Re: [PATCH v2 2/2] x86/tdx: Fix zero-extension for 32-bit port I/O

[Kiryl Shutsemau]

On Tue, May 12, 2026 at 06:14:13PM -0700, Dave Hansen wrote:
> On 4/28/26 05:56, Kiryl Shutsemau (Meta) wrote:
> > +	if (size == 4)
> > +		regs->ax = 0;
> > +	else
> > +		regs->ax &= ~mask;
> 
> I haven't thought about this _that_ much, but this feels wrong. Why is
> is 4 so special cased?
> 
> Also, what _are_ the limits on the registers that 'in' can be used on?
> 
> RAX - n/a, no 64-bit I/O
> EAX - size=4
> AX  - size=2
> AH  - n/a no encoding for inb
> AL  - size=1
> 
> I'd find this much easier to grasp if there was a nice table of what the
> registers, sizes, and masks ended up being usable. As usual, x86 is
> "fun" here.

How about this for the comment:

        /*
         * 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.
         */

-- 
  Kiryl Shutsemau / Kirill A. Shutemov