Re: [PATCH] x86/hap: be more selective with assisted TLB flush

["Roger Pau Monné" <roger.pau@citrix.com>]

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 <jbeulich@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.