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

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Date:   Wed, 30 Jan 2019 10:38:26 +0000
From:   "Kang, Luwei" <luwei.kang@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.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

> >> 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.

Hi Paolo,
    How about like this:

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 820a03b..dfc5e3d 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -1219,6 +1219,9 @@ static void vmx_vcpu_pi_load(struct kvm_vcpu *vcpu, int cpu)
                        new.ndst = (dest << 8) & 0xFF00;

                new.sn = 0;
+
+               if (!bitmap_empty((unsigned long *)new.pir, NR_VECTORS))
+                       new.on = 1;
        } while (cmpxchg64(&pi_desc->control, old.control,
                           new.control) != old.control);
 }

Thanks,
Luwei Kang