Re: [for-4.15][RESEND PATCH v4 1/2] xen/x86: iommu: Ignore IOMMU mapping requests when a domain is dying

[Jan Beulich <jbeulich@xxxxxxxx>]

On 25.02.2021 12:56, Julien Grall wrote:
> On 24/02/2021 14:07, Jan Beulich wrote:
>> On 24.02.2021 10:43, Julien Grall wrote:
>>> --- a/xen/drivers/passthrough/x86/iommu.c
>>> +++ b/xen/drivers/passthrough/x86/iommu.c
>>> @@ -267,6 +267,12 @@ int iommu_free_pgtables(struct domain *d)
>>>       struct page_info *pg;
>>>       unsigned int done = 0;
>>>   
>>> +    if ( !is_iommu_enabled(d) )
>>> +        return 0;
>>
>> Why is this addition needed? Hitting a not yet initialize spin lock
>> is - afaict - no worse than a not yet initialized list, so it would
>> seem to me that this can't be the reason. No other reason looks to
>> be called out by the description.
> 
> struct domain_iommu will be initially zeroed as it is part of struct domain.
> 
> For the list, we are so far fine because page_list_remove_head() 
> tolerates NULL. If we were using the normal list operations (e.g. 
> list_del), then this code would have segfaulted.

And so we do, in the CONFIG_BIGMEM case. May I suggest then to split
this out as a prereq patch, or add wording to the description
mentioning this additional effect?

Jan