Re: [PATCH v6 09/25] KVM: arm64: iommu: Add memory pool

[Mostafa Saleh <smostafa@google.com>]

On Tue, Jul 14, 2026 at 09:43:44AM +0100, Vincent Donnefort wrote:
> [...]
> 
> > > nit: kvm_iommu_init uses nr_pages. 
> > > 
> > > >  {
> > > >  	guard(mutex)(&kvm_iommu_reg_lock);
> > > >  
> > > > @@ -21,6 +22,36 @@ int kvm_iommu_register_driver(struct kvm_iommu_ops *hyp_ops)
> > > >  	if (kvm_nvhe_sym(kvm_iommu_ops))
> > > >  		return -EBUSY;
> > > >  
> > > > +	/* See kvm_iommu_pages() */
> > > > +	if (pool_pages > kvm_nvhe_sym(hyp_kvm_iommu_pages)) {
> > > > +		kvm_err("Not enough memory for the IOMMU pool, need 0x%x pages, check kvm-arm.hyp_iommu_pages",
> > > > +			pool_pages);
> > > > +		return -ENOMEM;
> > > > +	}
> > > > +
> > > >  	kvm_nvhe_sym(kvm_iommu_ops) = hyp_ops;
> > > >  	return 0;
> > > >  }
> > > > +
> > > > +unsigned int kvm_iommu_pages(void)
> > > > +{
> > > > +	/*
> > > > +	 * This is used very early during setup_arch() before any initcalls
> > > > +	 * or any drivers are registered.
> > > > +	 * This value is set by a command line option.
> > > > +	 * Later, when the driver is registered, it will pass the number
> > > > +	 * pages needed for it's page tables, if it was less that what
> > > > +	 * the system has already allocated, the registration will fail.
> > > > +	 */
> > > > +	return kvm_nvhe_sym(hyp_kvm_iommu_pages);
> > > > +}
> > > 
> > > Could we actually estimate the memory needed, based on the allocation for the
> > > host stage-2 and an estimation of MMIO coverage? This would allow the whole
> > > thing to run without setting this value as a best effort and to make it
> > > optional, which is surely more user-friendly?
> > 
> > Not really, this abstracts the IOMMU, so it does not have to match the
> > host stage-2, in addition that the SMMUv3 driver requires extra pages
> > for other in-memory data-structure.
> 
> What I thought would be to take __hyp_pgtable_total_pages() and then add on top
> of that memory to cover the MMIO region. As of "here's memory we think your
> driver will need" and of course that can't take into account any
> "driver"-specific struct.

I wanted to isolate the concept of IOMMU and SMMUv3 in the core code,
so we should not assume how large the page table would be (it does
not have to match the CPU topology at all)

> 
> To be fair, that is a bit weird to have memory reservation so early for a
> "driver" but perhaps that is okay because the pKVM protection is only complete
> if we have that "driver"?

Yes, there has been multiple attempts in this series, we used to
allocate this pool later which was worse in my opinion as it was
not part of the pKVM carveout and still needed some hacks to make
it work.

Thanks,
Mostafa