Re: [PATCH v4 14/22] x86/fpu/xstate: Expand the xstate buffer on the first use of dynamic user state

[Andy Lutomirski]

> On Mar 24, 2021, at 2:30 PM, Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
> 
> On 3/24/21 2:26 PM, Andy Lutomirski wrote:
>>> 3. user space always uses fully uncompacted XSAVE buffers.
>>> 
>> There is no reason we have to do this for new states. Arguably we
>> shouldn’t for AMX to avoid yet another altstack explosion.
> 
> The thing that's worried me is that the list of OS-enabled states is
> visible to apps via XGETBV.  It doesn't seem too much of a stretch to
> think that apps will see AMX enabled with XGETBV and them assume that
> it's on the signal stack.
> 
> Please tell me I'm being too paranoid.  If we can break this assumption,
> it would get rid of a lot of future pain.

There are no AMX apps. I sure hope that there are no apps that enumerate xfeatures with CPUID and try to decode the mess in the signal stack.

I do think we need to save AMX state *somewhere* if a signal happens unless userspace opts out, but I don’t think it needs to be in the nominally expected spot.