linux-kernel - Re: [PATCH v4 11/23] iommufd/viommu: Add IOMMUFD_CMD_HW_QUEUE

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Message-ID: <20250515185938.GT382960@nvidia.com>
Date: Thu, 15 May 2025 15:59:38 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: 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
Subject: Re: [PATCH v4 11/23] iommufd/viommu: Add IOMMUFD_CMD_HW_QUEUE_ALLOC
 ioctl

On Thu, May 15, 2025 at 11:16:45AM -0700, Nicolin Chen wrote:
> > As I understand AMD's system the iommu HW itself translates the
> > base_addr through the S2 page table automatically, so it doesn't need
> > pinned memory and physical addresses but just the IOVA.
> 
> Right. That's why we invented a flag, and it should be probably
> extended to cover the pin step as well.

Yes, no pin
 
> > Perhaps for this reason the pinning should be done with a function
> > call from the driver?
> 
> But the whole point of doing in the core was to avoid the entire
> iopt related structure/function from being exposed to the driver,
> which would otherwise hugely increase the size of the driver.o?

Ugh, yes, but also, maybe we need to figure something else out for
this. Pass down a function pointers struct to the driver or something
like that?

> > I don't think this actually works like this without an unmap
> > callback. unmap will break:
> > 
> > 			iommufd_access_notify_unmap(iopt, area_first, length);
> > 			/* Something is not responding to unmap requests. */
> > 			tries++;
> > 			if (WARN_ON(tries > 100))
> > 				return -EDEADLOCK;
> > 
> > If it can't shoot down the pinning.
> 
> Hmm, I thought we want the unmap to fail until VMM releases the HW
> QUEUE first? In what case, does VMM wants to unmap while holding
> the queue pages?

Well, if that is what we want to do then this needs to be revised
somehow..

> > This is more reason to put the pin/access in the driver so it can
> > provide an unmap callback that can fix it up.
> 
> As there are two types of "access" here.. you mean iommufd_access,
> i.e. vcmdq driver should hold an iommufd_access like an emulated
> vfio device driver?

Yes.

Jason