Re: [PATCH] KVM/x86: Do not clear SIPI while in SMM

[boris . ostrovsky]

On 4/16/24 4:53 PM, Paolo Bonzini wrote:
> On 4/16/24 22:47, Boris Ostrovsky wrote:
> > When a processor is running in SMM and receives INIT message the 
> > interrupt
> > is left pending until SMM is exited. On the other hand, SIPI, which
> > typically follows INIT, is discarded. This presents a problem since 
> > sender
> > has no way of knowing that its SIPI has been dropped, which results in
> > processor failing to come up.
> >
> > Keeping the SIPI pending avoids this scenario.
>
> This is incorrect - it's yet another ugly legacy facet of x86, but we 
> have to live with it.  SIPI is discarded because the code is supposed 
> to retry it if needed ("INIT-SIPI-SIPI").


I couldn't find in the SDM/APM a definitive statement about whether SIPI 
is supposed to be dropped.


> The sender should set a flag as early as possible in the SIPI code so 
> that it's clear that it was not received; and an extra SIPI is not a 
> problem, it will be ignored anyway and will not cause trouble if 
> there's a race.
>
> What is the reproducer for this?

Hotplugging/unplugging cpus in a loop, especially if you oversubscribe 
the guest, will get you there in 10-15 minutes.

Typically (although I think not always) this is happening when OVMF if 
trying to rendezvous and a processor is missing and is sent an extra SMI.


-boris