linux-kernel - Re: [RFC PATCH v2 40/58] KVM: arm64: smmu-v3: Add map/unmap pages and iova_to

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Message-ID: <Z1yPpEn_MElCRzLG@google.com>
Date: Fri, 13 Dec 2024 19:48:52 +0000
From: Mostafa Saleh <smostafa@...gle.com>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: iommu@...ts.linux.dev, kvmarm@...ts.linux.dev,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	catalin.marinas@....com, will@...nel.org, maz@...nel.org,
	oliver.upton@...ux.dev, joey.gouly@....com, suzuki.poulose@....com,
	yuzenghui@...wei.com, robdclark@...il.com, joro@...tes.org,
	robin.murphy@....com, jean-philippe@...aro.org, nicolinc@...dia.com,
	vdonnefort@...gle.com, qperret@...gle.com, tabba@...gle.com,
	danielmentz@...gle.com, tzukui@...gle.com
Subject: Re: [RFC PATCH v2 40/58] KVM: arm64: smmu-v3: Add map/unmap pages
 and iova_to_phys

Hi Jason,

On Thu, Dec 12, 2024 at 03:44:35PM -0400, Jason Gunthorpe wrote:
> On Thu, Dec 12, 2024 at 06:04:04PM +0000, Mostafa Saleh wrote:
> > Add map_pages and iova_to_phys HVC code, which
> > mainly calls the io-pgtable.
> > 
> > For unmap_pages, we rely on IO_PGTABLE_QUIRK_UNMAP_INVAL, where the
> > driver first calls unmap_pages which invalidate all the pages as
> > a typical unmap, issuing all the necessary TLB invalidations.
> > Then, we will start a page table with 2 callbacks:
> > - visit_leaf: for each unmapped leaf, it would decrement the refcount
> >   of the page using __pkvm_host_unuse_dma(), reversing the what IOMMU
> >   core does in map.
> > - visit_post_table: this would free any invalidated tables as they
> >   wouldn't be freed because of the quirk.
> 
> I don't know if the timelines will work out, but the pagetable stuff
> I'm working on will let you write a much more appropriate
> implementation for pkvm's usage than trying to hack it into the
> iopgtable code like this.

I didn’t check your new page table patches yet, but I would say it’s
more likely your patches would land first, because as mentioned in the
cover letter, there are still many dependencies for pKVM before IOMMU
support lands, so I don’t mind converging if possible.

Thanks,
Mostafa

> 
> Even the iommu focused routines I have got now would solve this
> problem because they allways spit out a linked list of all the memory
> to free after map/unmap and never internally free it..

> 
> Jason