
2020-12-07 23:29 ` Marcelo Tosatti
2020-12-08 14:50 ` Maxim Levitsky
2020-12-08 16:02 ` Thomas Gleixner
2020-12-08 16:25 ` Maxim Levitsky
2020-12-08 17:33 ` Andy Lutomirski
2020-12-08 21:25 ` Thomas Gleixner
2020-12-08 18:12 ` Marcelo Tosatti
2020-12-08 21:35 ` Thomas Gleixner
2020-12-08 21:20 ` Thomas Gleixner
2020-12-10 11:48 ` Paolo Bonzini
2020-12-10 14:52 ` Maxim Levitsky
2020-12-10 15:16 ` Andy Lutomirski
2020-12-10 17:59 ` Oliver Upton
2020-12-10 18:05 ` Paolo Bonzini
2020-12-10 18:13 ` Oliver Upton [this message]
2020-12-10 21:25 ` Thomas Gleixner
2020-12-10 22:01 ` Andy Lutomirski
2020-12-10 22:28 ` Thomas Gleixner
2020-12-10 23:19 ` Andy Lutomirski
2020-12-11 0:03 ` Thomas Gleixner
2020-12-08 18:11 ` Marcelo Tosatti
2020-12-08 21:33 ` Thomas Gleixner
2020-12-09 16:34 ` Marcelo Tosatti
2020-12-09 20:58 ` Thomas Gleixner
2020-12-10 15:26 ` Marcelo Tosatti
2020-12-10 21:48 ` Thomas Gleixner
2020-12-11 0:27 ` Marcelo Tosatti
2020-12-11 13:30 ` Thomas Gleixner
2020-12-11 14:18 ` Marcelo Tosatti
2020-12-11 21:04 ` Thomas Gleixner
2020-12-11 21:59 ` Paolo Bonzini
2020-12-12 13:03 ` Thomas Gleixner
2020-12-15 10:59 ` Marcelo Tosatti
2020-12-15 16:55 ` Andy Lutomirski
2020-12-15 22:34 ` Thomas Gleixner
2020-12-11 13:37 ` Paolo Bonzini
2020-12-08 17:35 ` Marcelo Tosatti
2020-12-03 17:11 ` [PATCH v2 2/3] KVM: x86: introduce KVM_X86_QUIRK_TSC_HOST_ACCESS Maxim Levitsky
2020-12-03 17:11 ` [PATCH v2 3/3] kvm/selftests: update tsc_msrs_test to cover KVM_X86_QUIRK_TSC_HOST_ACCESS Maxim Levitsky
2020-12-07 23:16 ` [PATCH v2 0/3] RFC: Precise TSC migration Marcelo Tosatti
2020-12-10 11:48 ` Paolo Bonzini
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