On 10/1/2021 2:29 PM, Thomas Gleixner wrote:
So we'd end up with two XSAVES on context switch. We can simply do:
XSAVES();
fpu.state.xtsate.uintr.uinv = 0;
I am a bit confused. Do we need to set UINV to 0 explicitly?
If XSAVES gets called twice during context switch then the UINV in the
XSTATE buffer automatically gets set to 0. Since XSAVES saves the
current UINV value in the MISC_MSR which was already set to 0 by the
previous XSAVES.
Though, this probably happens due to pure luck than intentional design :)
which allows to do as many XRSTORS in a row as we want. Only the final
one on the way to user space will have to restore the real vector if the
register state is not valid:
if (fpu_state_valid()) {
if (needs_uinv(current)
wrmsrl(UINV, vector);
} else {
if (needs_uinv(current)
fpu.state.xtsate.uintr.uinv = vector;
XRSTORS();
}
I might have missed some subtle difference. Has this logic changed from
what you previously suggested for arch_exit_to_user_mode_prepare()?
if (xrstors_pending)) {
// Update the saved xstate for xrstors
// Unconditionally update the UINV since it could have been
overwritten by calling XSAVES twice.
current->xstate.uintr.uinv = UINTR_NOTIFICATION_VECTOR;
current->xstate.uintr.uirr |= pir;
} else {
// Manually restore UIRR and UINV
rdmsrl(IA32_UINTR_RR, uirr);
wrmsrl(IA32_UINTR_RR, uirr | pir);
misc.val64 = 0;
misc.uittsz = current->uintr->uittsz;
misc.uinv = UINTR_NOTIFICATION_VECTOR;
wrmsrl(IA32_UINTR_MISC, misc.val64);
}
Hmm?
The one case I can see this failing is if there was another XRSTORS
after the "final" restore in arch_exit_to_user_mode_prepare()? I think
that is not possible but I am not an expert on this. Did I misunderstand
something?
Thanks,
Sohil
