xen/riscv: skip disabled CPU nodes when probing ISA extensions

riscv_fill_hwcap_from_isa_string() intersects the riscv,isa of every CPU node under /cpus, including the ones with status = "disabled". Such a hart is never used by Xen, yet it can still remove extensions from the host ISA: e.g. a disabled hart without the H extension makes Xen panic with "Xen requires extension: h", even though every hart Xen runs on implements it.

Skip CPU nodes which aren't available, both there and in has_isa_extensions_property(), so that the two agree on which harts are taken into account.

---
Changes in v2:
- Skip the disabled CPU nodes in has_isa_extensions_property() as well.
- Remove the comment, as the explanation in the commit description is sufficient and avoids duplicating the same comment in has_isa_extensions_property().
- Update the last paragraph of the commit message: add info about has_isa_extensions_property().
---

---
 xen/arch/riscv/cpufeature.c | 6 ++++++
 1 file changed, 6 insertions(+)

index dff590f68a30..9767ba0b5824 100644
@@ -477,6 +477,9 @@ static void __init riscv_fill_hwcap_from_isa_string(void)
         if ( !dt_device_type_is_equal(cpu, "cpu") )
             continue;
 
+        if ( !dt_device_is_available(cpu) )
+            continue;
+
         if ( dt_get_cpuid_from_node(cpu, &cpuid) < 0 )
             continue;
 

@@ -519,6 +522,9 @@ static bool __init has_isa_extensions_property(void)
         if ( !dt_device_type_is_equal(cpu, "cpu") )
             continue;
 
+        if ( !dt_device_is_available(cpu) )
+            continue;
+
         if ( dt_property_read_string(cpu, "riscv,isa-extensions", &isa) )
             continue;
 

2.56.0

riscv_isa_parse_string() is meant to reject a riscv,isa string which doesn't start with "rv32" or "rv64" (depending on Xen's bitness), but the checks are written with && instead of ||, so a string is rejected only when both characters of a pair are wrong. As a result, e.g. "xv64i" or "rv62i" is accepted on RV64. Also, for a string as short as "rv", isa[3] is read past the terminating NUL.

Use || so that each character is checked.

---
Changes in v2:
- Add A-by and R-by.
---

---
 xen/arch/riscv/cpufeature.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

index 9767ba0b5824..b1c614867cd8 100644
@@ -261,14 +261,14 @@ static void __init match_isa_ext(const char *name, const char *name_end,
 static int __init riscv_isa_parse_string(const char *isa,
                                          unsigned long *out_bitmap)
 {
-    if ( (isa[0] != 'r') && (isa[1] != 'v') )
+    if ( (isa[0] != 'r') || (isa[1] != 'v') )
         return -EINVAL;
 
 #if defined(CONFIG_RISCV_32)
-    if ( isa[2] != '3' && isa[3] != '2' )
+    if ( (isa[2] != '3') || (isa[3] != '2') )
         return -EINVAL;
 #elif defined(CONFIG_RISCV_64)
-    if ( isa[2] != '6' && isa[3] != '4' )
+    if ( (isa[2] != '6') || (isa[3] != '4') )
         return -EINVAL;
 #else
 # error "unsupported RISC-V bitness"

2.56.0