RE: [PATCH v5 0/6] Add Tegra241 (Grace) CMDQV Support (part 1/2)

[Shameerali Kolothum Thodi]

> -----Original Message-----
> From: Shameerali Kolothum Thodi
> Sent: Wednesday, April 17, 2024 9:01 AM
> To: 'Jason Gunthorpe' <jgg@xxxxxxxxxx>; Nicolin Chen <nicolinc@xxxxxxxxxx>
> Cc: will@xxxxxxxxxx; robin.murphy@xxxxxxx; joro@xxxxxxxxxx;
> thierry.reding@xxxxxxxxx; vdumpa@xxxxxxxxxx; jonathanh@xxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; iommu@xxxxxxxxxxxxxxx; linux-arm-
> kernel@xxxxxxxxxxxxxxxxxxx; linux-tegra@xxxxxxxxxxxxxxx; Jerry Snitselaar
> <jsnitsel@xxxxxxxxxx>
> Subject: RE: [PATCH v5 0/6] Add Tegra241 (Grace) CMDQV Support (part 1/2)
> 
> 
> 
> > -----Original Message-----
> > From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> > Sent: Monday, April 15, 2024 6:14 PM
> > To: Nicolin Chen <nicolinc@xxxxxxxxxx>
> > Cc: will@xxxxxxxxxx; robin.murphy@xxxxxxx; joro@xxxxxxxxxx;
> > thierry.reding@xxxxxxxxx; vdumpa@xxxxxxxxxx; jonathanh@xxxxxxxxxx;
> > linux-kernel@xxxxxxxxxxxxxxx; iommu@xxxxxxxxxxxxxxx; linux-arm-
> > kernel@xxxxxxxxxxxxxxxxxxx; linux-tegra@xxxxxxxxxxxxxxx; Jerry Snitselaar
> > <jsnitsel@xxxxxxxxxx>
> > Subject: Re: [PATCH v5 0/6] Add Tegra241 (Grace) CMDQV Support (part 1/2)
> >
> > On Fri, Apr 12, 2024 at 08:43:48PM -0700, Nicolin Chen wrote:
> >
> > > The user-space support is to provide uAPIs (via IOMMUFD) for hypervisors
> > > in user space to passthrough VCMDQs to VMs, allowing these VMs to
> > access
> > > the VCMDQs directly without trappings, i.e. no VM Exits. This gives huge
> > > performance improvements: 70% to 90% reductions of TLB invalidation
> > time
> > > were measured by various DMA unmap tests running in a guest OS,
> > compared
> > > to a nested SMMU CMDQ (with trappings).
> >
> > So everyone is on the same page, this is the primary point of this
> > series. The huge speed up of in-VM performance is necessary for the
> > workloads this chip is expected to be running. This series is unique
> > from all the rest because it runs inside a VM, often in the from of a
> > distro release.
> >
> > It doesn't need the other series or it's own part 2 as it entirely
> > stands alone on bare metal hardware or on top of commercial VM cloud
> > instances runing who-knows-what in their hypervisors.
> >
> > The other parts are substantially about enabling qemu and the open
> > ecosystem to have fully functional vSMMU3 virtualization.
> 
> Hi,
> 
> 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?
> 
> SMMUV3 spec does say,
> "Arm expects that the Non-secure Stream table, Command queue, Event queue
> and
> PRI queue are controlled by the most privileged Non-secure system software. "
> 
> Not clear to me what are the major concerns here and maybe we can come up
> with
> something to address that in kernel.

Just to add to that. One idea could be like to have a case where when ECMDQs are 
detected, use that for issuing limited set of cmds(like stage 1 TLBIs) and use the
normal cmdq for rest. Since we use stage 1 for both host and for Guest nested cases
and TLBIs are the bottlenecks in most cases I think this should give performance
benefits.

Thanks,
Shameer