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

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Date:   Wed, 7 Sep 2022 10:05:48 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     John Garry <john.garry@...wei.com>,
        Ethan Zhao <haifeng.zhao@...ux.intel.com>, joro@...tes.org,
        will@...nel.org
Cc:     iommu@...ts.linux.dev, linux-kernel@...r.kernel.org,
        linuxarm@...wei.com
Subject: Re: [PATCH v2 1/2] iova: Remove some magazine pointer NULL checks

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.