Re: [PATCH] iommu/amd-vi: fix checking for Invalidate All support in amd_iommu_resume()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 13.06.2023 12:33, Roger Pau Monné wrote:
> On Tue, Jun 13, 2023 at 12:23:37PM +0200, Jan Beulich wrote:
>> On 13.06.2023 12:13, Roger Pau Monne wrote:
>>> Do not rely on iommu local variable pointing to a valid amd_iommu
>>
>> "Do not rely" sounds like we might, but we choose not to. But we may
>> not rely on this, as the pointer simply isn't valid to deref past
>> the loop. Hence perhaps better "We cannot rely ..." or even "The
>> iommu local variable does not point to ..."?
> 
> "Xen cannot rely ..." doesn't modify the sentence too much and could
> likely be adjusted at commit if you agree?
> 
> Otherwise your last suggestion would also be OK by me.

I used that latter one, as being more concise.

>>> --- a/xen/drivers/passthrough/amd/iommu_init.c
>>> +++ b/xen/drivers/passthrough/amd/iommu_init.c
>>> @@ -1580,6 +1580,7 @@ void cf_check amd_iommu_crash_shutdown(void)
>>>  void cf_check amd_iommu_resume(void)
>>>  {
>>>      struct amd_iommu *iommu;
>>> +    bool invalidate_all = true;
>>>  
>>>      for_each_amd_iommu ( iommu )
>>>      {
>>> @@ -1589,10 +1590,12 @@ void cf_check amd_iommu_resume(void)
>>>          */
>>>          disable_iommu(iommu);
>>>          enable_iommu(iommu);
>>> +        if ( !iommu->features.flds.ia_sup && invalidate_all )
>>
>> You don't really need the right hand part of the condition, do you?
>>
>> Preferably with the adjustments (which I'd be happy to do while
>> committing, as long as you agree)
>> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> Wanted to avoid repeatedly setting `invalidate_all = false` if all the
> IOMMUs on the system don't support Invalidate All.
> 
> I don't have a strong opinion, my first (local) version didn't have
> the right hand side of the condition, but then I realized that setting
> this for every IOMMU on the system could be wasteful.

I've dropped that part: We don't care much about that tiny performance
difference here (and it's unclear whether the extra check actually is
a win or a loss), and imo the code is more clear with the simpler
conditional.

Jan