[RFC PATCH v2 08/10] kvm: pfncache: Support caching gmem pfns
From: Patrick Roy
Date: Tue Sep 10 2024 - 12:35:05 EST
Inside the `hva_to_pfn_retry` loop, for gpa based gpcs, check whether
the gpa has KVM_MEMORY_ATTRIBUTE_PRIVATE set, and if so, use
`kvm_gmem_get_pfn` with `KVM_GMEM_GET_PFN_SHARED` to resolve the pfn.
Ignore uhva based gpcs for now, as they are only used with Xen, and we
don't have guest_memfd there (yet). Gmem pfns that are cached by a gpc
have their sharing refcount elevated until the gpc gets invalidated (or
rather: until it gets refreshed after invalidation) or deactivated.
Since during the refresh loop the memory attributes could change between
private shared, store a uhva anyway, even if it will not be used in the
translation in the end.
Signed-off-by: Patrick Roy <roypat@xxxxxxxxxxxx>
---
include/linux/kvm_types.h | 1 +
virt/kvm/pfncache.c | 63 ++++++++++++++++++++++++++++++++++-----
2 files changed, 56 insertions(+), 8 deletions(-)
diff --git a/include/linux/kvm_types.h b/include/linux/kvm_types.h
index 827ecc0b7e10a..8903b8f46cf6c 100644
--- a/include/linux/kvm_types.h
+++ b/include/linux/kvm_types.h
@@ -70,6 +70,7 @@ struct gfn_to_pfn_cache {
kvm_pfn_t pfn;
bool active;
bool valid;
+ bool private;
};
#ifdef KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE
diff --git a/virt/kvm/pfncache.c b/virt/kvm/pfncache.c
index 6de934a8a153f..a4f935e80f545 100644
--- a/virt/kvm/pfncache.c
+++ b/virt/kvm/pfncache.c
@@ -16,6 +16,7 @@
#include <linux/highmem.h>
#include <linux/module.h>
#include <linux/errno.h>
+#include <linux/pagemap.h>
#include "kvm_mm.h"
@@ -145,13 +146,20 @@ static void *gpc_map(kvm_pfn_t pfn)
#endif
}
-static void gpc_unmap(kvm_pfn_t pfn, void *khva)
+static void gpc_unmap(kvm_pfn_t pfn, void *khva, bool private)
{
/* Unmap the old pfn/page if it was mapped before. */
if (is_error_noslot_pfn(pfn) || !khva)
return;
if (pfn_valid(pfn)) {
+ if (private) {
+ struct folio *folio = pfn_folio(pfn);
+
+ folio_lock(folio);
+ kvm_gmem_put_shared_pfn(pfn);
+ folio_unlock(folio);
+ }
kunmap(pfn_to_page(pfn));
return;
}
@@ -203,6 +211,7 @@ static kvm_pfn_t hva_to_pfn_retry(struct gfn_to_pfn_cache *gpc)
void *old_khva = (void *)PAGE_ALIGN_DOWN((uintptr_t)gpc->khva);
kvm_pfn_t new_pfn = KVM_PFN_ERR_FAULT;
void *new_khva = NULL;
+ bool private = gpc->private;
unsigned long mmu_seq;
lockdep_assert_held(&gpc->refresh_lock);
@@ -235,17 +244,43 @@ static kvm_pfn_t hva_to_pfn_retry(struct gfn_to_pfn_cache *gpc)
* the existing mapping and didn't create a new one.
*/
if (new_khva != old_khva)
- gpc_unmap(new_pfn, new_khva);
+ gpc_unmap(new_pfn, new_khva, private);
kvm_release_pfn_clean(new_pfn);
cond_resched();
}
- /* We always request a writeable mapping */
- new_pfn = hva_to_pfn(gpc->uhva, false, false, NULL, true, NULL);
- if (is_error_noslot_pfn(new_pfn))
- goto out_error;
+ /*
+ * If we do not have a GPA, we cannot immediately determine
+ * whether the area of guest memory gpc->uhva pointed to
+ * is currently set to shared. So assume that uhva-based gpcs
+ * never have their underlying guest memory switched to
+ * private (which we can do as uhva-based gpcs are only used
+ * with Xen, and guest_memfd is not supported there).
