Kernel patches for gfn_to_pfn_cache (including kvm-clock)

resources/hiding_ci/build_and_install_kernel.sh

@@ -79,7 +79,7 @@ confirm() {
 apply_patch_file() {
   echo "Applying patch:" $(basename $1)
 
-  git apply $1
+  git apply --allow-empty $1
 }
 
 apply_patch_or_series() {

resources/hiding_ci/linux_patches/11-kvm-clock/0000-cover-letter.patch

@@ -0,0 +1,70 @@
+From 363385a3c2cd4f7fe445ed71329e55d190cb14d5 Mon Sep 17 00:00:00 2001
+From: Takahiro Itazuri <itazur@amazon.com>
+Date: Tue, 2 Dec 2025 12:15:49 +0000
+Subject: [RFC PATCH 0/2] KVM: pfncache: Support guest_memfd without direct map
+
+[ based on kvm/next with [1] ]
+
+Recent work on guest_memfd [1] is introducing support for removing guest
+memory from the kernel direct map (Note that it hasn't been merged yet,
+and that is why this patch series is labelled RFC). The feature is
+useful for non-CoCo VMs to prevent the host kernel from accidentally or
+speculatively accessing guest memory as a general safety improvement.
+Pages for guest_memfd created with GUEST_MEMFD_FLAG_NO_DIRECT_MAP have
+their direct-map PTEs explicitly disabled, and thus cannot rely on the
+direct map.
+
+This breaks the facilities that use gfn_to_pfn_cache, including
+kvm-clock.  gfn_to_pfn_cache caches the pfn and kernel host virtual
+address (khva) for a given gfn so that KVM can repeatedly read or write
+the corresponding guest page.  The cached khva may be later dereferenced
+from atomic contexts in some cases.  Such contexts cannot tolerate
+sleeping or page faults, and therefore cannot use the userspace mapping
+(uhva), as those mappings may fault at any time. As a result,
+gfn_to_pfn_cache requires a stable, fault-free kernel virtual address
+for the backing pages, independent of the userspace page.
+
+This small patch series enables gfn_to_pfn_cache to work correctly when
+a memslot is backed by guest_memfd with GUEST_MEMFD_FLAG_NO_DIRECT_MAP.
+The first patch teaches gfn_to_pfn_cache to obtain pfn for guest_memfd-
+backed memslots via kvm_gmem_get_pfn() instead of GUP (hva_to_pfn()).
+The second patch makes gfn_to_pfn_cache use vmap()/vunmap() to create a
+fault-free kernel address for such pages.  We believe that establishing
+such mapping for paravirtual guest/host communication is acceptable
+since such pages do not contain sensitive data.
+
+Another considered idea was to use memremap() instead of vmap(), since
+gpc_map() already falls back to memremap() if pfn_valid() is false.
+However, vmap() was chosen for the following reason.  memremap() with
+MEMREMAP_WB first attempts to use the direct map via try_ram_remap(),
+and then falls back to arch_memremap_wb(), which explicitly refuses to
+map system RAM.  It would be possible to relax this restriction, but the
+side effects are unclear because memremap() is widely used throughout
+the kernel.  Changing memremap() to support system RAM without the
+direct map solely for gfn_to_pfn_cache feels disproportionate. If
+additional users appear that need to map system RAM without the direct
+map, revisiting and generalizing memremap() might make sense. For now,
+vmap()/vunmap() provides a contained and predictable solution.
+
+A possible approach in the future is to use the "ephmap" (or proclocal)
+proposed in [2], but it is not yet clear when that work will be merged.
+In constrast, the changes in this patch series are small and
+self-contained, yet immediately allow gfn_to_pfn_cache (including
+kvm-clock) to operate correctly with direct map-removed guest_memfd.
+Once ephmap eventually is merged, gfn_to_pfn_cache can be updated to
+make use of it as appropriate.
+
+[1]: https://lore.kernel.org/all/20250924151101.2225820-1-patrick.roy@campus.lmu.de/
+[2]: https://lore.kernel.org/all/20250812173109.295750-1-jackmanb@google.com/
+
+Takahiro Itazuri (2):
+  KVM: pfncache: Use kvm_gmem_get_pfn() for guest_memfd-backed memslots
+  KVM: pfncache: Use vmap() for guest_memfd pages without direct map
+
+ include/linux/kvm_host.h |  7 ++++++
+ virt/kvm/pfncache.c      | 52 +++++++++++++++++++++++++++++-----------
+ 2 files changed, 45 insertions(+), 14 deletions(-)
+
+--
+2.50.1
+

