Re: [Xen-devel] [PATCH v10 09/11] x86/ctxt: Issue a speculation barrier between vcpu contexts

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 25/01/18 16:31, Jan Beulich wrote:
>>>> On 25.01.18 at 17:09, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 25/01/18 15:57, Jan Beulich wrote:
>>>>>> On 24.01.18 at 14:12, <andrew.cooper3@xxxxxxxxxx> wrote:
>>>> @@ -1743,6 +1744,34 @@ void context_switch(struct vcpu *prev, struct vcpu 
>>>> *next)
>>>>          }
>>>>  
>>>>          ctxt_switch_levelling(next);
>>>> +
>>>> +        if ( opt_ibpb && !is_idle_domain(nextd) )
>>> Is the idle domain check here really useful?
>> Yes, because as you pointed out in v9, the outer condition isn't
>> sufficient to exclude nextd being idle.
> True, but then again - what's wrong with an idle vCPU making it
> into the block? It'll be a pointless barrier that you issue, but no
> other harm afaics. Remember that I complained about the missing
> check only because of the chosen variable naming, but you've
> renamed the variables in question, so I don't see why you've also
> added the extra condition.

The barrier would be pointless, yes, but at 8k cycles, the cost is massive.

~Andrew

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