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

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Message-ID: <aBEXJFnLalIh788i@Asurada-Nvidia>
Date: Tue, 29 Apr 2025 11:15:00 -0700
From: Nicolin Chen <nicolinc@...dia.com>
To: Pranjal Shrivastava <praan@...gle.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>, <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 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.

Nicolin