On Tue, 22 Jul 2025 at 06:41, Xiaoyao Li <xiaoyao.li@xxxxxxxxx> wrote:
>
> On 7/22/2025 12:47 AM, Sean Christopherson wrote:
> > On Thu, Jul 17, 2025, Fuad Tabba wrote:
> >> From: Ackerley Tng <ackerleytng@xxxxxxxxxx>
> >>
> >> Update the KVM MMU fault handler to service guest page faults
> >> for memory slots backed by guest_memfd with mmap support. For such
> >> slots, the MMU must always fault in pages directly from guest_memfd,
> >> bypassing the host's userspace_addr.
> >>
> >> This ensures that guest_memfd-backed memory is always handled through
> >> the guest_memfd specific faulting path, regardless of whether it's for
> >> private or non-private (shared) use cases.
> >>
> >> Additionally, rename kvm_mmu_faultin_pfn_private() to
> >> kvm_mmu_faultin_pfn_gmem(), as this function is now used to fault in
> >> pages from guest_memfd for both private and non-private memory,
> >> accommodating the new use cases.
> >>
> >> Co-developed-by: David Hildenbrand <david@xxxxxxxxxx>
> >> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> >> Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
> >> Co-developed-by: Fuad Tabba <tabba@xxxxxxxxxx>
> >> Signed-off-by: Fuad Tabba <tabba@xxxxxxxxxx>
> >> ---
> >> arch/x86/kvm/mmu/mmu.c | 13 +++++++++----
> >> 1 file changed, 9 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> >> index 94be15cde6da..ad5f337b496c 100644
> >> --- a/arch/x86/kvm/mmu/mmu.c
> >> +++ b/arch/x86/kvm/mmu/mmu.c
> >> @@ -4511,8 +4511,8 @@ static void kvm_mmu_finish_page_fault(struct kvm_vcpu *vcpu,
> >> r == RET_PF_RETRY, fault->map_writable);
> >> }
> >>
> >> -static int kvm_mmu_faultin_pfn_private(struct kvm_vcpu *vcpu,
> >> - struct kvm_page_fault *fault)
> >> +static int kvm_mmu_faultin_pfn_gmem(struct kvm_vcpu *vcpu,
> >> + struct kvm_page_fault *fault)
> >> {
> >> int max_order, r;
> >>
> >> @@ -4536,13 +4536,18 @@ static int kvm_mmu_faultin_pfn_private(struct kvm_vcpu *vcpu,
> >> return RET_PF_CONTINUE;
> >> }
> >>
> >> +static bool fault_from_gmem(struct kvm_page_fault *fault)
> >
> > Drop the helper. It has exactly one caller, and it makes the code *harder* to
> > read, e.g. raises the question of what "from gmem" even means. If a separate
> > series follows and needs/justifies this helper, then it can/should be added then.
>
> there is another place requires the same check introduced by your
> "KVM: x86/mmu: Extend guest_memfd's max mapping level to shared
> mappings" provided in [*]
>
> [*] https://lore.kernel.org/kvm/aH7KghhsjaiIL3En@xxxxxxxxxx/
I guess this is meant for the patch [*], which as far as I could tell,
isn't in the latest (-rc7) yet.
Thanks,
/fuad
> ---
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 1ff7582d5fae..2d1894ed1623 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
>
> @@ -3335,8 +3336,9 @@ int kvm_mmu_max_mapping_level(struct kvm *kvm,
> struct kvm_page_fault *fault,
> if (max_level == PG_LEVEL_4K)
> return PG_LEVEL_4K;
>
> - if (is_private)
> - host_level = kvm_max_private_mapping_level(kvm, fault,
> slot, gfn);
> + if (is_private || kvm_memslot_is_gmem_only(slot))
> + host_level = kvm_gmem_max_mapping_level(kvm, fault,
> slot, gfn,
> + is_private);
> else
> host_level = host_pfn_mapping_level(kvm, gfn, slot);
> return min(host_level, max_level);
>
> >> +{
> >> + return fault->is_private || kvm_memslot_is_gmem_only(fault->slot);
> >> +}
> >> +
> >> static int __kvm_mmu_faultin_pfn(struct kvm_vcpu *vcpu,
> >> struct kvm_page_fault *fault)
> >> {
> >> unsigned int foll = fault->write ? FOLL_WRITE : 0;
> >>
> >> - if (fault->is_private)
> >> - return kvm_mmu_faultin_pfn_private(vcpu, fault);
> >> + if (fault_from_gmem(fault))
> >> + return kvm_mmu_faultin_pfn_gmem(vcpu, fault);
> >>
> >> foll |= FOLL_NOWAIT;
> >> fault->pfn = __kvm_faultin_pfn(fault->slot, fault->gfn, foll,
> >> --
> >> 2.50.0.727.gbf7dc18ff4-goog
> >>
>
>
