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authorSean Christopherson <seanjc@google.com>2024-10-10 11:24:27 -0700
committerPaolo Bonzini <pbonzini@redhat.com>2024-10-25 13:01:35 -0400
commit8b15c3764c05ed8766709711d2054d96349dee8e (patch)
tree2b28e7b160c28d374682ecf290d2a26796c05044 /virt
parent93b7da404f5b0b02a4211bbb784889f001d27953 (diff)
downloadlwn-8b15c3764c05ed8766709711d2054d96349dee8e.tar.gz
lwn-8b15c3764c05ed8766709711d2054d96349dee8e.zip
KVM: Don't grab reference on VM_MIXEDMAP pfns that have a "struct page"
Now that KVM no longer relies on an ugly heuristic to find its struct page references, i.e. now that KVM can't get false positives on VM_MIXEDMAP pfns, remove KVM's hack to elevate the refcount for pfns that happen to have a valid struct page. In addition to removing a long-standing wart in KVM, this allows KVM to map non-refcounted struct page memory into the guest, e.g. for exposing GPU TTM buffers to KVM guests. Tested-by: Alex Bennée <alex.bennee@linaro.org> Signed-off-by: Sean Christopherson <seanjc@google.com> Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Message-ID: <20241010182427.1434605-86-seanjc@google.com>
Diffstat (limited to 'virt')
-rw-r--r--virt/kvm/kvm_main.c75
1 files changed, 2 insertions, 73 deletions
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 396ca14f18f3..b1b10dc408a0 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -160,52 +160,6 @@ __weak void kvm_arch_guest_memory_reclaimed(struct kvm *kvm)
{
}
-bool kvm_is_zone_device_page(struct page *page)
-{
- /*
- * The metadata used by is_zone_device_page() to determine whether or
- * not a page is ZONE_DEVICE is guaranteed to be valid if and only if
- * the device has been pinned, e.g. by get_user_pages(). WARN if the
- * page_count() is zero to help detect bad usage of this helper.
- */
- if (WARN_ON_ONCE(!page_count(page)))
- return false;
-
- return is_zone_device_page(page);
-}
-
-/*
- * Returns a 'struct page' if the pfn is "valid" and backed by a refcounted
- * page, NULL otherwise. Note, the list of refcounted PG_reserved page types
- * is likely incomplete, it has been compiled purely through people wanting to
- * back guest with a certain type of memory and encountering issues.
- */
-struct page *kvm_pfn_to_refcounted_page(kvm_pfn_t pfn)
-{
- struct page *page;
-
- if (!pfn_valid(pfn))
- return NULL;
-
- page = pfn_to_page(pfn);
- if (!PageReserved(page))
- return page;
-
- /* The ZERO_PAGE(s) is marked PG_reserved, but is refcounted. */
- if (is_zero_pfn(pfn))
- return page;
-
- /*
- * ZONE_DEVICE pages currently set PG_reserved, but from a refcounting
- * perspective they are "normal" pages, albeit with slightly different
- * usage rules.
- */
- if (kvm_is_zone_device_page(page))
- return page;
-
- return NULL;
-}
-
/*
* Switches to specified vcpu, until a matching vcpu_put()
*/
@@ -2804,35 +2758,10 @@ static kvm_pfn_t kvm_resolve_pfn(struct kvm_follow_pfn *kfp, struct page *page,
if (kfp->map_writable)
*kfp->map_writable = writable;
- /*
- * FIXME: Remove this once KVM no longer blindly calls put_page() on
- * every pfn that points at a struct page.
- *
- * Get a reference for follow_pte() pfns if they happen to point at a
- * struct page, as KVM will ultimately call kvm_release_pfn_clean() on
- * the returned pfn, i.e. KVM expects to have a reference.
- *
- * Certain IO or PFNMAP mappings can be backed with valid struct pages,
- * but be allocated without refcounting, e.g. tail pages of
- * non-compound higher order allocations. Grabbing and putting a
- * reference to such pages would cause KVM to prematurely free a page
- * it doesn't own (KVM gets and puts the one and only reference).
- * Don't allow those pages until the FIXME is resolved.
- *
- * Don't grab a reference for pins, callers that pin pages are required
- * to check refcounted_page, i.e. must not blindly release the pfn.
- */
- if (map) {
+ if (map)
pfn = map->pfn;
-
- if (!kfp->pin) {
- page = kvm_pfn_to_refcounted_page(pfn);
- if (page && !get_page_unless_zero(page))
- return KVM_PFN_ERR_FAULT;
- }
- } else {
+ else
pfn = page_to_pfn(page);
- }
*kfp->refcounted_page = page;