Message-ID: <aBPrYXixWhFRkM7q@google.com>
Date: Thu, 1 May 2025 21:45:05 +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,
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	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 21/22] iommu/tegra241-cmdqv: Add user-space use support

On Wed, Apr 30, 2025 at 03:39:15PM -0700, Nicolin Chen wrote:
> On Wed, Apr 30, 2025 at 09:59:13PM +0000, Pranjal Shrivastava wrote:
> > On Fri, Apr 25, 2025 at 10:58:16PM -0700, Nicolin Chen wrote:
> > > The CMDQV HW supports a user-space use for virtualization cases. It allows
> > > the VM to issue guest-level TLBI or ATC_INV commands directly to the queue
> > > and executes them without a VMEXIT, as HW will replace the VMID field in a
> > > TLBI command and the SID field in an ATC_INV command with the preset VMID
> > > and SID.
> > > 
> > > This is built upon the vIOMMU infrastructure by allowing VMM to allocate a
> > > VINTF (as a vIOMMU object) and assign VCMDQs (vCMDQ objects) to the VINTF.
> > > 
> > > So firstly, replace the standard vSMMU model with the VINTF implementation
> > > but reuse the standard cache_invalidate op (for unsupported commands) and
> > > the standard alloc_domain_nested op (for standard nested STE).
> > > 
> > > Each VINTF has two 64KB MMIO pages (128B per logical vCMDQ):
> > >  - Page0 (directly accessed by guest) has all the control and status bits.
> > >  - Page1 (trapped by VMM) has guest-owned queue memory location/size info.
> > > 
> > > VMM should trap the emulated VINTF0's page1 of the guest VM for the guest-
> > > level VCMDQ location/size info and forward that to the kernel to translate
> > > to a physical memory location to program the VCMDQ HW during an allocation
> > > call. Then, it should mmap the assigned VINTF's page0 to the VINTF0 page0
> > > of the guest VM. This allows the guest OS to read and write the guest-own
> > > VINTF's page0 for direct control of the VCMDQ HW.
> > > 
> > > For ATC invalidation commands that hold an SID, it requires all devices to
> > > register their virtual SIDs to the SID_MATCH registers and their physical
> > > SIDs to the pairing SID_REPLACE registers, so that HW can use those as a
> > > lookup table to replace those virtual SIDs with the correct physical SIDs.
> > > Thus, implement the driver-allocated vDEVICE op with a tegra241_vintf_sid
> > > structure to allocate SID_REPLACE and to program the SIDs accordingly.
> > > 
> > > This enables the HW accelerated feature for NVIDIA Grace CPU. Compared to
> > > the standard SMMUv3 operating in the nested translation mode trapping CMDQ
> > > for TLBI and ATC_INV commands, this gives a huge performance improvement:
> > > 70% to 90% reductions of invalidation time were measured by various DMA
> > > unmap tests running in a guest OS.
> > > 
> > 
> > The write-up is super helpful to understand how the HW works from a high
> > level. Thanks for explaining this well! :) 
> > 
> > I'm curious to know the DMA unmap tests that were run for perf?
> 
> tools/testing/selftests/dma/dma_map_benchmark.c
> 

Ahh okay.. I thought it was something else. I guess it's worth posting
some comparitive results in the cover letter if you prefer.

> > >  /**
> > >   * struct iommu_hw_info_arm_smmuv3 - ARM SMMUv3 hardware information
> > >   *                                   (IOMMU_HW_INFO_TYPE_ARM_SMMUV3)
> > >   *
> > > - * @flags: Must be set to 0
> > > - * @impl: Must be 0
> > > + * @flags: Combination of enum iommu_hw_info_arm_smmuv3_flags
> > > + * @impl: Implementation-defined bits when the following flags are set:
> > > + *        - IOMMU_HW_INFO_ARM_SMMUV3_HAS_TEGRA241_CMDQV
> > > + *          Bits[15:12] - Log2 of the total number of SID replacements
> > > + *          Bits[07:04] - Log2 of the total number of vCMDQs per vIOMMU
> > > + *          Bits[03:00] - Version number for the CMDQ-V HW
> > 
> > Nit: It seems that we deliberately chose not to reveal `NUM_VINTF_LOG2`
> > to the user-space. If so, maybe we shall mark those bitfields as unused
> > or reserved for clarity? Bits[11:08] - Reserved / Unused (even 31:16).
> 
> I think it should have been there, but kernel should just report 0.
>                  Bits[11:08] - Log2 of the total number of virtual interface
> 

Ack.

