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