[PATCH v2 0/5] ... and FWB for all

[PATCH v2 0/5] ... and FWB for all

[Marc Zyngier]

[Yes, I have since learned that FWB doesn't always mean "Force Write
 Back". I'm not judging.]

This is v2 for this small series teaching pKVM about the benefits
(haha) of using FWB on the host S2 page-tables.

* From v1 [1]:

  - Dropped the stage2_pte_cacheable() hack after both Will and Alex
    made the valid point that I can't read

  - Killed stage2_has_fwb() altogether

  - Collected RBs and TBs, thanks to Joey and Fuad!

[1] https://lore.kernel.org/r/20260119105651.255693-1-maz@kernel.org

Marc Zyngier (5):
  arm64: Add MT_S2{,_FWB}_AS_S1 encodings
  KVM: arm64: Add KVM_PGTABLE_S2_AS_S1 flag
  KVM: arm64: Switch pKVM host S2 over to KVM_PGTABLE_S2_AS_S1
  KVM: arm64: Kill KVM_PGTABLE_S2_NOFWB
  KVM: arm64: Simplify PAGE_S2_MEMATTR

 arch/arm64/include/asm/kvm_pgtable.h  |  7 +++----
 arch/arm64/include/asm/memory.h       | 11 ++++++++---
 arch/arm64/include/asm/pgtable-prot.h |  4 ++--
 arch/arm64/kvm/hyp/nvhe/mem_protect.c |  4 +++-
 arch/arm64/kvm/hyp/pgtable.c          | 26 ++++++++++++++------------
 5 files changed, 30 insertions(+), 22 deletions(-)

-- 
2.47.3

[PATCH v2 1/5] arm64: Add MT_S2{,_FWB}_AS_S1 encodings

[Marc Zyngier]

pKVM usage of S2 translation on the host is purely for isolation
purposes, not translation. To that effect, the memory attributes
being used must be that of S1.

With FWB=0, this is easily achieved by using the Normal Cacheable
type (which is the weakest possible memory type) at S2, and let S1
pick something stronger as required.

With FWB=1, the attributes are combined in a different way, and we
cannot arbitrarily use Normal Cacheable. We can, however, use a
memattr encoding that indicates that the final attributes are that
of Stage-1.

Add these encoding and a few pointers to the relevant parts of the
specification. It might come handy some day.

Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Reviewed-by: Fuad Tabba <tabba@google.com>
Tested-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 arch/arm64/include/asm/memory.h | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
index 9d54b2ea49d66..a2b7a33966ff1 100644
--- a/arch/arm64/include/asm/memory.h
+++ b/arch/arm64/include/asm/memory.h
@@ -175,19 +175,24 @@
 #define MT_DEVICE_nGnRE		4
 
 /*
- * Memory types for Stage-2 translation
+ * Memory types for Stage-2 translation when HCR_EL2.FWB=0. See R_HMNDG,
+ * R_TNHFM, R_GQFSF and I_MCQKW for the details on how these attributes get
+ * combined with Stage-1.
  */
 #define MT_S2_NORMAL		0xf
 #define MT_S2_NORMAL_NC		0x5
 #define MT_S2_DEVICE_nGnRE	0x1
+#define MT_S2_AS_S1		MT_S2_NORMAL
 
 /*
- * Memory types for Stage-2 translation when ID_AA64MMFR2_EL1.FWB is 0001
- * Stage-2 enforces Normal-WB and Device-nGnRE
+ * Memory types for Stage-2 translation when HCR_EL2.FWB=1. Stage-2 enforces
+ * Normal-WB and Device-nGnRE, unless we actively say that S1 wins. See
+ * R_VRJSW and R_RHWZM for details.
  */
 #define MT_S2_FWB_NORMAL	6
 #define MT_S2_FWB_NORMAL_NC	5
 #define MT_S2_FWB_DEVICE_nGnRE	1
+#define MT_S2_FWB_AS_S1		7
 
 #ifdef CONFIG_ARM64_4K_PAGES
 #define IOREMAP_MAX_ORDER	(PUD_SHIFT)
-- 
2.47.3

[PATCH v2 2/5] KVM: arm64: Add KVM_PGTABLE_S2_AS_S1 flag

[Marc Zyngier]

Plumb the MT_S2{,_FWB}_AS_S1 memory types into the KVM_S2_MEMATTR()
macro with a new KVM_PGTABLE_S2_AS_S1 flag.

Nobody selects it yet.

Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Reviewed-by: Fuad Tabba <tabba@google.com>
Tested-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 arch/arm64/include/asm/kvm_pgtable.h |  2 ++
 arch/arm64/kvm/hyp/pgtable.c         | 14 +++++++++++++-
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
index fc02de43c68dd..9ce51a637da0a 100644
--- a/arch/arm64/include/asm/kvm_pgtable.h
+++ b/arch/arm64/include/asm/kvm_pgtable.h
@@ -232,10 +232,12 @@ struct kvm_pgtable_mm_ops {
  * @KVM_PGTABLE_S2_NOFWB:	Don't enforce Normal-WB even if the CPUs have
  *				ARM64_HAS_STAGE2_FWB.
  * @KVM_PGTABLE_S2_IDMAP:	Only use identity mappings.
+ * @KVM_PGTABLE_S2_AS_S1:	Final memory attributes are that of Stage-1.
  */
 enum kvm_pgtable_stage2_flags {
 	KVM_PGTABLE_S2_NOFWB			= BIT(0),
 	KVM_PGTABLE_S2_IDMAP			= BIT(1),
+	KVM_PGTABLE_S2_AS_S1			= BIT(2),
 };
 
 /**
diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
index 947ac1a951a5b..c52a24c15ff28 100644
--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -659,7 +659,19 @@ void kvm_tlb_flush_vmid_range(struct kvm_s2_mmu *mmu,
 	}
 }
 
-#define KVM_S2_MEMATTR(pgt, attr) PAGE_S2_MEMATTR(attr, stage2_has_fwb(pgt))
+#define KVM_S2_MEMATTR(pgt, attr)					\
+	({								\
+		kvm_pte_t __attr;					\
+									\
+		if ((pgt)->flags & KVM_PGTABLE_S2_AS_S1)		\
+			__attr = PAGE_S2_MEMATTR(AS_S1,			\
+						 stage2_has_fwb(pgt));	\
+		else							\
+			__attr = PAGE_S2_MEMATTR(attr,			\
+						 stage2_has_fwb(pgt));	\
+									\
+		__attr;							\
+	})
 
 static int stage2_set_xn_attr(enum kvm_pgtable_prot prot, kvm_pte_t *attr)
 {
-- 
2.47.3

[PATCH v2 3/5] KVM: arm64: Switch pKVM host S2 over to KVM_PGTABLE_S2_AS_S1

[Marc Zyngier]

Since we have the basics to use the S1 memory attributes as the
final ones with FWB, flip the host over to that when FWB is present.

Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Tested-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 arch/arm64/kvm/hyp/nvhe/mem_protect.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
index 49db32f3ddf71..38f66a56a7665 100644
--- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
+++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
@@ -19,7 +19,7 @@
 #include <nvhe/mem_protect.h>
 #include <nvhe/mm.h>
 
-#define KVM_HOST_S2_FLAGS (KVM_PGTABLE_S2_NOFWB | KVM_PGTABLE_S2_IDMAP)
+#define KVM_HOST_S2_FLAGS (KVM_PGTABLE_S2_AS_S1 | KVM_PGTABLE_S2_IDMAP)
 
 struct host_mmu host_mmu;
 
@@ -324,6 +324,8 @@ int __pkvm_prot_finalize(void)
 	params->vttbr = kvm_get_vttbr(mmu);
 	params->vtcr = mmu->vtcr;
 	params->hcr_el2 |= HCR_VM;
+	if (cpus_have_final_cap(ARM64_HAS_STAGE2_FWB))
+		params->hcr_el2 |= HCR_FWB;
 
 	/*
 	 * The CMO below not only cleans the updated params to the
-- 
2.47.3

[PATCH v2 4/5] KVM: arm64: Kill KVM_PGTABLE_S2_NOFWB

[Marc Zyngier]

Nobody is using this flag anymore, so remove it. This allows
some cleanup by removing stage2_has_fwb(), which is can be replaced
by a direct check on the capability.

Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Reviewed-by: Fuad Tabba <tabba@google.com>
Tested-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 arch/arm64/include/asm/kvm_pgtable.h |  7 ++-----
 arch/arm64/kvm/hyp/pgtable.c         | 21 ++++++---------------
 2 files changed, 8 insertions(+), 20 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
index 9ce51a637da0a..2198b62428832 100644
--- a/arch/arm64/include/asm/kvm_pgtable.h
+++ b/arch/arm64/include/asm/kvm_pgtable.h
@@ -229,15 +229,12 @@ struct kvm_pgtable_mm_ops {
 
 /**
  * enum kvm_pgtable_stage2_flags - Stage-2 page-table flags.
- * @KVM_PGTABLE_S2_NOFWB:	Don't enforce Normal-WB even if the CPUs have
- *				ARM64_HAS_STAGE2_FWB.
  * @KVM_PGTABLE_S2_IDMAP:	Only use identity mappings.
  * @KVM_PGTABLE_S2_AS_S1:	Final memory attributes are that of Stage-1.
  */
 enum kvm_pgtable_stage2_flags {
-	KVM_PGTABLE_S2_NOFWB			= BIT(0),
-	KVM_PGTABLE_S2_IDMAP			= BIT(1),
-	KVM_PGTABLE_S2_AS_S1			= BIT(2),
+	KVM_PGTABLE_S2_IDMAP			= BIT(0),
+	KVM_PGTABLE_S2_AS_S1			= BIT(1),
 };
 
