Re: [PATCH v2] xen/riscv: add H extenstion to -march

[Jan Beulich <jbeulich@xxxxxxxx>]

On 25.03.2025 14:02, Oleksii Kurochko wrote:
> 
> On 3/25/25 12:52 PM, Jan Beulich wrote:
>> On 25.03.2025 12:48, Oleksii Kurochko wrote:
>>> On 3/24/25 1:31 PM, Jan Beulich wrote:
>>>> On 21.03.2025 17:17, Oleksii Kurochko wrote:
>>>>> H provides additional instructions and CSRs that control the new stage of
>>>>> address translation and support hosting a guest OS in virtual S-mode
>>>>> (VS-mode).
>>>>>
>>>>> According to the Unprivileged Architecture (version 20240411) 
>>>>> specification:
>>>>> ```
>>>>> Table 74 summarizes the standardized extension names. The table also 
>>>>> defines
>>>>> the canonical order in which extension names must appear in the name 
>>>>> string,
>>>>> with top-to-bottom in table indicating first-to-last in the name string, 
>>>>> e.g.,
>>>>> RV32IMACV is legal, whereas RV32IMAVC is not.
>>>>> ```
>>>>> According to Table 74, the h extension is placed last in the one-letter
>>>>> extensions name part of the ISA string.
>>>>>
>>>>> `h` is a standalone extension based on the patch [1] but it wasn't so
>>>>> before.
>>>>> As the minimal supported GCC version to build Xen for RISC-V is 12.2.0,
>>>>> and for that version it will be needed to encode H extensions instructions
>>>>> explicitly by checking if __risv_h is defined.
>>>> Leaving aside the typo, what is this about? There's no use of __riscv_h in
>>>> the patch here, and ...
>>> It is going to be used in future 
>>> patches:https://gitlab.com/xen-project/people/olkur/xen/-/blob/riscv-next-upstreaming/xen/arch/riscv/p2m.c?ref_type=heads#L32
>> For this and ...
>>
>>>>> @@ -25,10 +24,13 @@ $(eval $(1) := \
>>>>>           $(call as-insn,$(CC) $(riscv-generic-flags)_$(1),$(value 
>>>>> $(1)-insn),_$(1)))
>>>>>    endef
>>>>>    
>>>>> +h-insn := "hfence.gvma"
>>>>> +$(call check-extension,h)
>>>> ... this, if it fails, will not have any effect on the build right now
>>>> afaics.
>>> No, it won't have any affect now as instruction from H extension isn't used 
>>> now.
>>> But it will be 
>>> neededforhttps://lore.kernel.org/xen-devel/dae753618491b2a6e42f7ed3f24190d0dc13fe3f.1740754166.git.Slavisa.Petrovic@xxxxxxxxx/
>>> and for p2m changes mentioned above.
>> ... this both being future work, it might help if it could be made clear
>> right here how things are going to work (with both gcc12 and up-to-date
>> gcc).
> 
> I can update the commit message with the following:
> ```
> If 'H' extension is supported by compiler then __riscv_h will be defined by
> compiler (for gcc version >= 13.1).
> For gcc12 it will be needed to:
> #ifdef __riscv_h
>   asm volatile ("h extension instruction");
> #else
>   asm volatile ("|.insn ..."); #endif ```

Okay, that's what I was concerned about. __riscv_h is a compiler indication.
It means nothing about H extension insns being supported by the assembler
(except perhaps for Clang's integrated one). The check-extension thing in
the Makefile will actually check both in one go. Yet then the hfence.* insns
have been in binutils since 2.38, i.e. pre-dating gcc12.

> Or probably it will be easier not to ifdef-ing 
> everything with __riscv_h but just return back a workaround with the 
> following changes: ``` $ git diff diff --git a/xen/arch/riscv/arch.mk 
> b/xen/arch/riscv/arch.mk index f29ad332c1..3bd64e7e51 100644 --- 
> a/xen/arch/riscv/arch.mk +++ b/xen/arch/riscv/arch.mk @@ -24,13 +24,17 
> @@ $(eval $(1) := \ $(call as-insn,$(CC) 
> $(riscv-generic-flags)_$(1),$(value $(1)-insn),_$(1))) endef -h-insn := 
> "hfence.gvma" -$(call check-extension,h) + 
> +h-extension-name-$(CONFIG_CC_IS_GCC) := $(call cc-ifversion,-lt,1301, 
> hh, h) +h-extension-name-$(CONFIG_CC_IS_CLANG) := h + 
> +$(h-extension-name-y)-insn := "hfence.gvma" +$(call 
> check-extension,$(h-extension-name-y)) zihintpause-insn := "pause" 
> $(call check-extension,zihintpause) -extensions := $(h) $(zihintpause) 
> _zicsr_zifencei_zbb +extensions := $($(h-extension-name-y)) 
> $(zihintpause) _zicsr_zifencei_zbb extensions := $(subst 
> $(space),,$(extensions)) ``` I prefer more a little bit the second 
> option with having the workaround for GCC version. ~ Oleksii |

I fear this ended up unreadable.

Jan