linux-kernel - Re: [PATCH v2 10/22] iommufd/viommmu: Add IOMMUFD_CMD_VCMDQ

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Message-ID: <aBEhGmoh0E865PxF@google.com>
Date: Tue, 29 Apr 2025 18:57:30 +0000
From: Pranjal Shrivastava <praan@...gle.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,
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	shuah@...nel.org, jsnitsel@...hat.com, nathan@...nel.org,
	peterz@...radead.org, yi.l.liu@...el.com, mshavit@...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 v2 10/22] iommufd/viommmu: Add IOMMUFD_CMD_VCMDQ_ALLOC
 ioctl

On Tue, Apr 29, 2025 at 11:15:00AM -0700, Nicolin Chen wrote:
> On Tue, Apr 29, 2025 at 06:10:31PM +0000, Pranjal Shrivastava wrote:
> > On Tue, Apr 29, 2025 at 08:28:01AM +0000, Pranjal Shrivastava wrote:
> > > On Mon, Apr 28, 2025 at 03:44:08PM -0700, Nicolin Chen wrote:
> > > > On Mon, Apr 28, 2025 at 09:34:05PM +0000, Pranjal Shrivastava wrote:
> > > > > On Fri, Apr 25, 2025 at 10:58:05PM -0700, Nicolin Chen wrote:
> > > [...] 
> > > > > IIUC, one vintf can have multiple lvcmdqs and looking at the series
> > > > > it looks like the vcmdq_alloc allocates a single lvcmdq. Is the plan to
> > > > > dedicate one lvcmdq to per VM? Which means VMs can share a vintf?
> > > > 
> > > > VINTF is a vSMMU instance per SMMU. Each VINTF can have multiple
> > > > LVCMDQs. Each vCMDQ is allocated per IOMMUFD_CMD_VCMDQ_ALLOC. In
> > > > other word, VM can issue multiple IOMMUFD_CMD_VCMDQ_ALLOC calls
> > > > for each VTINF/vSMMU.
> > > > 
> > > 
> > > Ack. I'm just wondering why would a single VM want more than one vCMDQ
> > > per vSMMU?
> > > [...]
> > 
> > I guess the only thing on this patch from me was to understand why
> > would a single VM want more than one vCMDQ per vSMMU? (Just curious to
> > know :) )
> 
> It gives some perf gain since it has two portals to fill commands.

Ohh! I'm imagining concurrent invalidations / commands! Interesting!

> 
> Nicolin

Thanks!
Praan