linux-kernel - Re: [PATCH v5 14/29] iommufd/viommu: Add IOMMUFD_CMD_HW_QUEUE

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Message-ID: <20250530182519.GV233377@nvidia.com>
Date: Fri, 30 May 2025 15:25:19 -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,
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	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,
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Subject: Re: [PATCH v5 14/29] iommufd/viommu: Add IOMMUFD_CMD_HW_QUEUE_ALLOC
 ioctl

On Fri, May 30, 2025 at 11:23:02AM -0700, Nicolin Chen wrote:
> On Fri, May 30, 2025 at 02:40:37PM -0300, Jason Gunthorpe wrote:
> > On Fri, May 30, 2025 at 10:38:24AM -0700, Nicolin Chen wrote:
> > > On Fri, May 30, 2025 at 01:14:55PM -0300, Jason Gunthorpe wrote:
> > > > On Sat, May 17, 2025 at 08:21:31PM -0700, Nicolin Chen wrote:
> > > > > +	offset =
> > > > > +		cmd->nesting_parent_iova - PAGE_ALIGN(cmd->nesting_parent_iova);
> > > > > +	max_npages = DIV_ROUND_UP(offset + cmd->length, PAGE_SIZE);
> > > > 
> > > > This should probably be capped to PAGE_SIZE/sizeof(void *), return
> > > > EINVAL if not
> > > 
> > > Hmm, mind elaborating where this PAGE_SIZE/sizeof comes from?
> > 
> > We can usually allocate up to a PAGE_SIZE without too much
> > trouble. Beyond that it gets more likely to fail.
> 
> If PAGE_SIZE=4096, the upper limit for max_npages is 512, i.e. the
> max size of a guest queue is 2MB? It seems to be too small, as the
> VMM can use a larger huge page size to back the guest queue?

May need to make a new API that returns a bio_vec or something else
more efficient then :\

Jason