linux-kernel - Re: [PATCH v1 2/3] KVM: VMX: Do not change PID.NDST when loading a blocked vCPU

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Date:   Mon, 11 Nov 2019 14:48:44 +0000
From:   Joao Martins <joao.m.martins@...cle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Radim Krčmář <rkrcmar@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Liran Alon <liran.alon@...cle.com>,
        Jag Raman <jag.raman@...cle.com>
Subject: Re: [PATCH v1 2/3] KVM: VMX: Do not change PID.NDST when loading a
 blocked vCPU

On 11/11/19 2:39 PM, Paolo Bonzini wrote:
> On 06/11/19 18:56, Joao Martins wrote:
>> When vCPU enters block phase, pi_pre_block() inserts vCPU to a per pCPU
>> linked list of all vCPUs that are blocked on this pCPU. Afterwards, it
>> changes PID.NV to POSTED_INTR_WAKEUP_VECTOR which its handler
>> (wakeup_handler()) is responsible to kick (unblock) any vCPU on that
>> linked list that now has pending posted interrupts.
>>
>> While vCPU is blocked (in kvm_vcpu_block()), it may be preempted which
>> will cause vmx_vcpu_pi_put() to set PID.SN.  If later the vCPU will be
>> scheduled to run on a different pCPU, vmx_vcpu_pi_load() will clear
>> PID.SN but will also *overwrite PID.NDST to this different pCPU*.
>> Instead of keeping it with original pCPU which vCPU had entered block
>> phase on.
>>
>> This results in an issue because when a posted interrupt is delivered,
>> the wakeup_handler() will be executed and fail to find blocked vCPU on
>> its per pCPU linked list of all vCPUs that are blocked on this pCPU.
>> Which is due to the vCPU being placed on a *different* per pCPU
>> linked list than the original pCPU that it had entered block phase.
>>
>> The regression is introduced by commit c112b5f50232 ("KVM: x86:
>> Recompute PID.ON when clearing PID.SN"). Therefore, partially revert
>> it and reintroduce the condition in vmx_vcpu_pi_load() responsible for
>> avoiding changing PID.NDST when loading a blocked vCPU.
>>
>> Fixes: c112b5f50232 ("KVM: x86: Recompute PID.ON when clearing PID.SN")
>> Signed-off-by: Joao Martins <joao.m.martins@...cle.com>
>> Signed-off-by: Liran Alon <liran.alon@...cle.com>
> 
> Something wrong in the SoB line?
> 
I can't spot any mistake; at least it looks chained correctly for me. What's the
issue you see with the Sob line?

The only thing I forgot was a:

Tested-by: Nathan Ni <nathan.ni@...cle.com>

> Otherwise looks good.

If you want I can resubmit the series with the Tb and Jag Rb, unless you think
it's OK doing on commit? Otherwise I can resubmit.

	Joao