Re: [PATCH] x86/tdx: Fix zero-extension for CPUID emulation

[Edgecombe, Rick P]

On Tue, 2026-05-12 at 23:37 +0200, Carlos López wrote:
> In the x86 architecture, 32-bit operations zero-extend the result in the
> destination register to 64 bits. This includes the CPUID instruction,
> which writes 32-bit values EAX/EBX/ECX/EDX.
> 
> When handling the CPUID instruction via #VE, copy only the lower 32-bits
> provided by the hypervisor for the output registers, and zero out the
> upper half.
> 
> Fixes: c141fa2c2bba ("x86/tdx: Handle CPUID via #VE")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Carlos López <clopez@xxxxxxx>
> ---
>  arch/x86/coco/tdx/tdx.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
> index c8b9e86d0488..a2fe1ae019bd 100644
> --- a/arch/x86/coco/tdx/tdx.c
> +++ b/arch/x86/coco/tdx/tdx.c
> @@ -543,10 +543,10 @@ static int handle_cpuid(struct pt_regs *regs, struct ve_info *ve)
>  	 * EAX, EBX, ECX, EDX registers after the CPUID instruction execution.
>  	 * So copy the register contents back to pt_regs.
>  	 */
> -	regs->ax = args.r12;
> -	regs->bx = args.r13;
> -	regs->cx = args.r14;
> -	regs->dx = args.r15;
> +	regs->ax = lower_32_bits(args.r12);
> +	regs->bx = lower_32_bits(args.r13);
> +	regs->cx = lower_32_bits(args.r14);
> +	regs->dx = lower_32_bits(args.r15);
>  

Can you explain the impact here? Why should the guest fixup what the VMM
emulates?