linux-kernel - Re: [PATCH v7 27/28] iommu/tegra241-cmdqv: Add user-space use support

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <aGbQipeSn0aZnwZU@google.com>
Date: Thu, 3 Jul 2025 18:48:42 +0000
From: Pranjal Shrivastava <praan@...gle.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Nicolin Chen <nicolinc@...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, dwmw2@...radead.org, baolu.lu@...ux.intel.com
Subject: Re: [PATCH v7 27/28] iommu/tegra241-cmdqv: Add user-space use support

On Thu, Jul 03, 2025 at 02:55:32PM -0300, Jason Gunthorpe wrote:
> On Thu, Jul 03, 2025 at 02:46:03PM +0000, Pranjal Shrivastava wrote:
> 
> > Right.. I was however hoping we'd also trap commands like CMD_PRI_RESP
> > and CMD_RESUME...I'm not sure if they should be accelerated via CMDQV..
> > I guess I'll need to look and understand a little more if they are..
> 
> Right now these commands are not supported by vSMMUv3 in Linux.
> 
> They probably should be trapped, but completing a PRI (or resuming a
> stall which we will treat the same) will go through the PRI/page fault
> logic in iommufd not the cache invalidate.
> 

Ahh, thanks for this, that saved a lot of my time! And yes, I see some
functions in eventq.c calling the iopf_group_response which settles the
CMD_RESUME. So.. I assume these resume commands would be trapped and
*actually* executed through this or a similar path for vPRI. 

Meh, I had been putting off reading up the fault parts of iommufd, 
I guess I'll go through that too, now :) 

> > > The goal of the SMMU driver when it detects CMDQV support is to route
> > > all supported invalidations to CMDQV queues and then balance those
> > > queues across CPUs to reduce lock contention.
> > 
> > I see.. that makes sense.. so it's a relatively small gain (but a nice
> > one). Thanks for clarifying!
> 
> On bare metal the gain is small (due to locking and balancing), while
> on virtualization the gain is huge (due to no trapping).
> 

Ohh yes, I meant the bare metal gains here.. for virtualization, it's
definitely huge (as reported too).

> Regardless the SMMU driver uses cmdqv support if the HW says it is
> there.
>
> Jason
>

Thanks!
Praan