Re: Thoughts of AMX KVM support based on latest kernel

[Sean Christopherson <seanjc@google.com>]

On Tue, Nov 16, 2021, Paolo Bonzini wrote:
> On 11/16/21 19:55, Thomas Gleixner wrote:
> > 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.
> 
> I agree, this is how we handle SPEC_CTRL for example and it can be extended
> to XFD.  We should first do that, then switch to the MSR lists.  Hacking
> into schedule() should really be the last resort.

Agreed as well.

> >            local_irq_enable();     <- Problem starts here
> > 
> >            preempt_enable();	   <- Becomes wider here
> 
> It doesn't become that much wider because there's always preempt notifiers.
> So if it's okay to save XFD in the XSAVES wrapper and in
> kvm_arch_vcpu_put(), that might be already remove the need to do it
> schedule().

Assuming AMX can be accessed from (soft) IRQ context, hooking the preempt notifiers
isn't sufficient.  That's also why KVM waits until IRQs are disabled before
handling TIF_NEED_FPU_LOAD.