linux-kernel - RE: [PATCH v2 17/19] iommu/arm-smmu-v3: Add arm_smmu_viommu_cache

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Message-ID: <BN9PR11MB527623483A2C981ABCFEEFB38C9B2@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Wed, 11 Sep 2024 08:17:23 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Baolu Lu <baolu.lu@...ux.intel.com>, Nicolin Chen <nicolinc@...dia.com>
CC: Jason Gunthorpe <jgg@...dia.com>, "will@...nel.org" <will@...nel.org>,
	"joro@...tes.org" <joro@...tes.org>, "suravee.suthikulpanit@....com"
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	"shuah@...nel.org" <shuah@...nel.org>, "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-kselftest@...r.kernel.org"
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	<shameerali.kolothum.thodi@...wei.com>, "smostafa@...gle.com"
	<smostafa@...gle.com>, "Liu, Yi L" <yi.l.liu@...el.com>
Subject: RE: [PATCH v2 17/19] iommu/arm-smmu-v3: Add
 arm_smmu_viommu_cache_invalidate

> From: Baolu Lu <baolu.lu@...ux.intel.com>
> Sent: Wednesday, September 11, 2024 3:51 PM
> 
> On 2024/9/11 15:20, Nicolin Chen wrote:
> > On Wed, Sep 11, 2024 at 06:25:16AM +0000, Tian, Kevin wrote:
> >>> From: Jason Gunthorpe<jgg@...dia.com>
> >>> Sent: Friday, September 6, 2024 2:22 AM
> >>>
> >>> On Thu, Sep 05, 2024 at 11:00:49AM -0700, Nicolin Chen wrote:
> >>>> On Thu, Sep 05, 2024 at 01:20:39PM -0300, Jason Gunthorpe wrote:
> >>>>> On Tue, Aug 27, 2024 at 09:59:54AM -0700, Nicolin Chen wrote:
> >>>>>
> >>>>>> +static int arm_smmu_viommu_cache_invalidate(struct
> >>> iommufd_viommu *viommu,
> >>>>>> +                                           struct iommu_user_data_array
> >>> *array)
> >>>>>> +{
> >>>>>> +       struct iommu_domain *domain =
> >>> iommufd_viommu_to_parent_domain(viommu);
> >>>>>> +
> >>>>>> +       return __arm_smmu_cache_invalidate_user(
> >>>>>> +                       to_smmu_domain(domain), viommu, array);
> >>>>> I'd like to have the viommu struct directly hold the VMID. The nested
> >>>>> parent should be sharable between multiple viommus, it doesn't make
> >>>>> any sense that it would hold the vmid.
> >>>>>
> >>>>> This is struggling because it is trying too hard to not have the
> >>>>> driver allocate the viommu, and I think we should just go ahead and
> do
> >>>>> that. Store the vmid, today copied from the nesting parent in the vmid
> >>>>> private struct. No need for iommufd_viommu_to_parent_domain(),
> just
> >>>>> rework the APIs to pass the vmid down not a domain.
> >>>> OK. When I designed all this stuff, we still haven't made mind
> >>>> about sharing the s2 domain, i.e. moving the VMID, which might
> >>>> need a couple of more patches to achieve.
> >>> Yes, many more patches, and don't try to do it now.. But we can copy
> >>> the vmid from the s2 and place it in the viommu struct during
> >>> allocation time.
> >>>
> >> does it assume that a viommu object cannot span multiple physical
> >> IOMMUs so there is only one vmid per viommu?
> > I think so. One the reasons of introducing vIOMMU is to maintain
> > the shareability across physical IOMMUs at the s2 HWPT_PAGING.
> 
> My understanding of VMID is something like domain id in x86 arch's. Is
> my understanding correct?

yes

> 
> If a VMID for an S2 hwpt is valid on physical IOMMU A but has already
> been allocated for another purpose on physical IOMMU B, how can it be
> shared across both IOMMUs? Or the VMID is allocated globally?
> 

I'm not sure that's a problem. The point is that each vIOMMU object 
will get a VMID from the SMMU which it's associated to (assume
one vIOMMU cannot span multiple SMMU). Whether that VMID
is globally allocated or per-SMMU is the policy in the SMMU driver.

It's the driver's responsibility to ensure not using a conflicting VMID
when creating an vIOMMU instance.