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
