Re: [kvm-unit-tests PATCH v3 7/7] s390x: Add test for STFLE interpretive execution (format-0)

Nico Boehr
2024-08-27T14:08:50Z

Quoting Nina Schoetterl-Glausch (2024-06-20 16:17:00)
[...]
> diff --git a/lib/s390x/asm/facility.h b/lib/s390x/asm/facility.h
> index a66fe56a..2bad05c5 100644
> --- a/lib/s390x/asm/facility.h
> +++ b/lib/s390x/asm/facility.h
> @@ -27,12 +27,20 @@ static inline void stfl(void)
> asm volatile(" stfl 0(0)\n" : : : "memory");
> }
>
> -static inline void stfle(uint64_t *fac, unsigned int nb_doublewords)
> +static inline unsigned int stfle(uint64_t *fac, unsigned int nb_doublewords)

Why unsigned int?

[...]
> diff --git a/s390x/snippets/c/stfle.c b/s390x/snippets/c/stfle.c
> new file mode 100644
> index 00000000..eb024a6a
> --- /dev/null
> +++ b/s390x/snippets/c/stfle.c
[...]
> +int main(void)
> +{
> + const unsigned int max_fac_len = 8;
> + uint64_t res[max_fac_len + 1];
> +
> + res[0] = max_fac_len - 1;
> + asm volatile ( "lg 0,%[len]\n"
> + " stfle %[fac]\n"
> + " stg 0,%[len]\n"
> + : [fac] "=QS"(*(uint64_t(*)[max_fac_len])&res[1]),

Out of curiosity:

Q = Memory reference without index register and with short displacement
S = Memory reference without index register but with long displacement

Which one is it?

And: is long displacement even appropriate here?

The cast also is hard to understand. Since this is not super high
performance code, do we just want to clobber memory so this gets a bit
easier to understand?

> + [len] "+RT"(res[0])

Same question about RT as above.

[...]
> diff --git a/s390x/stfle-sie.c b/s390x/stfle-sie.c
> new file mode 100644
> index 00000000..a3e7f1c9
> --- /dev/null
> +++ b/s390x/stfle-sie.c
[...]
> +static struct guest_stfle_res run_guest(void)
> +{
> + struct guest_stfle_res res;
> + uint64_t guest_stfle_addr;
> +
> + sie(&vm);
> + assert(snippet_is_force_exit_value(&vm));
> + guest_stfle_addr = snippet_get_force_exit_value(&vm);
> + res.mem = &vm.guest_mem[guest_stfle_addr];
> + memcpy(&res.reg, res.mem, sizeof(res.reg));
> + res.len = (res.reg & 0xff) + 1;

If I'm not mistaken, you subtracted 1 in the guest. Here you add it again.
Is there a particular reason why?