Re: ...\n

Christophe de Dinechin
2022-06-01T08:25:19Z

> On 1 Jun 2022, at 10:03, Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>
> Peter Zijlstra <peterz@infradead.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
>