[PATCH 3/5] xen/riscv: make Svpbmt no longer a required extension

required_extensions[] panics at boot if Svpbmt is missing, which is a
problem on hardware that doesn't implement it. Xen already checks Svpbmt at
runtime in some places (vcpu_csr_init()), but not everywhere:
p2m_pte_from_mfn() and the PAGE_HYPERVISOR_NOCACHE/WC macros still set the
raw PTE_PBMT* encoding unconditionally.

Drop Svpbmt from required_extensions, and introduce pte_pbmt(), which masks
the requested PBMT encoding down to 0 when Svpbmt is unavailable, using it
in both remaining unguarded spots.

Assisted-by: Claude:claude-opus-5
Signed-off-by: Baptiste Le Duc <[email protected]>
---
 xen/arch/riscv/cpufeature.c       | 1 -
 xen/arch/riscv/include/asm/page.h | 8 ++++++--
 xen/arch/riscv/p2m.c              | 2 +-
 3 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/xen/arch/riscv/cpufeature.c b/xen/arch/riscv/cpufeature.c
index 92235fdfd5..900cb9d772 100644
--- a/xen/arch/riscv/cpufeature.c
+++ b/xen/arch/riscv/cpufeature.c
@@ -157,7 +157,6 @@ static const struct riscv_isa_ext_data __initconst required_extensions[] = {
     RISCV_ISA_EXT_DATA(zifencei),
     RISCV_ISA_EXT_DATA(zihintpause),
     RISCV_ISA_EXT_DATA(zbb),
-    RISCV_ISA_EXT_DATA(svpbmt),
 };
 
 static bool __init is_lowercase_extension_name(const char *str)
diff --git a/xen/arch/riscv/include/asm/page.h b/xen/arch/riscv/include/asm/page.h
index 5c02f64a17..6a3749526d 100644
--- a/xen/arch/riscv/include/asm/page.h
+++ b/xen/arch/riscv/include/asm/page.h
@@ -11,6 +11,7 @@
 #include <xen/types.h>
 
 #include <asm/atomic.h>
+#include <asm/cpufeature.h>
 #include <asm/page-bits.h>
 
 #define VPN_MASK                    (PAGETABLE_ENTRIES - 1UL)
@@ -54,6 +55,9 @@
 #define PAGE_HYPERVISOR_RX          (PTE_LEAF_DEFAULT | PTE_EXECUTABLE)
 
 #define PAGE_HYPERVISOR             PAGE_HYPERVISOR_RW
+
+#define pte_pbmt(pbmt) \
+    (riscv_isa_extension_available(NULL, RISCV_ISA_EXT_svpbmt) ? (pbmt) : 0UL)
 /*
  * PAGE_HYPERVISOR_NOCACHE is used for ioremap().
  *
@@ -61,8 +65,8 @@
  * is that IO is non-idempotent and strongly ordered, which makes it a good
  * candidate for mapping IOMEM.
  */
-#define PAGE_HYPERVISOR_NOCACHE     (PAGE_HYPERVISOR_RW | PTE_PBMT_IO)
-#define PAGE_HYPERVISOR_WC          (PAGE_HYPERVISOR_RW | PTE_PBMT_NOCACHE)
+#define PAGE_HYPERVISOR_NOCACHE     (PAGE_HYPERVISOR_RW | pte_pbmt(PTE_PBMT_IO))
+#define PAGE_HYPERVISOR_WC          (PAGE_HYPERVISOR_RW | pte_pbmt(PTE_PBMT_NOCACHE))
 
 /*
  * The PTE format does not contain the following bits within itself;
diff --git a/xen/arch/riscv/p2m.c b/xen/arch/riscv/p2m.c
index 11dc289f0f..f6e635ec1d 100644
--- a/xen/arch/riscv/p2m.c
+++ b/xen/arch/riscv/p2m.c
@@ -683,7 +683,7 @@ static pte_t p2m_pte_from_mfn(mfn_t mfn, p2m_type_t t,
         switch ( t )
         {
         case p2m_mmio_direct_io:
-            e.pte |= PTE_PBMT_IO;
+            e.pte |= pte_pbmt(PTE_PBMT_IO);
             break;
 
         default: