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

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Message-ID: <aIurlx5QzEtjpFLd@google.com>
Date: Thu, 31 Jul 2025 17:44:55 +0000
From: Mostafa Saleh <smostafa@...gle.com>
To: Jason Gunthorpe <jgg@...pe.ca>
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 Thu, Jul 31, 2025 at 01:57:57PM -0300, Jason Gunthorpe wrote:
> On Thu, Jul 31, 2025 at 02:17:17PM +0000, Mostafa Saleh wrote:
> > On Wed, Jul 30, 2025 at 01:47:52PM -0300, Jason Gunthorpe wrote:
> > > 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
> > 
> > I thought about that but I was worried the maintainers wouldn't like
> > introducing the infrastructure first in the hypervisor without a user.
> > I am open to split this, but let’s see what they think.
> 
> You can merge both series at the same time

Ok, I can split the last 12 patches which are the SMMUv3 driver, if the
maintainers are ok with that.

> 
> > Makes sense, from the kernel point of view it will be attached to
> > identity/blocking domains, but the hypervisor api is just enable/disable HVC
> > as it doesn’t know what is a domain. If terminology is really a problem,
> > I can make it one hypercall as “set_state” with on/off or identity/blocking
> 
> I would call it set_state with states IDENTITY/BLOCKING. That is
> clear. enable/disable is ambiguous.

Ok, will do that.

> 
> > TBH, I am not sure what hardware does that. So, another option is to fail
> > gracefully if RMR exists (which falls back to the current driver) and then
> > pKVM would run with DMA isolation, which is the status quo.
> 
> iGPUs either access the DRAM through the iommu or they use some OS
> invisible side band channel.
> 
> The ones that use the iommu have this quirk.

I see, I think that can be added later, and these devices can keep using the
current SMMU_V3 driver as it, I can add a check to elide this
registeration this driver if the platform have this quirk so the other
driver can probe the SMMUs after.

> 
> > They are not random, as part of this series the SMMUv3 driver is split
> > where some of the code goes to “arm-smmu-v3-common.c” which is used by
> > both drivers, this reduces a lot of duplication.
> 
> I find it very confusing.
> 
> It made sense to factor some of the code out so that pKVM can have
> it's own smmv3 HW driver, sure.
> 
> But I don't understand why a paravirtualized iommu driver for pKVM has
> any relation to smmuv3. Shouldn't it just be calling some hypercalls
> to set IDENTITY/BLOCKING?

Well it’s not really “paravirtualized” as virtio-iommu, this is an SMMUv3
driver (it uses the same binding a the smmu-v3)
It re-use the same probe code, fw/hw parsing and so on (inside the kernel),
also re-use the same structs to make that possible. The only difference is
that the page tables and STEs are managed by the hypervisor.
In part-2[1] I add event q parsing, which reuses 90% of the irq/evtq,
insert/remove_master logic, otherwise we have to duplicate all of that logic.

So, I think it makes sense to re-use as much logic as possible, as both drivers
are smmu-v3 drivers with one caveat about HW table management

As mentioned in the cover letter, we can also still build nesting on top of
this driver, and I plan to post an RFC for that, once this one is sorted.

[1] https://android-kvm.googlesource.com/linux/+log/refs/heads/for-upstream/pkvm-smmu-v3-part-2


> 
> > I am not sure if we need get_resv_regions, maybe it's useful for sysfs
> > "/sys/kernel/iommu_groups/reserved_regions"? I will double check.
> 
> It is important to get this info from the FW..
>
Yes, I think that should remain.

Thanks,
Mostafa
> Jason