linux-kernel - Re: [PATCH 1/2] KVM: x86: implement KVM_SET_TSC_PRECISE/KVM_GET_TSC

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Message-ID: <005aaf41-9376-d535-211f-9ff08e53bcc4@redhat.com>
Date:   Mon, 30 Nov 2020 18:01:16 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org
Cc:     Oliver Upton <oupton@...gle.com>, Ingo Molnar <mingo@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        open list <linux-kernel@...r.kernel.org>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Wanpeng Li <wanpengli@...cent.com>,
        Borislav Petkov <bp@...en8.de>,
        Jim Mattson <jmattson@...gle.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH 1/2] KVM: x86: implement
 KVM_SET_TSC_PRECISE/KVM_GET_TSC_PRECISE

On 30/11/20 16:58, Maxim Levitsky wrote:
>> This is mostly useful for userspace that doesn't disable the quirk, right?
> Isn't this the opposite? If I understand the original proposal correctly,
> the reason that we include the TSC_ADJUST in the new ioctl, is that
> we would like to disable the special kvm behavior (that is disable the quirk),
> which would mean that tsc will jump on regular host initiated TSC_ADJUST write.
> 
> To avoid this, userspace would set TSC_ADJUST through this new interface.

Yeah, that makes sense.  It removes the need to think "I have to set TSC 
adjust before TSC".

> Do you think that this is an issue? If so I can make the code work with
> signed numbers.

Not sure if it's an issue, but I prefer to make the API "less 
surprising" for userspace.  Who knows how it will be used.

> About nsec == 0, this is to allow to use this API for VM initialization.
> (That is to call KVM_SET_TSC_PRECISE prior to doing KVM_GET_TSC_PRECISE)

I prefer using flags for that purpose.

Paolo