Re: x86: Question about state of general purpose registers on switch to 64-bit mode

[Andy Lutomirski]

On Sat, May 23, 2020 at 8:57 AM Arvind Sankar <nivedita@xxxxxxxxxxxx> wrote:
>
> Hi,
>
> I have a question about the state of the upper 32 bits of the general
> purpose registers following a switch from/to 64-bit mode.
>
> Both the AMD [0] and Intel [1] manuals state that these bits are
> undefined following a switch from 64 to 32-bit mode. Since they can't be
> accessed in 32-bit mode, presumably this means they are undefined once
> you switch back to 64-bit mode and can see them again.

I would guess that all x86_64 CPUs actually preserve those registers
across mode changes and clear the high bits on 32-bit operations.  But
making the kernel boot code more robust sounds entirely sensible to
me.