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Message-ID: <20260114134510.1835-7-kalyazin@amazon.com>
Date: Wed, 14 Jan 2026 13:46:21 +0000
From: "Kalyazin, Nikita" <kalyazin@...zon.co.uk>
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Subject: [PATCH v9 06/13] KVM: arm64: define
kvm_arch_gmem_supports_no_direct_map()
From: Patrick Roy <patrick.roy@...ux.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@...nel.org>
Signed-off-by: Patrick Roy <patrick.roy@...ux.dev>
Signed-off-by: Nikita Kalyazin <kalyazin@...zon.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 ac7f970c7883..d431ca7d4fc9 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>
@@ -1654,5 +1655,17 @@ 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(void)
+{
+ /*
+ * 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 */
+
#endif /* __ARM64_KVM_HOST_H__ */
--
2.50.1
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