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

[Dave Hansen]

On 5/27/26 05:05, Kiryl Shutsemau (Meta) wrote:
...
> -	/* Update part of the register affected by the emulated instruction */
> -	regs->ax &= ~mask;
> +	/*
> +	 * 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)
> +	 *
> +	 * '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.
> +	 */
> +	if (size == 4)
> +		regs->ax = 0;
> +	else
> +		regs->ax &= ~mask;
> +

Is there any way we could do this with fewer comments and more code?

I mean, there's only three cases. Why have;

	u64 mask = GENMASK(BITS_PER_BYTE * size - 1, 0);

When there are only 3 possible cases:

	1 => 0xf
	2 => 0xff
	4 => 0xffff

and one of those cases needs a special case on top of it.

Maybe something like this?

	/* Clear out part of RAX so part of args.r11 can be OR'd in: */
	switch (size) {
	case 1:
		/* inb consumes lower 8 bits of r11: */
		regs->ax &= ~GENMASK_ULL(7, 0);
		args.r11 &=  GENMASK_ULL(7, 0);
		break;
	case 2:
		/* inw consumes lower 16 bits of r11: */
		regs->ax &= ~GENMASK_ULL(15, 0);
		args.r11 &=  GENMASK_ULL(15, 0);
		break;
	case 4:
		/* inl is weird and zeros the whole register: */
		regs->ax &= ~GENMASK_ULL(63, 0);
		/* But only consumes 32-bits from r11: */
		args.r11 &=  GENMASK_ULL(31, 0);
		break;
	default:
		/* Probable TDX module bug. Illegal in[bwl] size: */
		WARN_ON_ONCE(1);
		success = 0;
	}

	if (success)
		regs->ax |= args.r11;

It might need a temporary variable for args.r11, but you get the point.
That's basically the data from the comment but written as code. tdx.c |   29 ++++++++++++++++++++++++++++-
 1 file changed, 28 insertions(+), 1 deletion(-)