Re: [PATCH v2 1/2] iova: Remove some magazine pointer NULL checks

Robin Murphy
2022-09-07T09:05:59Z

On 2022-09-07 09:46, John Garry wrote:
> On 06/09/2022 19:25, Robin Murphy wrote:
>>>
>>> Caveat: on the chance that the IOVA domain init fails due to the
>>> rcache init failing, then, if there were another device in the group
>>> which probes later, its probe would be ok as the start_pfn is set.
>>> Not Good.
>>
>> Yeah, there's a lot not to like about iommu_dma_init_domain() - I've
>> been banking on it all getting cleaned up when I get to refactoring
>> that area of probing (remember the issue you reported years ago with
>> PCI groups being built in the wrong order? All related...), but in
>> fact since the cookie management got pulled into core code, we can
>> probably tie the IOVA domain setup to that right now without much
>> other involvement. That could be a cheap win, so I'll give it a go soon.
>
> ok, great.
>
> On a related topic, another thing to consider is that errors in IOVA
> domain init are not handled gracefully in terms of how we deal with the
> device probe and setting dma mapping ops, ref iommu_setup_dma_ops(). I
> assume you know all this.
>
>>
>>> - vdpa just fails to create the domain in vduse_domain_create()
>>>
>>>> That makes a fair amount of sense, but does mean that we're missing
>>>> the equivalent in iova_rcache_insert() for it to actually work. Or
>>>> we just remove it and tighten up the documentation to say that's not
>>>> valid
>>>
>>> I'd be more inclined to remove it. I would rather remove fathpath
>>> checks as much as possible and have robust error handling in the
>>> domain init.
>>>
>>> Afterall I do have the "remove check" craze going.
>>
>> Sure, like I say I'm happy to be consistent either way. If I do end up
>> reinstating such a check I think I'd prefer to have it explicit in
>> {alloc,free}_iova_fast() anyway, rather than buried in internal
>> implementation details.
>
> I'm not sure what you would like to see now, if anything.
>
> I could just remove the iovad->rcache check in iova_rcache_get().  It's
> pretty useless (on its own) since we don't have the same check on the
> "insert" path.

Yup, just remove it. Sorting iommu-dma is yet another issue, but let's
skip straight to fixing that properly by allocating the IOVA domain
up-front with the cookie (is this the last remnant of my 7-year-old
misunderstanding of dma_32bit_pfn? Let's hope so...)

Thanks,
Robin.