Re: [PATCH v2] x86/altcall: use an union as register type for function parameters

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 22/02/2024 4:55 pm, Jan Beulich wrote:
> On 22.02.2024 17:44, Roger Pau Monne wrote:
>> --- a/xen/arch/x86/include/asm/alternative.h
>> +++ b/xen/arch/x86/include/asm/alternative.h
>> @@ -167,9 +167,25 @@ extern void alternative_branches(void);
>>  #define ALT_CALL_arg5 "r8"
>>  #define ALT_CALL_arg6 "r9"
>>  
>> +#ifdef CONFIG_CC_IS_CLANG
>> +/*
>> + * Use an union with an unsigned long in order to prevent clang from skipping a
>> + * possible truncation of the value.  By using the union any truncation is
>> + * carried before the call instruction.
>> + * https://github.com/llvm/llvm-project/issues/82598
>> + */
> I think it needs saying that this is relying on compiler behavior not
> mandated by the standard, thus explaining why it's restricted to
> Clang (down the road we may even want to restrict to old versions,
> assuming they fix the issue at some point). Plus also giving future
> readers a clear understanding that if something breaks with this, it's
> not really a surprise.
>
> Aiui this bug is only a special case of the other, much older one, so
> referencing that one here too would seem advisable.
>
> As a nit: I think it is "a union" (spelling according to pronunciation),
> and I guess the title could now do with saying "optionally" or
> mentioning Clang or some such.

Yes.  "a union" is the right form here.

~Andrew

P.S. Spoken, "an union" would come across as "an onion", which is
something very different.

P.P.S.  As I noted to Matthew concerning
https://mjg59.dreamwidth.org/66907.html

"There's an interesting sidechannel in your blog post.  You seem to call
it an S-O-C and not a soc(k), based on the "an""