+ */
+ if (gpc->gpa != INVALID_GPA) {
+ /*
+ * mmu_notifier events can be due to shared/private conversions,
+ * thus recheck this every iteration.
+ */
+ private = kvm_mem_is_private(gpc->kvm, gpa_to_gfn(gpc->gpa));
+ } else {
+ private = false;
+ }
+
+ if (private) {
+ int r = kvm_gmem_get_pfn(gpc->kvm, gpc->memslot, gpa_to_gfn(gpc->gpa),
+ &new_pfn, NULL, KVM_GMEM_GET_PFN_SHARED);
+ if (r)
+ goto out_error;
+ } else {
+ /* We always request a writeable mapping */
+ new_pfn = hva_to_pfn(gpc->uhva, false, false, NULL,
+ true, NULL);
+ if (is_error_noslot_pfn(new_pfn))
+ goto out_error;
+ }
/*
* Obtain a new kernel mapping if KVM itself will access the
@@ -274,6 +309,7 @@ static kvm_pfn_t hva_to_pfn_retry(struct gfn_to_pfn_cache *gpc)
gpc->valid = true;
gpc->pfn = new_pfn;
gpc->khva = new_khva + offset_in_page(gpc->uhva);
+ gpc->private = private;
/*
* Put the reference to the _new_ pfn. The pfn is now tracked by the
@@ -298,6 +334,7 @@ static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned l
kvm_pfn_t old_pfn;
bool hva_change = false;
void *old_khva;
+ bool old_private;
int ret;
/* Either gpa or uhva must be valid, but not both */
@@ -316,6 +353,7 @@ static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned l
old_pfn = gpc->pfn;
old_khva = (void *)PAGE_ALIGN_DOWN((uintptr_t)gpc->khva);
old_uhva = PAGE_ALIGN_DOWN(gpc->uhva);
+ old_private = gpc->private;
if (kvm_is_error_gpa(gpa)) {
page_offset = offset_in_page(uhva);
@@ -338,6 +376,11 @@ static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned l
gpc->gpa = gpa;
gpc->generation = slots->generation;
gpc->memslot = __gfn_to_memslot(slots, gfn);
+ /*
+ * compute the uhva even for private memory, in case an
+ * invalidation event flips memory from private to
+ * shared while in hva_to_pfn_retry
+ */
gpc->uhva = gfn_to_hva_memslot(gpc->memslot, gfn);
if (kvm_is_error_hva(gpc->uhva)) {
@@ -395,7 +438,7 @@ static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned l
write_unlock_irq(&gpc->lock);
if (unmap_old)
- gpc_unmap(old_pfn, old_khva);
+ gpc_unmap(old_pfn, old_khva, old_private);
return ret;
}
@@ -486,6 +529,7 @@ void kvm_gpc_deactivate(struct gfn_to_pfn_cache *gpc)
struct kvm *kvm = gpc->kvm;
kvm_pfn_t old_pfn;
void *old_khva;
+ bool old_private;
guard(mutex)(&gpc->refresh_lock);
@@ -508,6 +552,9 @@ void kvm_gpc_deactivate(struct gfn_to_pfn_cache *gpc)
old_khva = gpc->khva - offset_in_page(gpc->khva);
gpc->khva = NULL;
+ old_private = gpc->private;
+ gpc->private = false;
+
old_pfn = gpc->pfn;
gpc->pfn = KVM_PFN_ERR_FAULT;
write_unlock_irq(&gpc->lock);
@@ -516,6 +563,6 @@ void kvm_gpc_deactivate(struct gfn_to_pfn_cache *gpc)
list_del(&gpc->list);
spin_unlock(&kvm->gpc_lock);
- gpc_unmap(old_pfn, old_khva);
+ gpc_unmap(old_pfn, old_khva, old_private);
}
}
--
2.46.0