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

[Jan Beulich <jbeulich@suse.com>]

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? 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.

Jan