linux-kernel - Re: [PATCH v4 01/28] KVM: arm64: Add a new function to donate memory with prot

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Message-ID: <aMcWIDQzi5CDNC4C@google.com>
Date: Sun, 14 Sep 2025 19:23:12 +0000
From: Pranjal Shrivastava <praan@...gle.com>
To: Will Deacon <will@...nel.org>
Cc: Mostafa Saleh <smostafa@...gle.com>, 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, robin.murphy@....com,
	jean-philippe@...aro.org, qperret@...gle.com, tabba@...gle.com,
	jgg@...pe.ca, mark.rutland@....com
Subject: Re: [PATCH v4 01/28] KVM: arm64: Add a new function to donate memory
 with prot

On Tue, Sep 09, 2025 at 02:46:42PM +0100, Will Deacon wrote:
> On Tue, Aug 19, 2025 at 09:51:29PM +0000, Mostafa Saleh wrote:
> > Soon, IOMMU drivers running in the hypervisor might interact with
> > non-coherent devices, so it needs a mechanism to map memory as
> > non cacheable.
> > Add ___pkvm_host_donate_hyp() which accepts a new argument for prot,
> > so the driver can add KVM_PGTABLE_PROT_NORMAL_NC.
> > 
> > Signed-off-by: Mostafa Saleh <smostafa@...gle.com>
> > ---
> >  arch/arm64/kvm/hyp/include/nvhe/mem_protect.h |  1 +
> >  arch/arm64/kvm/hyp/nvhe/mem_protect.c         | 11 +++++++++--
> >  2 files changed, 10 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h b/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
> > index 5f9d56754e39..52d7ee91e18c 100644
> > --- a/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
> > +++ b/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
> > @@ -36,6 +36,7 @@ int __pkvm_prot_finalize(void);
> >  int __pkvm_host_share_hyp(u64 pfn);
> >  int __pkvm_host_unshare_hyp(u64 pfn);
> >  int __pkvm_host_donate_hyp(u64 pfn, u64 nr_pages);
> > +int ___pkvm_host_donate_hyp(u64 pfn, u64 nr_pages, enum kvm_pgtable_prot prot);
> >  int __pkvm_hyp_donate_host(u64 pfn, u64 nr_pages);
> >  int __pkvm_host_share_ffa(u64 pfn, u64 nr_pages);
> >  int __pkvm_host_unshare_ffa(u64 pfn, u64 nr_pages);
> > diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> > index 8957734d6183..861e448183fd 100644
> > --- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> > +++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> > @@ -769,13 +769,15 @@ int __pkvm_host_unshare_hyp(u64 pfn)
> >  	return ret;
> >  }
> >  
> > -int __pkvm_host_donate_hyp(u64 pfn, u64 nr_pages)
> > +int ___pkvm_host_donate_hyp(u64 pfn, u64 nr_pages, enum kvm_pgtable_prot prot)
> >  {
> >  	u64 phys = hyp_pfn_to_phys(pfn);
> >  	u64 size = PAGE_SIZE * nr_pages;
> >  	void *virt = __hyp_va(phys);
> >  	int ret;
> >  
> > +	WARN_ON(prot & KVM_PGTABLE_PROT_X);
> 
> Should this actually just enforce that the permissions are
> KVM_PGTABLE_PROT_RW:
> 
> 	WARN_ON((prot & KVM_PGTABLE_PROT_RWX) != KVM_PGTABLE_PROT_RW);
> 
> ?
> 
> Since the motivation is about the memory type rather than the
> permissions, it would be best to preserve the current behaviour.

+1. I believe the current `WARN_ON(prot & KVM_PGTABLE_PROT_X);` check
would potentially allow "Read-only" or "Write-only" donations to slide
through silently.

> 
> Will

Thanks,
Praan