Re: Thoughts of AMX KVM support based on latest kernel

[Thomas Gleixner <tglx@linutronix.de>]

Sean,

On Tue, Nov 16 2021 at 16:05, Sean Christopherson wrote:
> On Tue, Nov 16, 2021, Thomas Gleixner wrote:
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 2686f2edb47c..9425fdbb4806 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -9576,6 +9576,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>>  	vcpu->arch.last_vmentry_cpu = vcpu->cpu;
>>  	vcpu->arch.last_guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc());
>>  
>> +	kvm_update_guest_xfd_state();
>
> Is there a reason the XFD switch can't key off TIF_NEED_FPU_LOAD a la the other
> FPU stuff?  I.e. piggyback this snippet in vcpu_enter_guest():

TIF_NEED_FPU_LOAD is not set here.

> 	if (test_thread_flag(TIF_NEED_FPU_LOAD))
> 		switch_fpu_return();

Assume guest has control of XFD and XFD writes are not trapped. That
means on vmexit the XFD state of the guest is unknown.

vcpu_run()
        kvm_load_guest_fpu()
            wrmsrl(XFD, guest_fpstate->xfd);
            XRSTORS
          
        do {

           local_irq_disable();

           // Covers the case of softirq usage and preemption
           if (test_thread_flag(TIF_NEED_FPU_LOAD))
		switch_fpu_return()
                  wrmsrl(XFD, guest_fpstate->xfd);

           do {
                vmenter();              // Guest modifies XFD
           } while (reenter);

           local_irq_enable();     <- Problem starts here

           preempt_enable();	   <- Becomes wider here

        } while (!breakout);

        kvm_put_guest_fpu();            // Switch back to user FPU state

So we have the following cases:

guest_fpstate.xfd        XFD at vmexit

       0                      0         // consistent state
       1                      0         // inconsistent state
       0                      1         // inconsistent state
       1                      1         // consistent state

Now assume that after reenabling interrupts a interrupt/softirq happens
which uses FPU. It will save the correct state because XFD is still
guest state, but the subsequent restore will operate on the stale
guest_fpstate.xfd value.

Same problem vs schedule after reenabling preemption or if not preempted
in kvm_put_guest_fpu()

Now you could argue that the interrupt/softirq XSAVES should also read
the XFD MSR and save it in guest_fpstate.xfd. Same in schedule()
and kvm_put_guest_fpu(), i.e:

      XSAVES
      if (fpstate->is_guest) {
            rdmsrl(XFD, xfd);
            fpstate->xfd = xfd;
            __this_cpu_write(..., xfd);
      }

We can do that, but I'm unhappy about this conditional in schedule(). So
I was asking for doing a simple KVM only solution first:

vcpu_run()
        kvm_load_guest_fpu()
            wrmsrl(XFD, guest_fpstate->xfd);
            XRSTORS
          
        do {

           local_irq_disable();

           if (test_thread_flag(TIF_NEED_FPU_LOAD))
		switch_fpu_return()
                  wrmsrl(XFD, guest_fpstate->xfd);

           do {
                vmenter();              // Guest modifies XFD
           } while (reenter);

           update_xfd_state();          // Restore consistency

           local_irq_enable();

and check how bad that is for KVM in terms of overhead on AMX systems.

If it really matters we can look at the conditional in XSAVES path.

Thanks,

        tglx