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Subject: Re: ...\n
From:   Christophe de Dinechin <dinechin@redhat.com>
In-Reply-To: <87r148olol.fsf@redhat.com>
Date:   Wed, 1 Jun 2022 10:25:01 +0200
Cc:     Peter Zijlstra <peterz@infradead.org>,
        "Durrant, Paul" <pdurrant@amazon.co.uk>,
        "Allister, Jack" <jalliste@amazon.com>,
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To:     Vitaly Kuznetsov <vkuznets@redhat.com>
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> On 1 Jun 2022, at 10:03, Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>=20
> Peter Zijlstra <peterz@infradead.org> writes:
>=20
>> On Tue, May 31, 2022 at 02:52:04PM +0000, Durrant, Paul wrote:
>=20
> ...
>=20
>>>=20
>>> 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?
>>=20
>> *why* would you want to do that? Everybody wants their stuff done
>> faster.
>>=20
>=20
> 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=E2=80=A6


>=20
> --=20
> Vitaly
>=20