Re: [PATCH v9 07/41] KVM: guest_memfd: Wire up core private/shared attribute interfaces

[Sean Christopherson <seanjc@google.com>]

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On Mon, Aug 03, 2026, Xiaoyao Li wrote:
> On 8/1/2026 12:26 AM, Sean Christopherson wrote:
> > On Fri, Jul 31, 2026, Xiaoyao Li wrote:
> > > On 7/31/2026 4:42 AM, Ackerley Tng wrote:
> > > If kvm_range_is_private() is useful/required by huge page support, then let
> > > the huge page series to introduce it. It has nothing to do with the in-place
> > > conversion series.
> > 
> > If it weren't for the fact that the range-based search is used later in this
> > series, I would 100% agree with Xiaoyao.
> > But since the core logic is used and needed elsewhere,
> 
> I don't see it a problem. Without introducing kvm_range_is_private() in this
> patch, the core logic of range-based search on gmem can still be introduced
> as kvm_gmem_range_has_attributes() directly in patch 10 or in a separate
> patch.
> 
> > and because kvm_range_has_vm_memory_attributes() takes a range, my vote is
> > to provide the plumbing now, even though a small portion of it isn't
> > strictly necessary.
> 
> So my initial feedback was "we can just use kvm_mem_is_private(kvm, gfn)".
> It makes code simpler.

My apologies, I hadn't actually applied v9 to look at the full context.  I agree,
providing kvm_range_is_private() is completely unnecessary.  It's not even really
a problem with this patch, it's a pre-existing issue that can and should be
addressed with a prep patch.

> And as a bonus, the logic to choose between gmem-based attribute query and
> vm-based attribute query is hidden from the static call.

I don't follow.  What do you mean by "hidden from the static call?"  The whole
point of using a static call is to select between gmem-based and vm-based private
memory.  Or are you saying the populate() flow doesn't need to manually check
gmem_in_place_conversion?

> The second bonus of dropping kvm_range_is_private() is that we can eliminate
> one more chunk in patch 20.

Ya.

Untested, but unless I'm missing something, patches 6 and 7 of this series can
be replaced with the attached patches (6 and 7 should be squashed together no
matter what).

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From 77bd0524535d65d7c6beb4a0cc70a65fbacc9f81 Mon Sep 17 00:00:00 2001
From: Sean Christopherson <seanjc@google.com>
Date: Mon, 3 Aug 2026 08:10:09 -0700
Subject: [PATCH 1/2] KVM: guest_memfd: Use kvm_mem_is_private() when
 populating guest_memfd memory

Using kvm_mem_is_private() when populating guest_memfd instead of using an
open coded equivalent.  In addition to simplifying the populate code *now*,
this avoids the need to provide a range-based gmem lookup API in the future
as well.

No functional change intended.

Suggested-by: Xiaoyao Li <xiaoyao.li@intel.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 virt/kvm/guest_memfd.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 8c7709a352cc..48c13d18af15 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -881,9 +881,7 @@ static long __kvm_gmem_populate(struct kvm *kvm, struct kvm_memory_slot *slot,
 
 	folio_unlock(folio);
 
-	if (!kvm_range_has_vm_memory_attributes(kvm, gfn, gfn + 1,
-						KVM_MEMORY_ATTRIBUTE_PRIVATE,
-						KVM_MEMORY_ATTRIBUTE_PRIVATE)) {
+	if (!kvm_mem_is_private(kvm, gfn)) {
 		ret = -EINVAL;
 		goto out_put_folio;
 	}

base-commit: 17548ee538ef4f476e16fb5dd3ca8adfd8a852ed
-- 
2.55.0.508.g3f0d502094-goog


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From 02db86dc193d969085bbc0c98c6117e59d469cb5 Mon Sep 17 00:00:00 2001
From: Sean Christopherson <seanjc@google.com>
Date: Mon, 3 Aug 2026 08:17:34 -0700
Subject: [PATCH 2/2] KVM: guest_memfd: Stub in ability to enable in-place
 shared<=>private conversion

Stub in global variable to enable in-place guest_memfd private<=>shared
memory conversion, which will eventually be exposed to userspace via a
module param, and wire up the __kvm_mem_is_private() static call to the
guest_memfd version when in-place conversion is enabled, i.e. when gmem is
the sole authority on private vs. shared memory.

