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

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Message-ID: <aDn7Y+3s+YD7qDlh@Asurada-Nvidia>
Date: Fri, 30 May 2025 11:39:31 -0700
From: Nicolin Chen <nicolinc@...dia.com>
To: Jason Gunthorpe <jgg@...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>,
	<dwmw2@...radead.org>, <baolu.lu@...ux.intel.com>
Subject: Re: [PATCH v5 14/29] iommufd/viommu: Add IOMMUFD_CMD_HW_QUEUE_ALLOC
 ioctl

On Fri, May 30, 2025 at 03:25:19PM -0300, Jason Gunthorpe wrote:
> 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 :\

Hmm, that sounds like a rabbit hole :-\

Let me leave a FIXME at this max_npages calculation instead..

Thanks
Nicolin