Re: [RFC PATCH 05/13] x86/irq: Reserve a user IPI notification vector

[Thomas Gleixner]

Sohil,

On Mon, Sep 27 2021 at 12:07, Sohil Mehta wrote:
> On 9/26/2021 5:39 AM, Thomas Gleixner wrote:
>
> The User-interrupt notification processing moves all the pending 
> interrupts from UPID.PIR to the UIRR.

Indeed that makes sense. Should have thought about that myself.

>> Also the restore portion on the way back to user space has to be coupled
>> more tightly:
>>
>> arch_exit_to_user_mode_prepare()
>> {
>>          ...
>>          if (unlikely(ti_work & _TIF_UPID))
>>          	uintr_restore_upid(ti_work & _TIF_NEED_FPU_LOAD);
>>          if (unlikely(ti_work & _TIF_NEED_FPU_LOAD))
>>          	switch_fpu_return();
>> }
>
> I am assuming _TIF_UPID would be set everytime SN is set and XSTATE is 
> saved.

Yes.

>> upid_set_ndst(upid)
>> {
>> 	apicid = __this_cpu_read(x86_cpu_to_apicid);
>>
>>          if (x2apic_enabled())
>>              upid->ndst.x2apic = apicid;
>>          else
>>              upid->ndst.apic = apicid;
>> }
>>
>> uintr_restore_upid(bool xrstors_pending)
>> {
>>          clear_thread_flag(TIF_UPID);
>>          
>> 	// Update destination
>>          upid_set_ndst(upid);
>>
>>          // Do we need something stronger here?
>>          barrier();
>>
>>          clear_bit(SN, upid->status);
>>
>>          // Any SENDUIPI after this point sends to this CPU
>>             
>>          // Any bit which was set in upid->pir after SN was set
>>          // and/or UINV was cleared by XSAVES up to the point
>>          // where SN was cleared above is not reflected in UIRR.
>>
>> 	// As this runs with interrupts disabled the current state
>>          // of upid->pir can be read and used for restore. A SENDUIPI
>>          // which sets a bit in upid->pir after that read will send
>>          // the notification vector which is going to be handled once
>>          // the task reenables interrupts on return to user space.
>>          // If the SENDUIPI set the bit before the read then the
>>          // notification vector handling will just observe the same
>>          // PIR state.
>>
>>          // Needs to be a locked access as there might be a
>>          // concurrent SENDUIPI modiying it.
>>          pir = read_locked(upid->pir);
>>
>>          if (xrstors_pending)) {
>>          	// Update the saved xstate for xrstors
>>             	current->xstate.uintr.uinv = UINTR_NOTIFICATION_VECTOR;
>
> XSAVES saves the UINV value into the XSTATE buffer. I am not sure if we 
> need this again. Is it because it could have been overwritten by calling 
> XSAVES twice?

Yes that can happen AFAICT. I haven't done a deep analysis, but this
needs to looked at.

>>                  current->xstate.uintr.uirr = pir;
>
> I believe PIR should be ORed. There could be some bits already set in 
> the UIRR.
>
> Also, shouldn't UPID->PIR be cleared? If not, we would detect these 
> interrupts all over again during the next ring transition.

Right. So that PIR read above needs to be a locked cmpxchg().

>>          } else {
>>                  // Manually restore UIRR and UINV
>>                  wrmsrl(IA32_UINTR_RR, pir);
> I believe read-modify-write here as well.

Sigh, yes.

Thanks,

        tglx