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KVM: guest_memfd: Track guest_memfd mmap support in memslot

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Add a new internal flag, KVM_MEMSLOT_GMEM_ONLY, to the top half of
memslot->flags (which makes it strictly for KVM's internal use). This
flag tracks when a guest_memfd-backed memory slot supports host
userspace mmap operations, which implies that all memory, not just
private memory for CoCo VMs, is consumed through guest_memfd: "gmem
only".

This optimization avoids repeatedly checking the underlying guest_memfd
file for mmap support, which would otherwise require taking and
releasing a reference on the file for each check. By caching this
information directly in the memslot, we reduce overhead and simplify the
logic involved in handling guest_memfd-backed pages for host mappings.

Reviewed-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: Shivank Garg <shivankg@amd.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Acked-by: David Hildenbrand <david@redhat.com>
Suggested-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20250729225455.670324-12-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Fuad Tabba
2025-07-29 15:54:42 -07:00
committed by Paolo Bonzini
parent a12578e147
commit 576d035e2a
2 changed files with 12 additions and 1 deletions
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11
include/linux/kvm_host.h
View File

@@ -54,7 +54,8 @@
* used in kvm, other bits are visible for userspace which are defined in
* include/uapi/linux/kvm.h.
*/
#define KVM_MEMSLOT_INVALID (1UL << 16)
#define KVM_MEMSLOT_INVALID (1UL << 16)
#define KVM_MEMSLOT_GMEM_ONLY (1UL << 17)
/*
* Bit 63 of the memslot generation number is an "update in-progress flag",
@@ -2490,6 +2491,14 @@ static inline void kvm_prepare_memory_fault_exit(struct kvm_vcpu *vcpu,
vcpu->run->memory_fault.flags |= KVM_MEMORY_EXIT_FLAG_PRIVATE;
}
static inline bool kvm_memslot_is_gmem_only(const struct kvm_memory_slot *slot)
{
if (!IS_ENABLED(CONFIG_KVM_GUEST_MEMFD))
return false;
return slot->flags & KVM_MEMSLOT_GMEM_ONLY;
}
#ifdef CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES
static inline unsigned long kvm_get_memory_attributes(struct kvm *kvm, gfn_t gfn)
{

2
virt/kvm/guest_memfd.c
View File

@@ -578,6 +578,8 @@ int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot,
*/
WRITE_ONCE(slot->gmem.file, file);
slot->gmem.pgoff = start;
if (kvm_gmem_supports_mmap(inode))
slot->flags |= KVM_MEMSLOT_GMEM_ONLY;
xa_store_range(&gmem->bindings, start, end - 1, slot, GFP_KERNEL);
filemap_invalidate_unlock(inode->i_mapping);