Re: [PATCH v7 04/20] xen/riscv: introduce guest riscv,isa string

[Jan Beulich <jbeulich@xxxxxxxx>]

On 13.08.2026 17:37, Oleksii Kurochko wrote:
> On 8/13/26 9:19 AM, Jan Beulich wrote:
>> On 04.08.2026 17:47, Oleksii Kurochko wrote:
>>> @@ -120,29 +148,30 @@ static int __init dt_get_cpuid_from_node(const struct 
>>> dt_device_node *cpu,
>>>    * and strncmp() is used in match_isa_ext() to compare extension names 
>>> instead
>>>    * of strncasecmp().
>>>    */
>>> -const struct riscv_isa_ext_data __initconst riscv_isa_ext[] = {
>>> -    RISCV_ISA_EXT_DATA(i),
>>> -    RISCV_ISA_EXT_DATA(m),
>>> -    RISCV_ISA_EXT_DATA(a),
>>> -    RISCV_ISA_EXT_DATA(f),
>>> -    RISCV_ISA_EXT_DATA(d),
>>> -    RISCV_ISA_EXT_DATA(q),
>>> -    RISCV_ISA_EXT_DATA(c),
>>> -    RISCV_ISA_EXT_DATA(h),
>>> -    RISCV_ISA_EXT_DATA(zicntr),
>>> -    RISCV_ISA_EXT_DATA(zicsr),
>>> -    RISCV_ISA_EXT_DATA(zifencei),
>>> -    RISCV_ISA_EXT_DATA(zihintpause),
>>> -    RISCV_ISA_EXT_DATA(zihpm),
>>> -    RISCV_ISA_EXT_DATA(zba),
>>> -    RISCV_ISA_EXT_DATA(zbb),
>>> -    RISCV_ISA_EXT_DATA(zbs),
>>> -    RISCV_ISA_EXT_DATA(smaia),
>>> -    RISCV_ISA_EXT_DATA(smstateen),
>>> -    RISCV_ISA_EXT_DATA(ssaia),
>>> -    RISCV_ISA_EXT_DATA(sstc),
>>> -    RISCV_ISA_EXT_DATA(svade),
>>> -    RISCV_ISA_EXT_DATA(svpbmt),
>>> +static const struct riscv_isa_ext_entry __initconstrel riscv_isa_ext[] = {
>>> +    RISCV_ISA_EXT_ENTRY(i,            true),
>>> +    RISCV_ISA_EXT_ENTRY(m,            true),
>>> +    RISCV_ISA_EXT_ENTRY(a,            true),
>>> +    RISCV_ISA_EXT_ENTRY(f,            false),
>>> +    RISCV_ISA_EXT_ENTRY(d,            false),
>>> +    RISCV_ISA_EXT_ENTRY(q,            false),
>>> +    RISCV_ISA_EXT_ENTRY(c,            true),
>>> +    RISCV_ISA_EXT_ENTRY(v,            false),
>>> +    RISCV_ISA_EXT_ENTRY(h,            false),
>>> +    RISCV_ISA_EXT_ENTRY(zicntr,       true),
>>> +    RISCV_ISA_EXT_ENTRY(zicsr,        true),
>>> +    RISCV_ISA_EXT_ENTRY(zifencei,     true),
>>> +    RISCV_ISA_EXT_ENTRY(zihintpause,  true),
>>> +    RISCV_ISA_EXT_ENTRY(zihpm,        true),
>>> +    RISCV_ISA_EXT_ENTRY(zba,          true),
>>> +    RISCV_ISA_EXT_ENTRY(zbb,          true),
>>> +    RISCV_ISA_EXT_ENTRY(zbs,          true),
>>> +    RISCV_ISA_EXT_ENTRY(smaia,        true),
>>> +    RISCV_ISA_EXT_ENTRY(smstateen,    true),
>>> +    RISCV_ISA_EXT_ENTRY(ssaia,        true),
>>> +    RISCV_ISA_EXT_ENTRY(sstc,         false),
>>> +    RISCV_ISA_EXT_ENTRY(svade,        false),
>>> +    RISCV_ISA_EXT_ENTRY(svpbmt,       false),
>>>   };
>>
>> Just as an independent, up front remark after having looked at patch 16/17 of
>> the other series: Is a mere boolean going to suffice in the longer run? I 
>> could
>> see some extensions wanting exposing to only RV32 or only RV64 guests. E.g.
>> Zilsd is RV32-only, while Zqinx quite likely would want restricting to RV64.
> 
> Good point generally.
> 
> Right now the distinction can't be observed: RV32 isn't buildable 
> (#error "RV32 isn't supported" in asm/config.h), and guest XLEN is 
> hard-wired to host XLEN — build_guest_isa_str() emits the rv32/rv64 
> prefix from the Kconfig symbol, and riscv_isa_parse_string() rejects a 
> host ISA string of the other width.
> 
> On top of that, extensions with an architectural XLEN restriction are 
> already filtered out for free: compute_guest_isa() masks the table 
> against the host bitmap, so an RV32-only extension like Zilsd can't have 
> its bit set on an RV64 build regardless of what the table says. The 
> boolean only ever subtracts from what the host actually reports.
> 
> That leaves purely policy-driven per-XLEN restrictions — e.g. exposing 
> Zqinx to RV64 guests but not RV32 ones, since Zqinx is architecturally 
> defined for both.

Is it? Can you point me at a spec, as I wasn't able to find any?

> Those only become meaningful once guest XLEN can 
> differ from host XLEN (i.e. hstatus.VSXL support), and at that point the 
> shared guest_isa bitmap has to become per-domain as well, as the comment 
> above it already notes.

Not necessarily - you could have an RV64 one and an RV32 one.

> So I'd rather keep the plain bool for now and widen it to a flags field 
> when there's an actual case; it's a mechanical change to the struct, the 
> macro and the single test in compute_guest_isa(), all local to 
> cpufeature.c. I can add a comment stating the "guest XLEN == host XLEN" 
> assumption so the reason is on record. If you'd prefer it as flags from 
> the start I don't mind doing it now either. I just don't have a way to 
> give either value a meaning yet. So if to do that now I would suggest 
> the following:

I'm fine with a comment, and I'd also be fine with the more extensive
logic. Main question being whether "guest XLEN < host XLEN" support is
meant to be added within the foreseeable future.

Jan