On Sat, May 23, 2020 at 8:57 AM Arvind Sankar <[email protected]> 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.
