linux-kernel - Re: [PATCH 0/2] RFC: Precise TSC migration

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Message-ID: <20201201150205.GA42117@fuller.cnet>
Date:   Tue, 1 Dec 2020 12:02:05 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Oliver Upton <oupton@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        open list <linux-kernel@...r.kernel.org>,
        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 0/2] RFC: Precise TSC migration

On Tue, Dec 01, 2020 at 02:48:11PM +0100, Thomas Gleixner wrote:
> On Mon, Nov 30 2020 at 16:16, Marcelo Tosatti wrote:
> >> Besides, Linux guests don't sync the TSC via IA32_TSC write,
> >> but rather use IA32_TSC_ADJUST which currently doesn't participate
> >> in the tsc sync heruistics.
> >
> > Linux should not try to sync the TSC with IA32_TSC_ADJUST. It expects
> > the BIOS to boot with synced TSCs.
> 
> That's wishful thinking.
> 
> Reality is that BIOS tinkerers fail to get it right. TSC_ADJUST allows
> us to undo the wreckage they create.
> 
> Thanks,
> 
>         tglx

Have not seen any multicore Dell/HP systems require that.

Anyway, for QEMU/KVM it should be synced (unless there is a bug
in the sync logic in the first place).