[PATCH v11 09/16] KVM: arm64: define kvm_arch_gmem_supports_no_direct_map()

Kalyazin, Nikita
2026-03-17T14:12:19Z

From: Patrick Roy <patrick.roy@linux.dev>

Support for GUEST_MEMFD_FLAG_NO_DIRECT_MAP on arm64 depends on 1) direct
map manipulations at 4k granularity being possible, and 2) FEAT_S2FWB.

1) is met whenever the direct map is set up at 4k granularity (e.g. not
with huge/gigantic pages) at boottime, as due to ARM's
break-before-make semantics, breaking huge mappings into 4k mappings in
the direct map is not possible (BBM would require temporary invalidation
of the entire huge mapping, even if only a 4k subrange should be zapped,
which will probably crash the kernel). However, the current default for
rodata_full is true, which forces a 4k direct map.

2) is required to allow KVM to elide cache coherency operations when
installing stage 2 page tables, which require the direct map to be
entry for the newly mapped memory to be present (which it will not be,
as guest_memfd would have removed direct map entries in
kvm_gmem_get_pfn()).

Cc: Will Deacon <will@kernel.org>
Signed-off-by: Patrick Roy <patrick.roy@linux.dev>
Reviewed-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Nikita Kalyazin <kalyazin@amazon.com>
---
arch/arm64/include/asm/kvm_host.h | 13 +++++++++++++
1 file changed, 13 insertions(+)

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 70cb9cfd760a..fbdd43e7e94e 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -19,6 +19,7 @@
#include <linux/maple_tree.h>
#include <linux/percpu.h>
#include <linux/psci.h>
+#include <linux/set_memory.h>
#include <asm/arch_gicv3.h>
#include <asm/barrier.h>
#include <asm/cpufeature.h>
@@ -1682,6 +1683,18 @@ static __always_inline enum fgt_group_id __fgt_reg_to_group_id(enum vcpu_sysreg
\
p; \
})
+#ifdef CONFIG_KVM_GUEST_MEMFD
+static inline bool kvm_arch_gmem_supports_no_direct_map(struct kvm *kvm)
+{
+ /*
+ * Without FWB, direct map access is needed in kvm_pgtable_stage2_map(),
+ * as it calls dcache_clean_inval_poc().
+ */
+ return can_set_direct_map() && cpus_have_final_cap(ARM64_HAS_STAGE2_FWB);
+}
+#define kvm_arch_gmem_supports_no_direct_map kvm_arch_gmem_supports_no_direct_map
+#endif /* CONFIG_KVM_GUEST_MEMFD */
+

long kvm_get_cap_for_kvm_ioctl(unsigned int ioctl, long *ext);

--
2.50.1