[PATCH v13 76/85] KVM: Add support for read-only usage of gfn_to_page()

Sean Christopherson
2024-10-10T22:16:35Z

Rework gfn_to_page() to support read-only accesses so that it can be used
by arm64 to get MTE tags out of guest memory.

Opportunistically rewrite the comment to be even more stern about using
gfn_to_page(), as there are very few scenarios where requiring a struct
page is actually the right thing to do (though there are such scenarios).
Add a FIXME to call out that KVM probably should be pinning pages, not
just getting pages.

Tested-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
include/linux/kvm_host.h | 7 ++++++-
virt/kvm/kvm_main.c | 15 ++++++++-------
2 files changed, 14 insertions(+), 8 deletions(-)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 9f7682ece4a1..af928b59b2ab 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -1213,7 +1213,12 @@ void kvm_arch_flush_shadow_memslot(struct kvm *kvm,
int kvm_prefetch_pages(struct kvm_memory_slot *slot, gfn_t gfn,
struct page **pages, int nr_pages);

-struct page *gfn_to_page(struct kvm *kvm, gfn_t gfn);
+struct page *__gfn_to_page(struct kvm *kvm, gfn_t gfn, bool write);
+static inline struct page *gfn_to_page(struct kvm *kvm, gfn_t gfn)
+{
+ return __gfn_to_page(kvm, gfn, true);
+}
+
unsigned long gfn_to_hva(struct kvm *kvm, gfn_t gfn);
unsigned long gfn_to_hva_prot(struct kvm *kvm, gfn_t gfn, bool *writable);
unsigned long gfn_to_hva_memslot(struct kvm_memory_slot *slot, gfn_t gfn);
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 1782242a4800..8f8b2cd01189 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -3138,25 +3138,26 @@ int kvm_prefetch_pages(struct kvm_memory_slot *slot, gfn_t gfn,
EXPORT_SYMBOL_GPL(kvm_prefetch_pages);

/*
- * Do not use this helper unless you are absolutely certain the gfn _must_ be
- * backed by 'struct page'. A valid example is if the backing memslot is
- * controlled by KVM. Note, if the returned page is valid, it's refcount has
- * been elevated by gfn_to_pfn().
+ * Don't use this API unless you are absolutely, positively certain that KVM
+ * needs to get a struct page, e.g. to pin the page for firmware DMA.
+ *
+ * FIXME: Users of this API likely need to FOLL_PIN the page, not just elevate
+ * its refcount.
*/
-struct page *gfn_to_page(struct kvm *kvm, gfn_t gfn)
+struct page *__gfn_to_page(struct kvm *kvm, gfn_t gfn, bool write)
{
struct page *refcounted_page = NULL;
struct kvm_follow_pfn kfp = {
.slot = gfn_to_memslot(kvm, gfn),
.gfn = gfn,
- .flags = FOLL_WRITE,
+ .flags = write ? FOLL_WRITE : 0,
.refcounted_page = &refcounted_page,
};

(void)kvm_follow_pfn(&kfp);
return refcounted_page;
}
-EXPORT_SYMBOL_GPL(gfn_to_page);
+EXPORT_SYMBOL_GPL(__gfn_to_page);

int __kvm_vcpu_map(struct kvm_vcpu *vcpu, gfn_t gfn, struct kvm_host_map *map,
bool writable)