linux-kernel - Re: [PATCH v8 12/14] iommu/arm-smmu-v3: Introduce struct arm_smmu

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Message-ID: <20250317154423.GI9311@nvidia.com>
Date: Mon, 17 Mar 2025 12:44:23 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: Will Deacon <will@...nel.org>, kevin.tian@...el.com, corbet@....net,
	joro@...tes.org, suravee.suthikulpanit@....com,
	robin.murphy@....com, dwmw2@...radead.org, baolu.lu@...ux.intel.com,
	linux-kernel@...r.kernel.org, iommu@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org,
	linux-kselftest@...r.kernel.org, linux-doc@...r.kernel.org,
	eric.auger@...hat.com, jean-philippe@...aro.org, mdf@...nel.org,
	mshavit@...gle.com, shameerali.kolothum.thodi@...wei.com,
	smostafa@...gle.com, ddutile@...hat.com, yi.l.liu@...el.com,
	praan@...gle.com, patches@...ts.linux.dev
Subject: Re: [PATCH v8 12/14] iommu/arm-smmu-v3: Introduce struct
 arm_smmu_vmaster

On Tue, Mar 11, 2025 at 10:43:08AM -0700, Nicolin Chen wrote:
> > > +int arm_smmu_attach_prepare_vmaster(struct arm_smmu_attach_state *state,
> > > +				    struct arm_smmu_nested_domain *nested_domain)
> > > +{
> > > +	struct arm_smmu_vmaster *vmaster;
> > > +	unsigned long vsid;
> > > +	int ret;
> > > +
> > > +	iommu_group_mutex_assert(state->master->dev);
> > > +
> > > +	/* Skip invalid vSTE */
> > > +	if (!(nested_domain->ste[0] & cpu_to_le64(STRTAB_STE_0_V)))
> > > +		return 0;
> > 
> > Ok, and we don't need to set 'state->vmaster' in this case because we
> > only report stage-1 faults back to the vSMMU?
> 
> This is a good question that I didn't ask myself hard enough..
> 
> I think we should probably drop it. An invalid STE should trigger
> a C_BAD_STE event that is in the supported vEVENT list. I'll run
> some test before removing this line from v9.

It won't trigger C_BAD_STE, recall Robin was opposed to thatm so we have this:

static void arm_smmu_make_nested_domain_ste(
	struct arm_smmu_ste *target, struct arm_smmu_master *master,
	struct arm_smmu_nested_domain *nested_domain, bool ats_enabled)
{
	unsigned int cfg =
		FIELD_GET(STRTAB_STE_0_CFG, le64_to_cpu(nested_domain->ste[0]));

	/*
	 * Userspace can request a non-valid STE through the nesting interface.
	 * We relay that into an abort physical STE with the intention that
	 * C_BAD_STE for this SID can be generated to userspace.
	 */
	if (!(nested_domain->ste[0] & cpu_to_le64(STRTAB_STE_0_V)))
		cfg = STRTAB_STE_0_CFG_ABORT;

So, in the case of a non-valid STE, and a device access, the HW will
generate one of the translation faults and that will be forwarded.

Some software component will have to transform those fault events into
C_BAD_STE for the VM.

I imagined userspace would do this, but it could be done in the kernel
too. Regardless, I think Will is right and the the viommu should be
set even in this case to capture the events.

Jason