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

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Date: Tue, 16 Apr 2024 16:57:42 -0400
From: boris.ostrovsky@...cle.com
To: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org
Cc: seanjc@...gle.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM/x86: Do not clear SIPI while in SMM


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