Re: [PATCH 7/9] x86/hvm: Disable MPX by default

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 17/06/2020 12:41, Jan Beulich wrote:
> On 17.06.2020 13:28, Andrew Cooper wrote:
>> We actually have AVX512 disabled by default in XenServer.  The perf
>> implications of letting 1 guest play with it is very severe.
>>
>> Now I think about it, I'm tempted to recommend it moves out of default
>> generally.
> Hmm, I'm tempted to ask whether you're kidding.

I'm very definitely not.

AVX512 is a disaster, perf wise on Skylake/CascadeLake, and its very
easy to cripple the entire system, including the other guest.

So much so that "better AVX512 frequency transitions" is a headline
feature in IceLake.

>  This is the kind of
> feature that I see no reason at all to move out of default. Imo we
> shouldn't put in place policy like this - if anything shouldn't be
> on by default, it should imo be because of limitations in our
> handling (I've recently revived my UMIP emulation patch, which
> comes to mind here) or because of uncertainty on some aspects (like
> is the case for MOVDIR / ENQCMD for example). Anything else should
> be left to the admins to decide.

"left to the admins to decide" does not mean "on by default".

"default" needs to be a sensible set, which migrates safely, and can't
be used to trivially DoS the rest of the system.  An admin can always
opt into allowing this DoS, but shouldn't have it by default.

~Andrew