resources/hiding_ci/linux_patches/11-kvm-clock/0001-KVM-pfncache-Use-kvm_gmem_get_pfn-for-guest_memfd-ba.patch

@@ -0,0 +1,99 @@
+From bebfd5914aae9d2eaec6b90b6408875f2aa40610 Mon Sep 17 00:00:00 2001
+From: Takahiro Itazuri <itazur@amazon.com>
+Date: Mon, 1 Dec 2025 14:58:44 +0000
+Subject: [RFC PATCH 1/2] KVM: pfncache: Use kvm_gmem_get_pfn() for guest_memfd-backed memslots
+
+gfn_to_pfn_cache currently relies on hva_to_pfn(), which resolves PFNs
+through GUP.  GUP assumes that the page has a valid direct-map PTE,
+which is not true for guest_memfd created with
+GUEST_MEMFD_FLAG_NO_DIRECT_MAP, because their direct-map PTEs are
+explicitly removed via set_direct_map_valid_noflush().
+
+Introduce a helper function, gpc_to_pfn(), that routes PFN lookup to
+kvm_gmem_get_pfn() for guest_memfd-backed memslots (regardless of
+whether GUEST_MEMFD_FLAG_NO_DIRECT_MAP is set), and otherwise falls
+back to the existing hva_to_pfn() path. Rename hva_to_pfn_retry() to
+gpc_to_pfn_retry() accordingly.
+
+Signed-off-by: Takahiro Itazuri <itazur@amazon.com>
+---
+ virt/kvm/pfncache.c | 34 +++++++++++++++++++++++-----------
+ 1 file changed, 23 insertions(+), 11 deletions(-)
+
+diff --git a/virt/kvm/pfncache.c b/virt/kvm/pfncache.c
+index 728d2c1b488a..bf8d6090e283 100644
+--- a/virt/kvm/pfncache.c
++++ b/virt/kvm/pfncache.c
+@@ -152,22 +152,34 @@ static inline bool mmu_notifier_retry_cache(struct kvm *kvm, unsigned long mmu_s
+ 	return kvm->mmu_invalidate_seq != mmu_seq;
+ }
+
+-static kvm_pfn_t hva_to_pfn_retry(struct gfn_to_pfn_cache *gpc)
++static kvm_pfn_t gpc_to_pfn(struct gfn_to_pfn_cache *gpc, struct page **page)
+ {
+-	/* Note, the new page offset may be different than the old! */
+-	void *old_khva = (void *)PAGE_ALIGN_DOWN((uintptr_t)gpc->khva);
+-	kvm_pfn_t new_pfn = KVM_PFN_ERR_FAULT;
+-	void *new_khva = NULL;
+-	unsigned long mmu_seq;
+-	struct page *page;
++	if (kvm_slot_has_gmem(gpc->memslot)) {
++		kvm_pfn_t pfn;
++
++		kvm_gmem_get_pfn(gpc->kvm, gpc->memslot, gpa_to_gfn(gpc->gpa),
++				 &pfn, page, NULL);
++		return pfn;
++	}
+
+ 	struct kvm_follow_pfn kfp = {
+ 		.slot = gpc->memslot,
+ 		.gfn = gpa_to_gfn(gpc->gpa),
+ 		.flags = FOLL_WRITE,
+ 		.hva = gpc->uhva,
+-		.refcounted_page = &page,
++		.refcounted_page = page,
+ 	};
++	return hva_to_pfn(&kfp);
++}
++
++static kvm_pfn_t gpc_to_pfn_retry(struct gfn_to_pfn_cache *gpc)
++{
++	/* Note, the new page offset may be different than the old! */
++	void *old_khva = (void *)PAGE_ALIGN_DOWN((uintptr_t)gpc->khva);
++	kvm_pfn_t new_pfn = KVM_PFN_ERR_FAULT;
++	void *new_khva = NULL;
++	unsigned long mmu_seq;
++	struct page *page;
+
+ 	lockdep_assert_held(&gpc->refresh_lock);
+
+@@ -206,7 +218,7 @@ static kvm_pfn_t hva_to_pfn_retry(struct gfn_to_pfn_cache *gpc)
+ 			cond_resched();
+ 		}
+
+-		new_pfn = hva_to_pfn(&kfp);
++		new_pfn = gpc_to_pfn(gpc, &page);
+ 		if (is_error_noslot_pfn(new_pfn))
+ 			goto out_error;
+
+@@ -319,7 +331,7 @@ static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned l
+ 		}
+ 	}
+
+-	/* Note: the offset must be correct before calling hva_to_pfn_retry() */
++	/* Note: the offset must be correct before calling gpc_to_pfn_retry() */
+ 	gpc->uhva += page_offset;
+
+ 	/*
+@@ -327,7 +339,7 @@ static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned l
+ 	 * drop the lock and do the HVA to PFN lookup again.
+ 	 */
+ 	if (!gpc->valid || hva_change) {
+-		ret = hva_to_pfn_retry(gpc);
++		ret = gpc_to_pfn_retry(gpc);
+ 	} else {
+ 		/*
+ 		 * If the HVA→PFN mapping was already valid, don't unmap it.
+-- 
+2.50.1
+

