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Peter Anvin" , Rick Edgecombe , Borys Tsyrulnikov , linux-kernel@vger.kernel.org, linux-coco@lists.linux.dev, kvm@vger.kernel.org, stable@vger.kernel.org References: <20260331112430.71425-1-kas@kernel.org> <20260331112430.71425-3-kas@kernel.org> Content-Language: en-US From: Kuppuswamy Sathyanarayanan In-Reply-To: <20260331112430.71425-3-kas@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Hi Kiril, On 3/31/2026 4:24 AM, 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. > > Update handle_in() to zero out the entire RAX register when the I/O size > is 4 bytes to ensure correct zero-extension. For smaller sizes (1 or 2 > bytes), continue to preserve the unaffected upper bits. > > Fixes: 03149948832a ("x86/tdx: Port I/O: Add runtime hypercalls") > Reported-by: Borys Tsyrulnikov > Signed-off-by: Kiryl Shutsemau (Meta) > Cc: stable@vger.kernel.org > --- If you have bug or discussion link, please include it. Reviewed-by: Kuppuswamy Sathyanarayanan > arch/x86/coco/tdx/tdx.c | 13 +++++++++++-- > 1 file changed, 11 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c > index 4d7f71d50122..b9b9a2d75119 100644 > --- a/arch/x86/coco/tdx/tdx.c > +++ b/arch/x86/coco/tdx/tdx.c > @@ -703,8 +703,17 @@ static bool handle_in(struct pt_regs *regs, int size, int port) > */ > success = !__tdx_hypercall(&args); > > - /* Update part of the register affected by the emulated instruction */ > - regs->ax &= ~mask; > + /* > + * Update part of the register affected by the emulated instruction. > + * > + * 32-bit operands generate a 32-bit result, zero-extended to a 64-bit > + * result. > + */ > + if (size < 4) > + regs->ax &= ~mask; > + else > + regs->ax = 0; The logic would be more readable as: if (size == 4) regs->ax = 0; else regs->ax &= ~mask; > + > if (success) > regs->ax |= args.r11 & mask; > -- Sathyanarayanan Kuppuswamy Linux Kernel Developer