On 13/10/21 16:06, Thomas Gleixner wrote:
- the guest value stored in vcpu->arch.
- the "QEMU" value attached to host_fpu. This one only becomes zero if
QEMU requires AMX (which shouldn't happen).
I don't think that makes sense.
First of all, if QEMU wants to expose AMX to guests, then it has to ask
for permission to do so as any other user space process. We're not going
to make that special just because.
Hmm, I would have preferred if there was no need to enable AMX for the
QEMU FPU. But you're saying that guest_fpu needs to swap out to
current->thread.fpu if the guest is preempted, which would require
XFD=0; and affect QEMU operation as well.
In principle I don't like it very much; it would be nicer to say "you
enable it for QEMU itself via arch_prctl(ARCH_SET_STATE_ENABLE), and for
the guests via ioctl(KVM_SET_CPUID2)". But I can see why you want to
keep things simple, so it's not a strong objection at all.
Anything else will just create more problems than it solves. Especially
#NM handling (think nested guest) and the XFD_ERR additive behaviour
will be a nasty playground and easy to get wrong.
Not having that at all makes life way simpler, right?
It is simpler indeed, and it makes sense to start simple. I am not sure
if it will hold, but I agree it's better for the first implementation.
Paolo
