On Tue, Dec 06, 2022 at 10:42:24PM -0800, Isaku Yamahata wrote:

On Tue, Dec 06, 2022 at 08:02:24PM +0800,
Chao Peng [off-list ref] wrote:

quoted

On Mon, Dec 05, 2022 at 02:49:59PM -0800, Isaku Yamahata wrote:

quoted

On Fri, Dec 02, 2022 at 02:13:45PM +0800,
Chao Peng [off-list ref] wrote:

quoted

A large page with mixed private/shared subpages can't be mapped as large
page since its sub private/shared pages are from different memory
backends and may also treated by architecture differently. When
private/shared memory are mixed in a large page, the current lpage_info
is not sufficient to decide whether the page can be mapped as large page
or not and additional private/shared mixed information is needed.

Tracking this 'mixed' information with the current 'count' like
disallow_lpage is a bit challenge so reserve a bit in 'disallow_lpage'
to indicate a large page has mixed private/share subpages and update
this 'mixed' bit whenever the memory attribute is changed between
private and shared.

Signed-off-by: Chao Peng <redacted>
---
 arch/x86/include/asm/kvm_host.h |   8 ++
 arch/x86/kvm/mmu/mmu.c          | 134 +++++++++++++++++++++++++++++++-
 arch/x86/kvm/x86.c              |   2 +
 include/linux/kvm_host.h        |  19 +++++
 virt/kvm/kvm_main.c             |   9 ++-
 5 files changed, 169 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 283cbb83d6ae..7772ab37ac89 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h

@@ -38,6 +38,7 @@
 #include <asm/hyperv-tlfs.h>
 
 #define __KVM_HAVE_ARCH_VCPU_DEBUGFS
+#define __KVM_HAVE_ARCH_SET_MEMORY_ATTRIBUTES
 
 #define KVM_MAX_VCPUS 1024
 

@@ -1011,6 +1012,13 @@ struct kvm_vcpu_arch {
 #endif
 };
 
+/*
+ * Use a bit in disallow_lpage to indicate private/shared pages mixed at the
+ * level. The remaining bits are used as a reference count.
+ */
+#define KVM_LPAGE_PRIVATE_SHARED_MIXED		(1U << 31)
+#define KVM_LPAGE_COUNT_MAX			((1U << 31) - 1)
+
 struct kvm_lpage_info {
 	int disallow_lpage;
 };

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index e2c70b5afa3e..2190fd8c95c0 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c

@@ -763,11 +763,16 @@ static void update_gfn_disallow_lpage_count(const struct kvm_memory_slot *slot,
 {
 	struct kvm_lpage_info *linfo;
 	int i;
+	int disallow_count;
 
 	for (i = PG_LEVEL_2M; i <= KVM_MAX_HUGEPAGE_LEVEL; ++i) {
 		linfo = lpage_info_slot(gfn, slot, i);
+
+		disallow_count = linfo->disallow_lpage & KVM_LPAGE_COUNT_MAX;
+		WARN_ON(disallow_count + count < 0 ||
+			disallow_count > KVM_LPAGE_COUNT_MAX - count);
+
 		linfo->disallow_lpage += count;
-		WARN_ON(linfo->disallow_lpage < 0);
 	}
 }
 

