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

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Message-ID: <aGRIctg4T6cQECx4@google.com>
Date: Tue, 1 Jul 2025 20:43:30 +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,
	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 Tue, Jul 01, 2025 at 01:23:17PM -0700, Nicolin Chen wrote:
> On Tue, Jul 01, 2025 at 08:03:35PM +0000, Pranjal Shrivastava wrote:
> > On Tue, Jul 01, 2025 at 12:42:32PM -0700, Nicolin Chen wrote:
> > > On Tue, Jul 01, 2025 at 04:02:35PM +0000, Pranjal Shrivastava wrote:
> > > > On Thu, Jun 26, 2025 at 12:34:58PM -0700, Nicolin Chen wrote:
> > > > > +/**
> > > > > + * struct tegra241_vintf_sid - Virtual Interface Stream ID Replacement
> > > > > + * @core: Embedded iommufd_vdevice structure, holding virtual Stream ID
> > > > > + * @vintf: Parent VINTF pointer
> > > > > + * @sid: Physical Stream ID
> > > > > + * @idx: Replacement index in the VINTF
> > > > > + */
> > > > > +struct tegra241_vintf_sid {
> > > > > +	struct iommufd_vdevice core;
> > > > > +	struct tegra241_vintf *vintf;
> > > > > +	u32 sid;
> > > > > +	u8 idx;
> > > > >  };
> > > > 
> > > > AFAIU, This seems to be a handle for sid -> vintf mapping.. it yes, then
> > > > I'm not sure if "Virtual Interface Stream ID Replacement" clarifies that?
> > > 
> > > No. It's for vSID to pSID mappings. I had it explained in commit log:
> > > 
> > 
> > I get that, it's for vSID -> pSID mapping which also "happens to" point
> > to the vintf.. all I wanted to say was that the description is unclear..
> > We could've described it as "Vintf SID map" or something, but I guess
> > it's fine the way it is too.. your call.
> 
> The "replace" word is borrowed from the "SID_REPLACE" HW register.
> 
> But I think it's okay to call it just "mapping", if that makes it
> clearer.
> 

Anything works. Maybe let it be as is.

> > > > > +static struct iommufd_viommu_ops tegra241_cmdqv_viommu_ops = {
> > > > > +	.destroy = tegra241_cmdqv_destroy_vintf_user,
> > > > > +	.alloc_domain_nested = arm_vsmmu_alloc_domain_nested,
> > > > > +	.cache_invalidate = arm_vsmmu_cache_invalidate,
> > > > 
> > > > I see that we currently use the main cmdq to issue these cache
> > > > invalidations (there's a FIXME in arm_vsmmu_cache_invalidate). I was
> > > > hoping for this series to change that but I'm assuming there's another
> > > > series coming for that?
> > > > 
> > > > Meanwhile, I guess it'd be good to call that out for folks who have
> > > > Grace and start trying out this feature.. I'm assuming they won't see
> > > > as much perf improvement with this series alone since we're still using
> > > > the main CMDQ in the upstream code?
> > > 
> > > VCMDQ only accelerates invalidation commands.
> > > 
> > 
> > I get that.. but I see we're using `arm_vsmmu_cache_invalidate` here
> > from arm-smmu-v3-iommufd.c which seems to issue all commands to
> > smmu->cmdq as of now (the code has a FIXME as well), per the code:
> > 
> > 	/* FIXME always uses the main cmdq rather than trying to group by type */
> >         ret = arm_smmu_cmdq_issue_cmdlist(smmu, &smmu->cmdq, last->cmd,
> > 					  cur - last, true);
> > 
> > I was hoping this FIXME to be addressed in this series..
> 
> Oh, that's not related.
> 
> The main goal of this series is to route all invalidation commands
> to the VCMDQ HW. And this is where Grace users can see perf gains
> mentioned in the cover letter or commit log, from eliminating the
> VM Exits at those most frequently used commands.
> 
> Any non-invalidation commands will just reuse what we have with the
> standard SMMU nesting. And even if we did something to that FIXME,
> there is no significant perf gain as it's going down the trapping
> pathway, i.e. the VM Exits are always there.
> 
> > > That is for non-invalidation commands that VCMDQ doesn't support,
> > > so they still have to go in the standard nesting pathway.
> > > 
> > > Let's add a line:
> > > 	/* for non-invalidation commands use */
> > 
> > Umm.. I was talking about the cache_invalidate op? I think there's some
> > misunderstanding here? What am I missing?
> 
> That line is exactly for cache_invalidate. All the non-invalidation
> commands will be sent to he arm_vsmmu_cache_invalidate() by VMM, as
> it means.
> 
> Or perhaps calling them "non-accelerated commands" would be nicer.

Uhh okay, so there'll be a separate driver in the VM issuing invalidation
commands directly to the CMDQV thus we don't see any of it's part here?

AND for non-invalidation commands, we trap out and the VMM ends up
calling the `cache_invalidate` op of the viommu?

Is that understanding correct?

> 
> Thanks
> Nicolin

Thanks
Praan