On Thu, Apr 30, 2020 at 09:20:58AM +0200, Jan Beulich wrote: > On 29.04.2020 19:36, Roger Pau Monne wrote: > > When doing an assisted flush on HAP the purpose of the > > on_selected_cpus is just to trigger a vmexit on remote CPUs that are > > in guest context, and hence just using is_vcpu_dirty_cpu is too lax, > > also check that the vCPU is running. > > Am I right to understand that the change is relevant only to > cover the period of time between ->is_running becoming false > and ->dirty_cpu becoming VCPU_CPU_CLEAN? I.e. ... > > > --- a/xen/arch/x86/mm/hap/hap.c > > +++ b/xen/arch/x86/mm/hap/hap.c > > @@ -719,7 +719,7 @@ static bool flush_tlb(bool (*flush_vcpu)(void *ctxt, > > struct vcpu *v), > > hvm_asid_flush_vcpu(v); > > > > cpu = read_atomic(&v->dirty_cpu); > > - if ( cpu != this_cpu && is_vcpu_dirty_cpu(cpu) ) > > + if ( cpu != this_cpu && is_vcpu_dirty_cpu(cpu) && v->is_running ) > > ... the previous logic would have suitably covered the switch-to > path, but doesn't properly cover the switch-from one, due to our > lazy context switch approach?
Yes. Also __context_switch is not called from context_switch when switching to the idle vcpu, and hence dirty_cpu is not cleared. > If so, I agree with the change: > Reviewed-by: Jan Beulich <jbeul...@suse.com> > It might be worth mentioning this detail in the description then, > though. Would you mind adding to the commit message if you agree: "Due to the lazy context switching done by Xen dirty_cpu won't always be cleared when the guest vCPU is not running, and hence relying on is_running allows more fine grained control of whether the vCPU is actually running." Thanks, Roger.
