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

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Message-ID: <77e48fe09de55fa77a9e33a2c6212e42c83556be.camel@redhat.com>
Date:   Thu, 03 Dec 2020 13:51:18 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     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, 2020-12-01 at 12:02 -0300, Marcelo Tosatti wrote:
> 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).
> 

I agree with that, and that is why I suggested to make the guest
avoid TSC syncing when KVM is detected.
 
I don't mind how to implement this.
 
It can be either done with new CPUID bit, 
or always when KVM
is detected, 
(or even when *any* hypervisor is detected)
 
I also don't mind if we only disable tsc sync logic or
set X86_FEATURE_TSC_RELIABLE which will disable it
and the clocksource watchdog.


Best regards,
	Maxim Levitsky