linux-kernel - Re: [PATCH v6 10/25] iommufd/viommu: Add IOMMUFD_CMD_HW_QUEUE

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Message-ID: <b9a7ae65-edc6-46e9-bea0-163a51459884@linux.intel.com>
Date: Mon, 16 Jun 2025 14:54:10 +0800
From: Baolu Lu <baolu.lu@...ux.intel.com>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: jgg@...dia.com, kevin.tian@...el.com, corbet@....net, will@...nel.org,
 bagasdotme@...il.com, robin.murphy@....com, joro@...tes.org,
 thierry.reding@...il.com, vdumpa@...dia.com, jonathanh@...dia.com,
 shuah@...nel.org, jsnitsel@...hat.com, nathan@...nel.org,
 peterz@...radead.org, yi.l.liu@...el.com, mshavit@...gle.com,
 praan@...gle.com, zhangzekun11@...wei.com, iommu@...ts.linux.dev,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, linux-tegra@...r.kernel.org,
 linux-kselftest@...r.kernel.org, patches@...ts.linux.dev, mochs@...dia.com,
 alok.a.tiwari@...cle.com, vasant.hegde@....com, dwmw2@...radead.org
Subject: Re: [PATCH v6 10/25] iommufd/viommu: Add IOMMUFD_CMD_HW_QUEUE_ALLOC
 ioctl

On 6/16/25 14:47, Nicolin Chen wrote:
> On Mon, Jun 16, 2025 at 02:12:04PM +0800, Baolu Lu wrote:
>> On 6/14/25 15:14, Nicolin Chen wrote:
>>> +	if (!viommu->ops || !viommu->ops->get_hw_queue_size ||
>>> +	    !viommu->ops->hw_queue_init_phys) {
>>> +		rc = -EOPNOTSUPP;
>>> +		goto out_put_viommu;
>>> +	}
> Hmm, here it does abort when !viommu->ops->hw_queue_init_phys ..
> 
>>> +	/*
>>> +	 * FIXME once ops->hw_queue_init is introduced, this should check "if
>>> +	 * ops->hw_queue_init_phys". And "access" should be initialized to NULL.
>>> +	 */
>> I just don't follow here. Up until now, only viommu->ops->
>> hw_queue_init_phys has been added, which means the current code only
>> supports hardware queues that access guest memory using physical
>> addresses. The access object is not needed for the other type of
>> hardware queue that uses guest IOVA.
>>
>> So, why not just abort here if ops->hw_queue_init_phys is not supported
>> by the IOMMU driver?
> .. so, it already does.
> 
>> Leave other logics to the patches that introduce
>> ops->hw_queue_init? I guess that would make this patch more readible.
> The patch is doing in the way to support the hw_queue_init_phys
> case only. It is just adding some extra FIXMEs as the guideline
> for the future patch adding hw_queue_init op.
> 
> I personally don't feel these are confusing. Maybe you can help
> point out what specific wording feels odd here? Maybe "FIXME"s
> should be "TODO"s?

Oh, I probably misunderstood the logic here. For both hw_queue_init and
hw_queue_init_phys, using an access object to pin the pages for hardware
access is necessary, right? My understanding was that pinning pages is
only required for hw_queue_init_phys.

> 
> I could also drop all of these guideline if they are considered
> very unnecessary.
> 
> Thanks
> Nicolin

Thanks,
baolu