Re: [PATCH v13 16/20] KVM: arm64: Handle guest_memfd-backed guest page faults

[Marc Zyngier <maz@kernel.org>]

On Fri, 11 Jul 2025 10:59:39 +0100,
"Roy, Patrick" <roypat@amazon.co.uk> wrote:
> 
> 
> Hi Fuad,
> 
> On Wed, 2025-07-09 at 11:59 +0100, Fuad Tabba wrote:> -snip-
> > +#define KVM_PGTABLE_WALK_MEMABORT_FLAGS (KVM_PGTABLE_WALK_HANDLE_FAULT | KVM_PGTABLE_WALK_SHARED)
> > +
> > +static int gmem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> > +                     struct kvm_s2_trans *nested,
> > +                     struct kvm_memory_slot *memslot, bool is_perm)
> > +{
> > +       bool write_fault, exec_fault, writable;
> > +       enum kvm_pgtable_walk_flags flags = KVM_PGTABLE_WALK_MEMABORT_FLAGS;
> > +       enum kvm_pgtable_prot prot = KVM_PGTABLE_PROT_R;
> > +       struct kvm_pgtable *pgt = vcpu->arch.hw_mmu->pgt;
> > +       struct page *page;
> > +       struct kvm *kvm = vcpu->kvm;
> > +       void *memcache;
> > +       kvm_pfn_t pfn;
> > +       gfn_t gfn;
> > +       int ret;
> > +
> > +       ret = prepare_mmu_memcache(vcpu, true, &memcache);
> > +       if (ret)
> > +               return ret;
> > +
> > +       if (nested)
> > +               gfn = kvm_s2_trans_output(nested) >> PAGE_SHIFT;
> > +       else
> > +               gfn = fault_ipa >> PAGE_SHIFT;
> > +
> > +       write_fault = kvm_is_write_fault(vcpu);
> > +       exec_fault = kvm_vcpu_trap_is_exec_fault(vcpu);
> > +
> > +       if (write_fault && exec_fault) {
> > +               kvm_err("Simultaneous write and execution fault\n");
> > +               return -EFAULT;
> > +       }
> > +
> > +       if (is_perm && !write_fault && !exec_fault) {
> > +               kvm_err("Unexpected L2 read permission error\n");
> > +               return -EFAULT;
> > +       }
> > +
> > +       ret = kvm_gmem_get_pfn(kvm, memslot, gfn, &pfn, &page, NULL);
> > +       if (ret) {
> > +               kvm_prepare_memory_fault_exit(vcpu, fault_ipa, PAGE_SIZE,
> > +                                             write_fault, exec_fault, false);
> > +               return ret;
> > +       }
> > +
> > +       writable = !(memslot->flags & KVM_MEM_READONLY);
> > +
> > +       if (nested)
> > +               adjust_nested_fault_perms(nested, &prot, &writable);
> > +
> > +       if (writable)
> > +               prot |= KVM_PGTABLE_PROT_W;
> > +
> > +       if (exec_fault ||
> > +           (cpus_have_final_cap(ARM64_HAS_CACHE_DIC) &&
> > +            (!nested || kvm_s2_trans_executable(nested))))
> > +               prot |= KVM_PGTABLE_PROT_X;
> > +
> > +       kvm_fault_lock(kvm);
> 
> Doesn't this race with gmem invalidations (e.g. fallocate(PUNCH_HOLE))?
> E.g. if between kvm_gmem_get_pfn() above and this kvm_fault_lock() a
> gmem invalidation occurs, don't we end up with stage-2 page tables
> refering to a stale host page? In user_mem_abort() there's the "grab
> mmu_invalidate_seq before dropping mmap_lock and check it hasnt changed
> after grabbing mmu_lock" which prevents this, but I don't really see an
> equivalent here.

Indeed. We have a similar construct in kvm_translate_vncr() as well,
and I'd definitely expect something of the sort 'round here. If for
some reason this is not needed, then a comment explaining why would be
welcome.

But this brings me to another interesting bit: kvm_translate_vncr() is
another path that deals with a guest translation fault (despite being
caught as an EL2 S1 fault), and calls kvm_faultin_pfn(). What happens
when the backing store is gmem? Probably nothin

I don't immediately see why NV and gmem should be incompatible, so
something must be done on that front too (including the return to
userspace if the page is gone).

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.