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

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Message-ID: <aE/B40NRb+GecpRi@nvidia.com>
Date: Mon, 16 Jun 2025 00:04:03 -0700
From: Nicolin Chen <nicolinc@...dia.com>
To: Baolu Lu <baolu.lu@...ux.intel.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 Mon, Jun 16, 2025 at 02:54:10PM +0800, Baolu Lu wrote:
> 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.

No. The access is only used by the ops->hw_queue_init_phys case.

The ops->hw_queue_init case will use the cmd->nesting_parent_iova
directly without calling iommufd_hw_queue_alloc_phys().

This FIXME means that, when adding ops->hw_queue_init, add this:

-	struct iommufd_access *access;
+	struct iommufd_access *access = NULL;
...
-	access = iommufd_hw_queue_alloc_phys(cmd, viommu, &base_pa);
+	if (ops->hw_queue_init_phys) {
+		access = iommufd_hw_queue_alloc_phys(cmd, viommu, &base_pa);

Also, the other FIXME guideline states that these two ops should be
mutually exclusive. So, add this too:
+	if (WARN_ON_ONCE(ops->hw_queue_init &&
+			 ops->hw_queue_init_phys)) {
+		rc = -EOPNOTSUPP;

Thanks
Nicolin