 /**
diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
index c52a24c15ff28..00e33a16494bd 100644
--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -631,14 +631,6 @@ u64 kvm_get_vtcr(u64 mmfr0, u64 mmfr1, u32 phys_shift)
 	return vtcr;
 }
 
-static bool stage2_has_fwb(struct kvm_pgtable *pgt)
-{
-	if (!cpus_have_final_cap(ARM64_HAS_STAGE2_FWB))
-		return false;
-
-	return !(pgt->flags & KVM_PGTABLE_S2_NOFWB);
-}
-
 void kvm_tlb_flush_vmid_range(struct kvm_s2_mmu *mmu,
 				phys_addr_t addr, size_t size)
 {
@@ -661,14 +653,13 @@ void kvm_tlb_flush_vmid_range(struct kvm_s2_mmu *mmu,
 
 #define KVM_S2_MEMATTR(pgt, attr)					\
 	({								\
+		bool __fwb = cpus_have_final_cap(ARM64_HAS_STAGE2_FWB);	\
 		kvm_pte_t __attr;					\
 									\
 		if ((pgt)->flags & KVM_PGTABLE_S2_AS_S1)		\
-			__attr = PAGE_S2_MEMATTR(AS_S1,			\
-						 stage2_has_fwb(pgt));	\
+			__attr = PAGE_S2_MEMATTR(AS_S1,	__fwb);		\
 		else							\
-			__attr = PAGE_S2_MEMATTR(attr,			\
-						 stage2_has_fwb(pgt));	\
+			__attr = PAGE_S2_MEMATTR(attr, __fwb);		\
 									\
 		__attr;							\
 	})
@@ -880,7 +871,7 @@ static bool stage2_unmap_defer_tlb_flush(struct kvm_pgtable *pgt)
 	 * system supporting FWB as the optimization is entirely
 	 * pointless when the unmap walker needs to perform CMOs.
 	 */
-	return system_supports_tlb_range() && stage2_has_fwb(pgt);
+	return system_supports_tlb_range() && cpus_have_final_cap(ARM64_HAS_STAGE2_FWB);
 }
 
 static void stage2_unmap_put_pte(const struct kvm_pgtable_visit_ctx *ctx,
@@ -1160,7 +1151,7 @@ static int stage2_unmap_walker(const struct kvm_pgtable_visit_ctx *ctx,
 		if (mm_ops->page_count(childp) != 1)
 			return 0;
 	} else if (stage2_pte_cacheable(pgt, ctx->old)) {
-		need_flush = !stage2_has_fwb(pgt);
+		need_flush = !cpus_have_final_cap(ARM64_HAS_STAGE2_FWB);
 	}
 
 	/*
@@ -1390,7 +1381,7 @@ int kvm_pgtable_stage2_flush(struct kvm_pgtable *pgt, u64 addr, u64 size)
 		.arg	= pgt,
 	};
 
-	if (stage2_has_fwb(pgt))
+	if (cpus_have_final_cap(ARM64_HAS_STAGE2_FWB))
 		return 0;
 
 	return kvm_pgtable_walk(pgt, addr, size, &walker);
-- 
2.47.3

[PATCH v2 5/5] KVM: arm64: Simplify PAGE_S2_MEMATTR

[Marc Zyngier]

Restore PAGE_S2_MEMATTR() to its former glory, keeping the use of
FWB as an implementation detail.

Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Reviewed-by: Fuad Tabba <tabba@google.com>
Tested-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 arch/arm64/include/asm/pgtable-prot.h | 4 ++--
 arch/arm64/kvm/hyp/pgtable.c          | 5 ++---
 2 files changed, 4 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/include/asm/pgtable-prot.h b/arch/arm64/include/asm/pgtable-prot.h
index 161e8660edddc..d27e8872fe3c8 100644
--- a/arch/arm64/include/asm/pgtable-prot.h
+++ b/arch/arm64/include/asm/pgtable-prot.h
@@ -109,10 +109,10 @@ static inline bool __pure lpa2_is_enabled(void)
 #define PAGE_KERNEL_EXEC	__pgprot(_PAGE_KERNEL_EXEC)
 #define PAGE_KERNEL_EXEC_CONT	__pgprot(_PAGE_KERNEL_EXEC_CONT)
 
