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Message-ID: <a876245b-6094-1767-ac63-dc8d31de9b84@redhat.com>
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
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