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

[Mostafa Saleh <smostafa@google.com>]

On Sun, Sep 14, 2025 at 07:23:12PM +0000, Pranjal Shrivastava wrote:
> 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@google.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.

True, this can only be done from the hypervisor code though, I will
make the check stricter as Will suggested, and if needed we can relax
that later.

Thanks,
Mostafa

> 
> > 
> > Will
> 
> Thanks,
> Praan