Re: [PATCH v9 1/3] x86/tlb: introduce a flush HVM ASIDs flag

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.04.2020 10:01, Roger Pau Monné wrote:
> On Thu, Apr 09, 2020 at 01:16:57PM +0200, Jan Beulich wrote:
>> On 08.04.2020 17:10, Roger Pau Monné wrote:
>>> On Wed, Apr 08, 2020 at 01:25:14PM +0200, Jan Beulich wrote:
>>>> On 06.04.2020 12:57, Roger Pau Monne wrote:
>>>>> --- a/xen/arch/x86/mm/paging.c
>>>>> +++ b/xen/arch/x86/mm/paging.c
>>>>> @@ -613,7 +613,8 @@ void paging_log_dirty_range(struct domain *d,
>>>>>  
>>>>>      p2m_unlock(p2m);
>>>>>  
>>>>> -    flush_tlb_mask(d->dirty_cpumask);
>>>>> +    flush_mask(d->dirty_cpumask, (!hap_enabled(d) ? FLUSH_TLB : 0) |
>>>>> +                                 FLUSH_HVM_ASID_CORE);
>>>>
>>>> In cases where one case is assumed to be more likely than the other
>>>> putting the more likely one first can be viewed as a mild hint to
>>>> the compiler, and hence an extra ! may be warranted in an if() or
>>>> a conditional expression. Here, however, I don't think we can
>>>> really consider one case more likely than the other, and hence I'd
>>>> suggest to avoid the !, flipping the other two expressions
>>>> accordingly. I may take the liberty to adjust this while committing
>>>> (if I'm to be the one).
>>>
>>> That's fine, thanks. Somehow '!hap -> flush' was clearer in my mind.
>>
>> Thinking about it with the other HVM-related changes in v9, shouldn't
>> this then be
>>
>>     flush_mask(d->dirty_cpumask, (hap_enabled(d) ? 0 : FLUSH_TLB) |
>>                                  (is_hvm_domain(d) ? FLUSH_HVM_ASID_CORE : 
>> 0));
>>
>> Or wait - the only caller lives in hap.c. As a result the FLUSH_TLB
>> part can be dropped altogether. And I question the need of flushing
>> guest TLBs - this is purely a p2m operation. I'll go look at the
>> history of this function, but for now I think the call should be
>> dropped (albeit then maybe better in a separate patch).
> 
> The ASID flush needs to stay unless it's moved into p2m_pt_set_entry,
> as p2m_pt_set_entry itself doesn't perform any ASID flush and won't
> work correctly.

Just like for said in the other reply sent a few minutes ago - yes
for NPT, but no for EPT.

> I think it's safe to remove the TLB flush, as the code is only called
> from HAP, and hence is not used by shadow (which is what would require
> a plain TLB flush). The placement of this function seems misleading to
> me, as it looks like it's used by both shadow and HAP. It might be
> better to move it to hap.c if it's only to be used by HAP code.

Either placement has its problems, I think. The function is meant to
be a paging layer one, but is needed by HAP only right now. I'm
pondering whether to wrap it in #ifdef CONFIG_HVM (plus perhaps a
respective ASSERT_UNREACHABLE()).

In the end, just like in the other cases, this may be a valid further
user of the more generic helper that I did suggest (resulting in no
flushing on EPT and an ASID-based one on NPT).

Jan