linux-kernel - Re: [PATCH v3 29/29] iommu/arm-smmu-v3-kvm: Add IOMMU ops

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Message-ID: <20250730164752.GO26511@ziepe.ca>
Date: Wed, 30 Jul 2025 13:47:52 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Mostafa Saleh <smostafa@...gle.com>
Cc: linux-kernel@...r.kernel.org, kvmarm@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org, iommu@...ts.linux.dev,
	maz@...nel.org, oliver.upton@...ux.dev, joey.gouly@....com,
	suzuki.poulose@....com, yuzenghui@...wei.com,
	catalin.marinas@....com, will@...nel.org, robin.murphy@....com,
	jean-philippe@...aro.org, qperret@...gle.com, tabba@...gle.com,
	mark.rutland@....com, praan@...gle.com
Subject: Re: [PATCH v3 29/29] iommu/arm-smmu-v3-kvm: Add IOMMU ops

On Wed, Jul 30, 2025 at 03:07:14PM +0000, Mostafa Saleh wrote:
> On Wed, Jul 30, 2025 at 11:42:53AM -0300, Jason Gunthorpe wrote:
> > On Mon, Jul 28, 2025 at 05:53:16PM +0000, Mostafa Saleh wrote:
> > > Register the SMMUv3 through IOMMU ops, that only support identity
> > > domains. This allows the driver to know which device are currently used
> > > to properly enable/disable then.
> > > 
> > > Signed-off-by: Mostafa Saleh <smostafa@...gle.com>
> > > ---
> > >  .../iommu/arm/arm-smmu-v3/arm-smmu-v3-kvm.c   | 92 ++++++++++++++++++-
> > >  1 file changed, 91 insertions(+), 1 deletion(-)
> > 
> > Can you split the new iommu subysstem driver out please? I think I
> > asked this before.
> 
> Sorry, maybe I misunderstood, do you mean split this patch into multiple
> patches or split all KVM SMMUv3 driver out of this series?

Yes the latter, the iommu driver introduction is best as its own
series

> >  - Domain attachment looks questionable. Please do not have
> >    attach/detach language at all in the hypervisor facing API.
> 
> I am not sure I understand this one, the hypervisor API has no
> attach/detach APIs?
> We only notify the hypervisor via “enable/disable” hypercalls when
> devices are attached or released (as only IDENTITY DOMAIN is supported)
> so it can enable or disable translation.

Same difference, different words.. We've had trouble with this kind of
ambiguous language.

attach identity
attach blocking

Keep it clean and simple

> >  - Get the ordering and APIs right so replace works. You need this to support RMRs
> 
> I see, we can’t support bypass for security reasons, but we can enable
> the identity map for such devices.

You will have a small issue if the bootup has the pkvm side put the
device into a blocking translation then later switches to an
"identity".

As far as I understand it display scan out buffers have to be
continuously hitlessly mapped. So you want all the transitions from
bootup bypass, pkvm isolation, "identity", etc to continuously
hitlessly map the RMRs containing the buffers.

I think :)

> >  - Use a blocking domain not some unclear detatch idea
> 
> There is not a single detach in this driver, we only support attach and release
> operation, which just goes to the hypervisor as enable/disable
> identity.

'disable' then.

> >  - Use a blocking domain for release
> 
> I see, I can add “blocked_domain” similar to “arm_smmu_blocked_domain”
> which just disables the device translation, I will look into, I am just
> concerned it's more code as this driver only supports a single domain
> type.

Yes, this is the right thing. There should not be operations in the
driver that change the underyling translation that are anything other
than attaching domains. It becomes too hard to maintain.

 
> >  - Use the smmu-v3 approach for the fwspec, don't store things in the drvdata.
> 
> This is using “dev_iommu_fwspec_get” similar to the SMMUv3 driver,
> am I missing something?

Oh I got it confused, you are just calling 

+       .device_group           = arm_smmu_device_group,
+       .of_xlate               = arm_smmu_of_xlate,
+       .get_resv_regions       = arm_smmu_get_resv_regions,

Which is also weird, why does an entirely new driver call 4 random functions
in arm smmu?

Maybe don't, they are all trivial functions, or you don't need
them. For example you don't need MSI_IOVA_BASE if the driver doesn't
support paging.

Jason