Re: [PATCH v3 2/2] x86/tdx: Fix zero-extension for 32-bit port I/O

[Edgecombe, Rick P]

On Wed, 2026-05-27 at 13:05 +0100, Kiryl Shutsemau (Meta) wrote:
> +	/*
> +	 * IN writes the result into a sub-register of RAX. Only the
> +	 * 32-bit form zero-extends; the smaller forms leave the upper
> +	 * bits untouched:
> +	 *
> +	 *   insn  dest  size  bits written     bits preserved
> +	 *   inb   AL    1     RAX[ 7: 0]       RAX[63: 8]
> +	 *   inw   AX    2     RAX[15: 0]       RAX[63:16]
> +	 *   inl   EAX   4     RAX[63: 0]       (none, zero-extended)

We are working on getting the GHCI spec amended to clarify who is supposed to do
this zero-extending and masking, host or guest. For this and the similar
tdvmcalls. The process involves getting all VMMs in agreement.

Today I think the spec doesn't say to *not* do it, so I think it is reasonable
to merge this, but there is some small risk of complications depending on how
that discussion goes.

> +	 *
> +	 * 'mask' only covers the low 'size' bytes, which is exactly the
> +	 * range affected for size 1 and 2. For size 4 the write also
> +	 * clears RAX[63:32], so widen the clear-mask.
> +	 */