> > >   * @idr: Implemented features for ARM SMMU Non-secure programming interface
> > >   * @iidr: Information about the implementation and implementer of ARM SMMU,
> > >   *        and architecture version supported
> > > @@ -952,10 +965,28 @@ struct iommu_fault_alloc {
> > >   * enum iommu_viommu_type - Virtual IOMMU Type
> > >   * @IOMMU_VIOMMU_TYPE_DEFAULT: Reserved for future use
> > >   * @IOMMU_VIOMMU_TYPE_ARM_SMMUV3: ARM SMMUv3 driver specific type
> > > + * @IOMMU_VIOMMU_TYPE_TEGRA241_CMDQV: NVIDIA Tegra241 CMDQV Extension for SMMUv3
> > >   */
> > >  enum iommu_viommu_type {
> > >  	IOMMU_VIOMMU_TYPE_DEFAULT = 0,
> > >  	IOMMU_VIOMMU_TYPE_ARM_SMMUV3 = 1,
> > > +	IOMMU_VIOMMU_TYPE_TEGRA241_CMDQV = 2,
> > > +};
> > 
> > This is a little confusing.. I understand that we need a new viommu type
> > to copy the new struct iommu_viommu_tegra241_cmdqv b/w the user & kernel
> > 
> > But, in a previous patch (Add vsmmu_alloc impl op), we add a check to
> > fallback to the standard type SMMUv3, if the impl_ops->vsmmu_alloc 
> > returns -EOPNOTSUPP:
> > 
> > 	if (master->smmu->impl_ops && master->smmu->impl_ops->vsmmu_alloc)
> > 		vsmmu = master->smmu->impl_ops->vsmmu_alloc(
> > 			master->smmu, s2_parent, ictx, viommu_type, user_data);
> > 	if (PTR_ERR(vsmmu) == -EOPNOTSUPP) {
> > 		if (viommu_type != IOMMU_VIOMMU_TYPE_ARM_SMMUV3)
> > 			return ERR_PTR(-EOPNOTSUPP);
> > 		/* Fallback to standard SMMUv3 type if viommu_type matches */
> > 		vsmmu = iommufd_viommu_alloc(ictx, struct arm_vsmmu, core,
> > 					     &arm_vsmmu_ops);
> > 
> > Now, if we'll ALWAYS try to allocate an impl-specified vsmmu first, even
> > when the viommu_type == IOMMU_VIOMMU_TYPE_ARM_SMMUV3, we are anyways
> > going to return back from the impl_ops->vsmmu_alloc with -EOPNOTSUPP.
> 
> That's not necessarily true. An impl_ops->vsmmu_alloc can support
> IOMMU_VIOMMU_TYPE_ARM_SMMUV3 potentially, e.g. an impl could just
> toggle a few special bits in a register and return a valid vsmmu
> pointer.
> 
> It doesn't work like this with VCMDQ as it supports its own type,
> but for the long run I think we should pass in the standard type
> to impl_ops->vsmmu_alloc too.
> 

That makes sense. I only considered CMDQV. Thanks for the clarification!

> > Then we'll again check if the retval was -EOPNOTSUPP and re-check the
> > viommu_type requested.. which seems a little counter intuitive.
> 
> It's just prioritizing the impl_ops->vsmmu_alloc. Similar to the
> probe, if VCMDQ is missing or encountering some initialization
> problem, give it a chance to fallback to the standard SMMU.
> 

Ack.

> > > +	/*
> > > +	 * @length must be a power of 2, in range of
> > > +	 *   [ 32, 1 ^ (idr[1].CMDQS + CMDQ_ENT_SZ_SHIFT) ]
> > > +	 */
> > 
> > Nit: 2 ^ (idr[1].CMDQS + CMDQ_ENT_SZ_SHIFT) to match the comment in uapi
> 
> Alok pointed it out too. Fixed.
> 

Thanks! I had only skimmed through Alok's comments and felt he only
pointed it out in the uapi and not here. Sorry for overlooking that :)

> > > +	vcmdq = iommufd_vcmdq_alloc(viommu, struct tegra241_vcmdq, core);
> > > +	if (!vcmdq)
> > > +		return ERR_PTR(-ENOMEM);
> > > +
> > > +	/*
> > > +	 * HW requires to unmap LVCMDQs in descending order, so destroy() must
> > > +	 * follow this rule. Set a dependency on its previous LVCMDQ so iommufd
> > > +	 * core will help enforce it.
> > > +	 */
> > > +	if (prev) {
> > > +		ret = iommufd_vcmdq_depend(vcmdq, prev, core);
> > > +		if (ret)
> > > +			goto free_vcmdq;
> > > +	}
> > > +	vcmdq->prev = prev;
> > > +
> > > +	ret = tegra241_vintf_init_lvcmdq(vintf, index, vcmdq);
> > > +	if (ret)
> > > +		goto free_vcmdq;
> > > +
> > > +	dev_dbg(cmdqv->dev, "%sallocated\n",
> > > +		lvcmdq_error_header(vcmdq, header, 64));
> > > +
> > > +	tegra241_vcmdq_map_lvcmdq(vcmdq);
> > > +
> > > +	vcmdq->cmdq.q.q_base = q_base & VCMDQ_ADDR;
> > > +	vcmdq->cmdq.q.q_base |= log2size;
> > > +
> > > +	ret = tegra241_vcmdq_hw_init_user(vcmdq);
> > > +	if (ret)
> > > +		goto free_vcmdq;
> > > +	vintf->lvcmdqs[index] = vcmdq;
> > > +
> > > +	return &vcmdq->core;
> > > +free_vcmdq:
> > > +	iommufd_struct_destroy(viommu->ictx, vcmdq, core);
> > > +	return ERR_PTR(ret);
> > 
> > Are we missing an undepend here?
> 
> Right. The iommufd_struct_destroy doesn't invoke obj->ops.abort().
> 
> The whole revert flow is wonky, missing all the unmap/deinit steps.
> 

Right.

> > > +static void tegra241_vintf_destroy_vdevice(struct iommufd_vdevice *vdev)
> > > +{
> > > +	struct tegra241_vintf_sid *vsid =
> > > +		container_of(vdev, struct tegra241_vintf_sid, core);
> > > +	struct tegra241_vintf *vintf = vsid->vintf;
> > > +
> > > +	writel_relaxed(0, REG_VINTF(vintf, SID_REPLACE(vsid->idx)));
> > > +	writel_relaxed(0, REG_VINTF(vintf, SID_MATCH(vsid->idx)));
> > 
> > Just a thought: Should these be writel to avoid races?
> > Although I believe all user-queues would be free-d by this point?
> 
> Yea. They should be. I will change them.
> 

Ack.

> Thanks
> Nicolin

Thanks
Praan

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