linux-kernel - Re: ...\n

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Message-Id: <F662DF4F-930A-486E-86FB-97D54E535114@redhat.com>
Date:   Wed, 1 Jun 2022 10:25:01 +0200
From:   Christophe de Dinechin <dinechin@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        "Durrant, Paul" <pdurrant@...zon.co.uk>,
        "Allister, Jack" <jalliste@...zon.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "diapop@...zon.co.uk" <diapop@...zon.co.uk>,
        "hpa@...or.com" <hpa@...or.com>,
        "jmattson@...gle.com" <jmattson@...gle.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "metikaya@...zon.co.uk" <metikaya@...zon.co.uk>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "sean.j.christopherson@...el.com" <sean.j.christopherson@...el.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "wanpengli@...cent.com" <wanpengli@...cent.com>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: ...\n



> On 1 Jun 2022, at 10:03, Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
> 
> Peter Zijlstra <peterz@...radead.org> writes:
> 
>> On Tue, May 31, 2022 at 02:52:04PM +0000, Durrant, Paul wrote:
> 
> ...
> 
>>> 
>>> I'll bite... What's ludicrous about wanting to run a guest at a lower
>>> CPU freq to minimize observable change in whatever workload it is
>>> running?
>> 
>> *why* would you want to do that? Everybody wants their stuff done
>> faster.
>> 
> 
> FWIW, I can see a valid use-case: imagine you're running some software
> which calibrates itself in the beginning to run at some desired real
> time speed but then the VM running it has to be migrated to a host with
> faster (newer) CPUs. I don't have a real world examples out of top of my
> head but I remember some old DOS era games were impossible to play on
> newer CPUs because everything was happenning too fast. Maybe that's the
> case :-)

The PC version of Alpha Waves was such an example, but Frederick Raynal,
who did the port, said it was the last time he made the mistake. That was 1990 :-)

More seriously, what about mitigating timing-based remote attacks by
arbitrarily changing the CPU frequency and injecting noise in the timing?
That could be a valid use case, no? Although I can think of about a
million other ways of doing this more efficiently…


> 
> -- 
> Vitaly
>