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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 30.04.2020 10:28, Roger Pau Monné wrote:
> 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@xxxxxxxx>
>> 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."

Sure; I'll give it over the weekend though for others to comment, if
so desired.

Jan