resources/hiding_ci/linux_patches/11-kvm-clock/0002-KVM-pfncache-Use-vmap-for-guest_memfd-pages-without-.patch

@@ -0,0 +1,115 @@
+From 6c90c75b15ba48cc5e0a1e74224c041c3c45668b Mon Sep 17 00:00:00 2001
+From: Takahiro Itazuri <itazur@amazon.com>
+Date: Mon, 1 Dec 2025 16:47:05 +0000
+Subject: [RFC PATCH 2/2] KVM: pfncache: Use vmap() for guest_memfd pages without direct map
+
+gfn_to_pfn_cache currently maps RAM PFNs with kmap(), which relies on
+the direct map.  guest_memfd created with GUEST_MEMFD_FLAG_NO_DIRECT_MAP
+disable their direct-map PTEs via set_direct_map_valid_noflush(), so the
+linear address returned by kmap()/page_address() will fault if
+dereferenced.
+
+In some cases, gfn_to_pfn_cache dereferences the cached kernel address
+(khva) from atomic contexts where page faults cannot be tolerated.
+Therefore khva must always refer to a fault-free kernel mapping.  Since
+mapping and unmapping happen exclusively in the refresh path, which may
+sleep, using vmap()/vunmap() for these pages is safe and sufficient.
+
+Introduce kvm_slot_no_direct_map() to detect guest_memfd slots without
+the direct map, and make gpc_map()/gpc_unmap() use vmap()/vunmap() for
+such pages.
+
+This allows the facilities based on gfn_to_pfn_cache (e.g. kvm-clock) to
+work correctly with guest_memfd regardless of whether its direct-map
+PTEs are valid.
+
+Signed-off-by: Takahiro Itazuri <itazur@amazon.com>
+---
+ include/linux/kvm_host.h |  7 +++++++
+ virt/kvm/pfncache.c      | 26 ++++++++++++++++++++------
+ 2 files changed, 27 insertions(+), 6 deletions(-)
+
+diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
+index 70e6a5210ceb..793d98f97928 100644
+--- a/include/linux/kvm_host.h
++++ b/include/linux/kvm_host.h
+@@ -15,6 +15,7 @@
+ #include <linux/minmax.h>
+ #include <linux/mm.h>
+ #include <linux/mmu_notifier.h>
++#include <linux/pagemap.h>
+ #include <linux/preempt.h>
+ #include <linux/msi.h>
+ #include <linux/slab.h>
+@@ -628,6 +629,12 @@ static inline bool kvm_slot_dirty_track_enabled(const struct kvm_memory_slot *sl
+ 	return slot->flags & KVM_MEM_LOG_DIRTY_PAGES;
