linux-kernel - Re: [PATCH v2] KVM: x86: Sync the pending Posted-Interrupts

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Date:   Wed, 30 Jan 2019 10:31:27 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Kang, Luwei" <luwei.kang@...el.com>,
        "rkrcmar@...hat.com" <rkrcmar@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>, "hpa@...or.com" <hpa@...or.com>,
        "x86@...nel.org" <x86@...nel.org>
Cc:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] KVM: x86: Sync the pending Posted-Interrupts

On 30/01/19 10:01, Kang, Luwei wrote:
>> This is not what I asked.  You should instead do the check after pi_clear_sn.
>>
> 
> I think the SN has been cleared here before test the bitmap.
> The SN will be set when the  vCPU is schedule out. ID: 28b835d60fcc2498e717cf5e6f0c3691c24546f7
> But SN will be cleared when sched in.
> 
> Another place is when vCPU run out of the vcpu_run() function:
>      kvm_arch_vcpu_ioctl_run()
>          vcpu_load(vcpu); -> kvm_arch_vcpu_load -> vmx_vcpu_load -> vmx_vcpu_pi_load -> new.sn = 0;
>          vcpu_run(vcpu);
>              for(;;)
>          vcpu_put(vcpu); -> kvm_arch_vcpu_put -> vmx_vcpu_put -> vmx_vcpu_pi_put -> pi_set_sn()
> But SN will be cleared in vcpu_load() before back to vcpu_run()

Yes, but you're changing the wrong path.  The patch is affecting _all_
vmentries, not just those after PID.SN has been cleared.

As I mentioned in the previous email, KVM relies on the SDM's invariant
that ON where PID.ON=1 whenever PID.PIR!=0.  Invariants are your best
friend when dealing with complicated multi-processor code so I don't
want to change that.

It's the VT-d pi_clear_sn path that I want to be changed, because it's
VT-d and specifically SN that complicates the very simple definition in
the SDM.  By modifying the pi_clear_sn path, you ensure the invariant is
respected and everyone is happy.

Paolo