Re: [PATCH 1/5] x86/altcall: allow compound types to be passed

["Daniel P. Smith" <dpsmith@xxxxxxxxxxxxxxxxxxxx>]

On 11/5/21 9:55 AM, Andrew Cooper wrote:
> From: Jan Beulich <jbeulich@xxxxxxxx>
>
> Replace the conditional operator in ALT_CALL_ARG(), which was intended
> to limit usable types to scalar ones, by a size check. Some restriction
> here is necessary to make sure we don't violate the ABI's calling
> conventions, but limiting to scalar types was both too restrictive
> (disallowing e.g. guest handles) and too permissive (allowing e.g.
> __int128_t).
>
> Note that there was some anomaly with that conditional operator anyway:
> Something - I don't recall what - made it impossible to omit the middle
> operand.
>
> Code-generation-wise this has the effect of removing certain zero- or
> sign-extending in some altcall invocations. This ought to be fine as the
> ABI doesn't require sub-sizeof(int) values to be extended, except when
> passed through an ellipsis. No functions subject to altcall patching has
> a variable number of arguments, though.
>
> Requested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Reviewed-by: Daniel P. Smith <dpsmith@xxxxxxxxxxxxxxxxxxxx>