target/riscv: implement svade

'svade' is a RVA22S64 profile requirement, a profile we're going to add
shortly. It is a named feature (i.e. not a formal extension, not defined
in riscv,isa DT at this moment) defined in [1] as:

"Page-fault exceptions are raised when a page is accessed when A bit is
clear, or written when D bit is clear.".

As far as the spec goes, 'svade' is one of the two distinct modes of
handling PTE_A and PTE_D. The other way, i.e. update PTE_A/PTE_D when
they're cleared, is defined by the 'svadu' extension. Checking
cpu_helper.c, get_physical_address(), we can verify that QEMU is
compliant with that: we will update PTE_A/PTE_D if 'svadu' is enabled,
or throw a page-fault exception if 'svadu' isn't enabled.

So, as far as we're concerned, 'svade' translates to 'svadu must be
disabled'.

We'll implement it like 'zic64b': an internal flag that profiles can
enable. The flag will not be exposed to users.

[1] https://github.com/riscv/riscv-profiles/blob/main/profiles.adoc

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20231218125334.37184-20-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/cpu.c

@@ -1444,6 +1444,7 @@ const RISCVCPUMultiExtConfig riscv_cpu_experimental_exts[] = {
 };
 
 const RISCVCPUMultiExtConfig riscv_cpu_named_features[] = {
+    MULTI_EXT_CFG_BOOL("svade", svade, true),
     MULTI_EXT_CFG_BOOL("zic64b", zic64b, true),
 
     DEFINE_PROP_END_OF_LIST(),