-#define PAGE_S2_MEMATTR(attr, has_fwb)					\
+#define PAGE_S2_MEMATTR(attr)						\
 	({								\
 		u64 __val;						\
-		if (has_fwb)						\
+		if (cpus_have_final_cap(ARM64_HAS_STAGE2_FWB))		\
 			__val = PTE_S2_MEMATTR(MT_S2_FWB_ ## attr);	\
 		else							\
 			__val = PTE_S2_MEMATTR(MT_S2_ ## attr);		\
diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
index 00e33a16494bd..a6f7533ce4974 100644
--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -653,13 +653,12 @@ void kvm_tlb_flush_vmid_range(struct kvm_s2_mmu *mmu,
 
 #define KVM_S2_MEMATTR(pgt, attr)					\
 	({								\
-		bool __fwb = cpus_have_final_cap(ARM64_HAS_STAGE2_FWB);	\
 		kvm_pte_t __attr;					\
 									\
 		if ((pgt)->flags & KVM_PGTABLE_S2_AS_S1)		\
-			__attr = PAGE_S2_MEMATTR(AS_S1,	__fwb);		\
+			__attr = PAGE_S2_MEMATTR(AS_S1);		\
 		else							\
-			__attr = PAGE_S2_MEMATTR(attr, __fwb);		\
+			__attr = PAGE_S2_MEMATTR(attr);			\
 									\
 		__attr;							\
 	})
-- 
2.47.3

Re: [PATCH v2 3/5] KVM: arm64: Switch pKVM host S2 over to KVM_PGTABLE_S2_AS_S1

[Fuad Tabba]

On Fri, 23 Jan 2026 at 19:16, Marc Zyngier <maz@kernel.org> wrote:
>
> Since we have the basics to use the S1 memory attributes as the
> final ones with FWB, flip the host over to that when FWB is present.
>
> Reviewed-by: Joey Gouly <joey.gouly@arm.com>
> Tested-by: Fuad Tabba <tabba@google.com>
> Signed-off-by: Marc Zyngier <maz@kernel.org>

Reviewed-by: Fuad Tabba <tabba@google.com>

Cheers,
/fuad
> ---
>  arch/arm64/kvm/hyp/nvhe/mem_protect.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> index 49db32f3ddf71..38f66a56a7665 100644
> --- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> +++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> @@ -19,7 +19,7 @@
>  #include <nvhe/mem_protect.h>
>  #include <nvhe/mm.h>
>
> -#define KVM_HOST_S2_FLAGS (KVM_PGTABLE_S2_NOFWB | KVM_PGTABLE_S2_IDMAP)
> +#define KVM_HOST_S2_FLAGS (KVM_PGTABLE_S2_AS_S1 | KVM_PGTABLE_S2_IDMAP)
>
>  struct host_mmu host_mmu;
>
> @@ -324,6 +324,8 @@ int __pkvm_prot_finalize(void)
>         params->vttbr = kvm_get_vttbr(mmu);
>         params->vtcr = mmu->vtcr;
>         params->hcr_el2 |= HCR_VM;
> +       if (cpus_have_final_cap(ARM64_HAS_STAGE2_FWB))
> +               params->hcr_el2 |= HCR_FWB;
>
>         /*
>          * The CMO below not only cleans the updated params to the
> --
> 2.47.3
>

Re: [PATCH v2 0/5] ... and FWB for all

[Marc Zyngier]

On Fri, 23 Jan 2026 19:16:32 +0000, Marc Zyngier wrote:
> [Yes, I have since learned that FWB doesn't always mean "Force Write
>  Back". I'm not judging.]
> 
> This is v2 for this small series teaching pKVM about the benefits
> (haha) of using FWB on the host S2 page-tables.
> 
> * From v1 [1]:
> 
> [...]

Applied to next, thanks!

[1/5] arm64: Add MT_S2{,_FWB}_AS_S1 encodings
      commit: d4236f1ef270347b26ed34bcd0eb28d285b86bcb
[2/5] KVM: arm64: Add KVM_PGTABLE_S2_AS_S1 flag
      commit: 17d7b15131f3ed7fff29f2a43b6ead547e495653
[3/5] KVM: arm64: Switch pKVM host S2 over to KVM_PGTABLE_S2_AS_S1
      commit: a373930ec9406aeb9f82b5b78db56ca8d0919d99
[4/5] KVM: arm64: Kill KVM_PGTABLE_S2_NOFWB
      commit: 4f27fe82aa304c50b24f92da72b4895cf73b54ba
[5/5] KVM: arm64: Simplify PAGE_S2_MEMATTR
      commit: 65d00e37b17f46a21e7a25e0fb211a4c33274a83

Cheers,

	M.
-- 
Without deviation from the norm, progress is not possible.