linux-kernel - Re: [PATCH v1 3/4] iommu/arm-smmu-v3: Decouple vmid from S2 nest

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Message-ID: <Z_Uww4PdJesOoA8N@google.com>
Date: Tue, 8 Apr 2025 14:20:51 +0000
From: Pranjal Shrivastava <praan@...gle.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Nicolin Chen <nicolinc@...dia.com>, will@...nel.org,
	robin.murphy@....com, joro@...tes.org,
	linux-arm-kernel@...ts.infradead.org, iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org, shameerali.kolothum.thodi@...wei.com
Subject: Re: [PATCH v1 3/4] iommu/arm-smmu-v3: Decouple vmid from S2
 nest_parent domain

On Mon, Apr 07, 2025 at 01:52:20PM -0300, Jason Gunthorpe wrote:
> On Mon, Apr 07, 2025 at 10:51:24AM +0000, Pranjal Shrivastava wrote:
> > > @@ -381,15 +401,24 @@ struct iommufd_viommu *arm_vsmmu_alloc(struct device *dev,
> > >  	    !(smmu->features & ARM_SMMU_FEAT_S2FWB))
> > >  		return ERR_PTR(-EOPNOTSUPP);
> > >  
> > > +	vmid = ida_alloc_range(&smmu->vmid_map, 1, (1 << smmu->vmid_bits) - 1,
> > > +			       GFP_KERNEL);
> > > +	if (vmid < 0)
> > > +		return ERR_PTR(vmid);
> > > +
> > 
> > Probably a basic question, I hope we'll have one vSMMU per VM? 
> 
> A VIOMMU is tied to the physical SMMU, it cannot be shared across
> physical SMMU, so this is the right sort of way to get the ID
> 
> > Even if that's not the case then the VMM should take care of
> > invalidating contexts of all associated vSMMUs anyway? (Just
> > thinking if we should allocate a VMID per VM or per vSMMU)
> 
> If the VMM wants to present a single vSMMU to the VM then the VMM
> needs to replicate invalidations as required to all the physical
> VIOMMU objects. This will prevent using the HW accelerated
> invalidation paths, so I expect that the VMM will have one vSMM per
> physical.
> 

Makes sense. Thanks!

> > Nit: Does it makes sense to create a helper like `arm_smmu_vmid_alloc`
> > and call it here and finalise_s2?
> 
> Maybe so
> 

I recently saw Shameer's patch [1] using a different vmid allocation
scheme, so I guess it's okay if we don't share this function..


Thanks,
Praan

[1] https://lore.kernel.org/linux-arm-kernel/20250319173202.78988-5-shameerali.kolothum.thodi@huawei.com/