Re: Thoughts of AMX KVM support based on latest kernel

[Nakajima, Jun]

> 
> On Nov 16, 2021, at 12:36 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> 
> Paolo,
> 
> On Tue, Nov 16 2021 at 20:49, 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.
> 
> SPEC_CTRL is different because it's done right after each VMEXIT.
> 
> XFD can be done lazy when breaking out of the exit fastpath loop before
> enabling interrupts.

I agree. The XFD features are for user-space.


> 
>> We should first do that, then switch to the MSR lists. 
>>  Hacking into schedule() should really be the last resort.
>> 
>>>           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().
> 
> Did not think about preemption notifiers. Probably because I hate
> notifiers with a passion since I had to deal with the CPU hotplug
> notifier trainwreck.
> 
> But yes that would work. So the places to do that would be:
> 
> 1) kvm_sched_out() -> kvm_arch_vcpu_put()
> 2) kernel_fpu_begin_mask()
> 3) kvm_put_guest_fpu()
> 
> But I really would start with the trivial version I suggested because
> that's already in the slow path and not at every VMEXIT.
> 
> I'd be really surprised if that RDMSR is truly noticeable within all the
> other crud this path is doing.
> 

I also agree here, and we’ll measure the effect to double-check.

We don’t want to complicate or optimize the system for very rare cases.
I like your "trivial version" because all the things KVM needs to do is just restore the consistent state.


--- 
Jun