@@ -6986,3 +6991,130 @@ void kvm_mmu_pre_destroy_vm(struct kvm *kvm)
 	if (kvm->arch.nx_huge_page_recovery_thread)
 		kthread_stop(kvm->arch.nx_huge_page_recovery_thread);
 }
+
+static bool linfo_is_mixed(struct kvm_lpage_info *linfo)
+{
+	return linfo->disallow_lpage & KVM_LPAGE_PRIVATE_SHARED_MIXED;
+}
+
+static void linfo_set_mixed(gfn_t gfn, struct kvm_memory_slot *slot,
+			    int level, bool mixed)
+{
+	struct kvm_lpage_info *linfo = lpage_info_slot(gfn, slot, level);
+
+	if (mixed)
+		linfo->disallow_lpage |= KVM_LPAGE_PRIVATE_SHARED_MIXED;
+	else
+		linfo->disallow_lpage &= ~KVM_LPAGE_PRIVATE_SHARED_MIXED;
+}
+
+static bool is_expected_attr_entry(void *entry, unsigned long expected_attrs)
+{
+	bool expect_private = expected_attrs & KVM_MEMORY_ATTRIBUTE_PRIVATE;
+
+	if (xa_to_value(entry) & KVM_MEMORY_ATTRIBUTE_PRIVATE) {
+		if (!expect_private)
+			return false;
+	} else if (expect_private)
+		return false;
+
+	return true;
+}
+
+static bool mem_attrs_mixed_2m(struct kvm *kvm, unsigned long attrs,
+			       gfn_t start, gfn_t end)
+{
+	XA_STATE(xas, &kvm->mem_attr_array, start);
+	gfn_t gfn = start;
+	void *entry;
+	bool mixed = false;
+
+	rcu_read_lock();
+	entry = xas_load(&xas);
+	while (gfn < end) {
+		if (xas_retry(&xas, entry))
+			continue;
+
+		KVM_BUG_ON(gfn != xas.xa_index, kvm);
+
+		if (!is_expected_attr_entry(entry, attrs)) {
+			mixed = true;
+			break;
+		}
+
+		entry = xas_next(&xas);
+		gfn++;
+	}
+
+	rcu_read_unlock();
+	return mixed;
+}
+
+static bool mem_attrs_mixed(struct kvm *kvm, struct kvm_memory_slot *slot,
+			    int level, unsigned long attrs,
+			    gfn_t start, gfn_t end)
+{
+	unsigned long gfn;
+
+	if (level == PG_LEVEL_2M)
+		return mem_attrs_mixed_2m(kvm, attrs, start, end);
+
+	for (gfn = start; gfn < end; gfn += KVM_PAGES_PER_HPAGE(level - 1))
+		if (linfo_is_mixed(lpage_info_slot(gfn, slot, level - 1)) ||
+		    !is_expected_attr_entry(xa_load(&kvm->mem_attr_array, gfn),
+					    attrs))
+			return true;
+	return false;
+}
+
+static void kvm_update_lpage_private_shared_mixed(struct kvm *kvm,
+						  struct kvm_memory_slot *slot,
+						  unsigned long attrs,
+						  gfn_t start, gfn_t end)
+{
+	unsigned long pages, mask;
+	gfn_t gfn, gfn_end, first, last;
+	int level;
+	bool mixed;
+
+	/*
+	 * The sequence matters here: we set the higher level basing on the
+	 * lower level's scanning result.
+	 */
+	for (level = PG_LEVEL_2M; level <= KVM_MAX_HUGEPAGE_LEVEL; level++) {
+		pages = KVM_PAGES_PER_HPAGE(level);
+		mask = ~(pages - 1);
+		first = start & mask;
+		last = (end - 1) & mask;
+
+		/*
+		 * We only need to scan the head and tail page, for middle pages
+		 * we know they will not be mixed.
+		 */
+		gfn = max(first, slot->base_gfn);
+		gfn_end = min(first + pages, slot->base_gfn + slot->npages);
+		mixed = mem_attrs_mixed(kvm, slot, level, attrs, gfn, gfn_end);
+		linfo_set_mixed(gfn, slot, level, mixed);
+
+		if (first == last)
+			return;

continue.

Ya!

quoted

quoted

+
+		for (gfn = first + pages; gfn < last; gfn += pages)
+			linfo_set_mixed(gfn, slot, level, false);
+
+		gfn = last;
+		gfn_end = min(last + pages, slot->base_gfn + slot->npages);

if (gfn == gfn_end) continue.

Do you see a case where gfn can equal to gfn_end? Though it does not
hurt to add a check.

If last == base_gfn + npages, gfn == gfn_end can occur.

'end' is guaranteed <=  base_gfn + npages in kvm_unmap_mem_range():
	gfn_range.end = min(end, slot->base_gfn + slot->npages);

And 'last' is defined as:
	last = (end - 1) & mask;

Then the math is:
	last = (end - 1) & mask
	     <= end - 1
	     <= base_gfn + npages - 1
	     <  base_gfn + npages

Thanks,
Chao

quoted

quoted

quoted

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 9a07380f8d3c..5aefcff614d2 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c

@@ -12362,6 +12362,8 @@ static int kvm_alloc_memslot_metadata(struct kvm *kvm,
 		if ((slot->base_gfn + npages) & (KVM_PAGES_PER_HPAGE(level) - 1))
 			linfo[lpages - 1].disallow_lpage = 1;
 		ugfn = slot->userspace_addr >> PAGE_SHIFT;
+		if (kvm_slot_can_be_private(slot))
+			ugfn |= slot->restricted_offset >> PAGE_SHIFT;

Is there any alignment restriction? If no, It should be +=.
In practice, alignment will hold though.

All we need here is checking whether both userspace_addr and
restricted_offset are aligned to HPAGE_SIZE or not. '+=' actually can
yield wrong value in cases when userspace_addr + restricted_offset is
aligned to HPAGE_SIZE but individually they may not align to HPAGE_SIZE.

Ah, got it. The blow comment explains it.

quoted

Thanks,
Chao

quoted

Thanks,

quoted

 		/*
 		 * If the gfn and userspace address are not aligned wrt each
 		 * other, disable large page support for this slot.

-- 
Isaku Yamahata [off-list ref]