Re: [Xen-devel] [PATCH] x86: don't clear high 32 bits of RAX on sub-word guest I/O port reads

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Fri, May 08, 2015 at 01:08:05PM +0100, Jan Beulich wrote:
> 1- or 2-byte operations never alter the high halves of registers.
> 

That is impressive. How possibly did you spot this?

Was it due to reviewing some other code that got you looking
at this?

Thanks!

> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> --- a/xen/arch/x86/traps.c
> +++ b/xen/arch/x86/traps.c
> @@ -2234,7 +2234,7 @@ static int emulate_privileged_op(struct 
>              if ( op_bytes == 4 )
>                  regs->eax = 0;
>              else
> -                regs->eax &= ~((1u << (op_bytes * 8)) - 1);
> +                regs->eax &= ~((1 << (op_bytes * 8)) - 1);
>              regs->eax |= guest_io_read(port, op_bytes, v, regs);
>          }
>          bpmatch = check_guest_io_breakpoint(v, port, op_bytes);
> 
> 
> 

> x86: don't clear high 32 bits of RAX on sub-word guest I/O port reads
> 
> 1- or 2-byte operations never alter the high halves of registers.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> --- a/xen/arch/x86/traps.c
> +++ b/xen/arch/x86/traps.c
> @@ -2234,7 +2234,7 @@ static int emulate_privileged_op(struct 
>              if ( op_bytes == 4 )
>                  regs->eax = 0;
>              else
> -                regs->eax &= ~((1u << (op_bytes * 8)) - 1);
> +                regs->eax &= ~((1 << (op_bytes * 8)) - 1);
>              regs->eax |= guest_io_read(port, op_bytes, v, regs);
>          }
>          bpmatch = check_guest_io_breakpoint(v, port, op_bytes);

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