[Eclair false positive] Re: [PATCH] x86/msr: Rework wrmsr_safe() using asm goto()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 21/05/2025 3:36 pm, Andrew Cooper wrote:
> diff --git a/xen/arch/x86/include/asm/msr.h b/xen/arch/x86/include/asm/msr.h
> index 0d3b1d637488..4c4f18b3a54d 100644
> --- a/xen/arch/x86/include/asm/msr.h
> +++ b/xen/arch/x86/include/asm/msr.h
> @@ -69,20 +69,20 @@ static inline void wrmsr_ns(uint32_t msr, uint32_t lo, 
> uint32_t hi)
>  /* wrmsr with exception handling */
>  static inline int wrmsr_safe(unsigned int msr, uint64_t val)
>  {
> -    int rc;
> -    uint32_t lo, hi;
> -    lo = (uint32_t)val;
> -    hi = (uint32_t)(val >> 32);
> -
> -    __asm__ __volatile__(
> -        "1: wrmsr\n2:\n"
> -        ".section .fixup,\"ax\"\n"
> -        "3: movl %5,%0\n; jmp 2b\n"
> -        ".previous\n"
> -        _ASM_EXTABLE(1b, 3b)
> -        : "=&r" (rc)
> -        : "c" (msr), "a" (lo), "d" (hi), "0" (0), "i" (-EFAULT));
> -    return rc;
> +    uint32_t lo = val, hi = val >> 32;
> +
> +    asm_inline goto (
> +        "1: wrmsr\n\t"
> +        _ASM_EXTABLE(1b, %l[fault])
> +        :
> +        : "a" (lo), "c" (msr), "d" (hi)
> +        :
> +        : fault );
> +
> +    return 0;
> +
> + fault:
> +    return -EFAULT;
>  }

It turns out this is the first piece of Eclair-scanned code using asm goto.

https://gitlab.com/xen-project/hardware/xen-staging/-/jobs/10108558677
(The run also contained an equivalent change to xsetbv())

We're getting R1.1 and R2.6 violations.

R1.1 complains about [STD.adrslabl] "label address" not being valid C99.

R2.6 complains about unused labels.

I expect this means that Eclair doesn't know how to interpret asm goto()
yet.  The labels listed are reachable from inside the asm block.

>From a qualification point of view, this allows for some extensive
optimisations dropping emitted code.

~Andrew