Re: [PATCH v4 15/18] KVM: guest_memfd: Fold __kvm_gmem_prepare_folio() into its sole caller

[Sean Christopherson <seanjc@google.com>]

On Thu, Jul 09, 2026, sashiko-bot@kernel.org wrote:
> > diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> > --- a/virt/kvm/guest_memfd.c
> > +++ b/virt/kvm/guest_memfd.c
> 
> [ ... ]
> 
> > @@ -82,6 +69,7 @@ static int __kvm_gmem_prepare_folio(struct kvm *kvm, struct kvm_memory_slot *slo
> >  static int kvm_gmem_prepare_folio(struct kvm *kvm, struct kvm_memory_slot *slot,
> >  				  gfn_t gfn, struct folio *folio)
> >  {
> > +#ifdef CONFIG_HAVE_KVM_ARCH_GMEM_PREPARE
> >  	pgoff_t index;
> >  
> >  	/*
> > @@ -101,7 +89,12 @@ static int kvm_gmem_prepare_folio(struct kvm *kvm, struct kvm_memory_slot *slot,
> >  	index = kvm_gmem_get_index(slot, gfn);
> >  	index = ALIGN_DOWN(index, folio_nr_pages(folio));
> >  
> > -	return __kvm_gmem_prepare_folio(kvm, slot, index, folio);
> > +	return kvm_arch_gmem_prepare(kvm, gfn_to_gpa(gfn),
> > +				     folio_file_pfn(folio, index),
> > +				     folio_order(folio));
> 
> [Severity: Medium]
> When folding this helper, did we lose the GFN alignment for huge folios?

Ugh, I totally missed that, obviously.  Aligning the index in the caller using
a gfn, and then subtly computing a new gfn is just mean.  I'll fix this by
squashing in "Align the gfn as well as the pfn when "preparing" a folio"

> In the old __kvm_gmem_prepare_folio() helper, the gfn was recalculated from
> the aligned index:
> 
> 	gfn_t gfn = slot->base_gfn + index - slot->gmem.pgoff;
> 
> By passing the original, unaligned faulting gfn to kvm_arch_gmem_prepare()
> alongside the aligned base PFN from folio_file_pfn(), does this create a
> mismatch between the unaligned GPA and the aligned base PFN for huge folios?
> 
> While guest_memfd currently restricts allocations to order-0 folios, this would
> pass an offset GPA with a base PFN to the architecture hooks once huge folios
> are supported, violating the API contract and potentially corrupting the guest
> mapping (e.g., if a backend like SEV-SNP falls back to 4K mappings for a huge
> folio).
> 
> > +#else
> > +	return 0;
> > +#endif
> >  }
> 
> -- 
> Sashiko AI review · https://sashiko.dev/#/patchset/20260709204948.1988414-1-seanjc@google.com?part=15