[PATCH v15 05/27] riscv: usercfi state for task and save/restore of CSR_SSP on trap entry/exit

[Radim Krčmář]

2025-05-15T09:28:25+02:00, Alexandre Ghiti <alex at ghiti.fr>:
> On 06/05/2025 12:10, Radim Krčmář wrote:
>> 2025-05-02T16:30:36-07:00, Deepak Gupta <debug at rivosinc.com>:
>>> diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
>>> @@ -91,6 +91,32 @@
>>> +.macro restore_userssp tmp
>>> +	ALTERNATIVE("nops(2)",
>>> +		__stringify(				\
>>> +		REG_L \tmp, TASK_TI_USER_SSP(tp);	\
>>> +		csrw CSR_SSP, \tmp),
>>> +		0,
>>> +		RISCV_ISA_EXT_ZICFISS,
>>> +		CONFIG_RISCV_USER_CFI)
>>> +.endm
>> Do we need to emit the nops when CONFIG_RISCV_USER_CFI isn't selected?
>>
>> (Why not put #ifdef CONFIG_RISCV_USER_CFI around the ALTERNATIVES?)
>
> The alternatives are used to create a generic kernel that contains the 
> code for a large number of extensions and only enable it at runtime 
> depending on the platform capabilities. This way distros can ship a 
> single kernel that works on all platforms.

Yup, and if a kernel is compiled without CONFIG_RISCV_USER_CFI, the nops
will only enlarge the binary and potentially slow down execution.
In other words, why we don't do something like this

 (!CONFIG_RISCV_USER_CFI ? "" :
   (RISCV_ISA_EXT_ZICFISS ? __stringify(...) : "nops(x)"))

instead of the current

 (CONFIG_RISCV_USER_CFI &&
    RISCV_ISA_EXT_ZICFISS ? __stringify(...) : "nops(x)")

It could be a new preprocessor macro in case we wanted to make it nice,
but it's probably not a common case, so an ifdef could work as well.

Do we just generally not care about such minor optimizations?

(If we wanted to go an extra mile, we could also keep the nops when both
 CONFIG_RISCV_USER_CFI and RISCV_ISA_EXT_ZICFISS are present, but
 command line riscv_nousercfi disabled backward cfi.)

Thanks.