+ }
+
++static inline bool kvm_slot_no_direct_map(const struct kvm_memory_slot *slot)
++{
++	return slot && kvm_slot_has_gmem(slot) &&
++	       mapping_no_direct_map(slot->gmem.file->f_mapping);
++}
++
+ static inline unsigned long kvm_dirty_bitmap_bytes(struct kvm_memory_slot *memslot)
+ {
+ 	return ALIGN(memslot->npages, BITS_PER_LONG) / 8;
+diff --git a/virt/kvm/pfncache.c b/virt/kvm/pfncache.c
+index bf8d6090e283..ae6d8699e536 100644
+--- a/virt/kvm/pfncache.c
++++ b/virt/kvm/pfncache.c
+@@ -96,10 +96,16 @@ bool kvm_gpc_check(struct gfn_to_pfn_cache *gpc, unsigned long len)
+ 	return true;
+ }
+
+-static void *gpc_map(kvm_pfn_t pfn)
++static void *gpc_map(struct gfn_to_pfn_cache *gpc, kvm_pfn_t pfn)
+ {
+-	if (pfn_valid(pfn))
+-		return kmap(pfn_to_page(pfn));
++	if (pfn_valid(pfn)) {
++		struct page *page = pfn_to_page(pfn);
++
++		if (kvm_slot_no_direct_map(gpc->memslot))
++			return vmap(&page, 1, VM_MAP, PAGE_KERNEL);
++
++		return kmap(page);
++	}
+
+ #ifdef CONFIG_HAS_IOMEM
+ 	return memremap(pfn_to_hpa(pfn), PAGE_SIZE, MEMREMAP_WB);
+@@ -115,6 +121,11 @@ static void gpc_unmap(kvm_pfn_t pfn, void *khva)
+ 		return;
+
+ 	if (pfn_valid(pfn)) {
++		if (is_vmalloc_addr(khva)) {
++			vunmap(khva);
++			return;
++		}
++
+ 		kunmap(pfn_to_page(pfn));
+ 		return;
+ 	}
+@@ -224,13 +235,16 @@ static kvm_pfn_t gpc_to_pfn_retry(struct gfn_to_pfn_cache *gpc)
+
+ 		/*
+ 		 * Obtain a new kernel mapping if KVM itself will access the
+-		 * pfn.  Note, kmap() and memremap() can both sleep, so this
+-		 * too must be done outside of gpc->lock!
++		 * pfn.  Note, kmap(), vmap() and memremap() can sleep, so this
++		 * too must be done outside of gpc->lock! Note that even though
++		 * the rwlock is dropped, it's still fine to access gpc->pfn and
++		 * other fields because gpc->fresh_lock mutex prevents those
++		 * from being changed.
+ 		 */
+ 		if (new_pfn == gpc->pfn)
+ 			new_khva = old_khva;
+ 		else
+-			new_khva = gpc_map(new_pfn);
++			new_khva = gpc_map(gpc, new_pfn);
+
+ 		if (!new_khva) {
+ 			kvm_release_page_unused(page);
+-- 
+2.50.1
+