Re: [PATCH v4 3/3] x86/tdx: Fix zero-extension for 32-bit port I/O

[Binbin Wu]

On 6/4/2026 10:47 PM, Kiryl Shutsemau (Meta) wrote:
> According to x86 architecture rules, 32-bit operations zero-extend the
> result to 64 bits. The current implementation of handle_in() only masks
> the lower 32 bits, which preserves the upper 32 bits of RAX when a
> 32-bit port IN instruction is emulated.
> 
> Use insn_assign_reg() to write the result back into RAX with proper
> partial-register-write semantics: 1- and 2-byte forms leave the upper
> bits untouched, the 4-byte form zero-extends to the full register.
> 
> Fixes: 03149948832a ("x86/tdx: Port I/O: Add runtime hypercalls")
> Reported-by: Borys Tsyrulnikov <tsyrulnikov.borys@xxxxxxxxx>
> Link: https://lore.kernel.org/all/CAKw_Dz96rfSQc6Rn+9QBcUFHhmkK+9zu+P=bxowfZwxrATCBRg@xxxxxxxxxxxxxx/
> Signed-off-by: Kiryl Shutsemau <kas@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx

I think the concern sashiko commented in patch 2 is valid.

But for this patch itself,
Reviewed-by: Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>

> ---
>  arch/x86/coco/tdx/tdx.c | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
> index 65119362f9a2..41cc23cc63dd 100644
> --- a/arch/x86/coco/tdx/tdx.c
> +++ b/arch/x86/coco/tdx/tdx.c
> @@ -693,8 +693,8 @@ static bool handle_in(struct pt_regs *regs, int size, int port)
>  		.r13 = PORT_READ,
>  		.r14 = port,
>  	};
> -	u64 mask = GENMASK(BITS_PER_BYTE * size - 1, 0);
>  	bool success;
> +	u64 val;
>  
>  	/*
>  	 * Emulate the I/O read via hypercall. More info about ABI can be found
> @@ -702,11 +702,9 @@ static bool handle_in(struct pt_regs *regs, int size, int port)
>  	 * "TDG.VP.VMCALL<Instruction.IO>".
>  	 */
>  	success = !__tdx_hypercall(&args);
> +	val = success ? args.r11 : 0;
>  
> -	/* Update part of the register affected by the emulated instruction */
> -	regs->ax &= ~mask;
> -	if (success)
> -		regs->ax |= args.r11 & mask;
> +	insn_assign_reg(&regs->ax, val, size);
>  
>  	return success;
>  }