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

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Message-ID: <a9f959ed-b528-681d-f941-d615360c0be3@linux.intel.com>
Date:   Wed, 7 Sep 2022 17:58:53 +0800
From:   Ethan Zhao <haifeng.zhao@...ux.intel.com>
To:     John Garry <john.garry@...wei.com>,
        Robin Murphy <robin.murphy@....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

John,

在 2022/9/7 16:46, John Garry 写道:
> 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.
>
> Or also add this:
>
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index 0d6d8edf782d..e8f0b8f47f45 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -578,6 +578,12 @@ static int iommu_dma_init_domain(struct 
> iommu_domain *domain, dma_addr_t base,
>              goto done_unlock;
>          }
>
> +        if (!iovad->rcaches) {
> +            pr_warn("IOVA domain rcache not properly initialised\n");
> +            ret = -EFAULT;
> +            goto done_unlock;
> +        }
> +
>          ret = 0;
>          goto done_unlock;
>
If the iovad->rcaches allocation failed, will skip iommu domain dma ops, 
so no need *any* iovad,->rcaches check, right ?

and there is already warning about the fallback.

Thanks,

Ethan

>
> But I figure that you don't want more crud there now, considering the 
> work you mention above.
>
> Thanks,
> John
>
>
>
-- 
"firm, enduring, strong, and long-lived"