linux-kernel - Re: [PATCH v5 0/6] Add Tegra241 (Grace) CMDQV Support (part 1/2)

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Date: Wed, 17 Apr 2024 09:24:45 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Shameerali Kolothum Thodi <shameerali.kolothum.thodi@...wei.com>
Cc: Nicolin Chen <nicolinc@...dia.com>, "will@...nel.org" <will@...nel.org>,
	"robin.murphy@....com" <robin.murphy@....com>,
	"joro@...tes.org" <joro@...tes.org>,
	"thierry.reding@...il.com" <thierry.reding@...il.com>,
	"vdumpa@...dia.com" <vdumpa@...dia.com>,
	"jonathanh@...dia.com" <jonathanh@...dia.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	Jerry Snitselaar <jsnitsel@...hat.com>
Subject: Re: [PATCH v5 0/6] Add Tegra241 (Grace) CMDQV Support (part 1/2)

On Wed, Apr 17, 2024 at 08:01:10AM +0000, Shameerali Kolothum Thodi wrote:
> We do have plans to revive the SMMUv3 ECMDQ series posted a while back[0]
> and looking at this series, I am just wondering whether it makes sense to have
> a similar one with ECMDQ as well?  I see that the NVIDIA VCMDQ has a special bit 
> to restrict the commands that can be issued from user space. If we end up assigning
> a ECMDQ to user space, is there any potential risk in doing so?

I think there is some risk/trouble, ECMDQ needs some enhancement
before it can be really safe to use from less privileged software, and
it wasn't designed to have an isolated doorbell page either.

> Not clear to me what are the major concerns here and maybe we can come up with 
> something to address that in kernel.

I haven't looked deeply but my impression has been the ECMDQ is not
workable to support virtualization. At a minimum it has no way to
constrain the command flow to a VMID and to do VSID -> PSID
translation.

I suggest you talk directly to ARM on this if you are interested in
this.

Jason