> On Mar 24, 2021, at 2:30 PM, Dave Hansen <dave.han...@intel.com> 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.