Cc: Fuad Tabba <tabba@google.com>
Cc: Xiaoyao Li <xiaoyao.li@intel.com>
Co-developed-by: Ackerley Tng <ackerleytng@google.com>
Signed-off-by: Ackerley Tng <ackerleytng@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 include/linux/kvm_host.h |  6 ++++++
 virt/kvm/guest_memfd.c   | 26 ++++++++++++++++++++++++++
 virt/kvm/kvm_main.c      | 12 +++++++++++-
 3 files changed, 43 insertions(+), 1 deletion(-)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 65fbce46b63f..9477ecebbbce 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -2580,6 +2580,8 @@ static inline bool kvm_vm_mem_is_private(struct kvm *kvm, gfn_t gfn)
 #endif  /* CONFIG_KVM_VM_MEMORY_ATTRIBUTES */
 
 #ifdef kvm_arch_has_private_mem
+extern bool gmem_in_place_conversion;
+
 typedef bool (kvm_mem_is_private_t)(struct kvm *kvm, gfn_t gfn);
 DECLARE_STATIC_CALL(__kvm_mem_is_private, kvm_mem_is_private_t);
 
@@ -2588,6 +2590,8 @@ static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
 	return static_call(__kvm_mem_is_private)(kvm, gfn);
 }
 #else
+#define gmem_in_place_conversion false
+
 static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
 {
 	return false;
@@ -2595,6 +2599,8 @@ static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
 #endif /* kvm_arch_has_private_mem */
 
 #ifdef CONFIG_KVM_GUEST_MEMFD
+bool kvm_gmem_is_private(struct kvm *kvm, gfn_t gfn);
+
 int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
 		     gfn_t gfn, kvm_pfn_t *pfn, struct page **page,
 		     int *max_order);
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 48c13d18af15..e21c7122539a 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -474,6 +474,32 @@ static int kvm_gmem_mmap(struct file *file, struct vm_area_struct *vma)
 	return 0;
 }
 
+bool kvm_gmem_is_private(struct kvm *kvm, gfn_t gfn)
+{
+	struct kvm_memory_slot *slot = gfn_to_memslot(kvm, gfn);
+	struct inode *inode;
+
+	if (!slot)
+		return 0;
+
+	CLASS(gmem_get_file, file)(slot);
+	if (!file)
+		return 0;
+
+	inode = file_inode(file);
+
+	/*
+	 * Rely on the maple tree's internal RCU lock to ensure a stable result.
+	 * This result can become stale as soon as the lock is dropped, so the
+	 * caller _must_ protect consumption of private vs. shared either by
+	 * holding guest_memfd's invalidate lock for the entire duration, or by
+	 * checking mmu_invalidate_retry_gfn() under mmu_lock to serialize
+	 * against concurrent attribute updates.
+	 */
+	return kvm_gmem_is_private_mem(inode, kvm_gmem_get_index(slot, gfn));
+}
+EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_gmem_is_private);
+
 static struct file_operations kvm_gmem_fops = {
 	.mmap		= kvm_gmem_mmap,
 	.open		= generic_file_open,
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 081e3309359d..e5654baa8895 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -101,6 +101,10 @@ EXPORT_SYMBOL_FOR_KVM_INTERNAL(halt_poll_ns_shrink);
 static bool __ro_after_init allow_unsafe_mappings;
 module_param(allow_unsafe_mappings, bool, 0444);
 
+#ifdef kvm_arch_has_private_mem
+bool __ro_after_init gmem_in_place_conversion = false;
+#endif
+
 /*
  * Ordering of locks:
  *
@@ -2422,6 +2426,9 @@ static int kvm_vm_ioctl_clear_dirty_log(struct kvm *kvm,
 static u64 kvm_supported_vm_mem_attributes(struct kvm *kvm)
 {
 #ifdef kvm_arch_has_private_mem
+	if (gmem_in_place_conversion)
+		return 0;
+
 	if (!kvm || kvm_arch_has_private_mem(kvm))
 		return KVM_MEMORY_ATTRIBUTE_PRIVATE;
 #endif
@@ -2633,8 +2640,11 @@ EXPORT_STATIC_CALL_GPL(__kvm_mem_is_private);
 
 static void kvm_init_memory_attributes(void)
 {
+	if (gmem_in_place_conversion)
+		static_call_update(__kvm_mem_is_private, kvm_gmem_is_private);
 #ifdef CONFIG_KVM_VM_MEMORY_ATTRIBUTES
-	static_call_update(__kvm_mem_is_private, kvm_vm_mem_is_private);
+	else
+		static_call_update(__kvm_mem_is_private, kvm_vm_mem_is_private);
 #endif
 }
 #else
-- 
2.55.0.508.g3f0d502094-goog