On Fri, 28 Jul 2023 at 02:17, Richard Henderson <richard.hender...@linaro.org> wrote: > > On 7/27/23 14:36, Ard Biesheuvel wrote: > > On Thu, 27 Jul 2023 at 19:56, Richard Henderson > > <richard.hender...@linaro.org> wrote: > >> > >> On 7/26/23 08:01, Richard Henderson wrote: > >>> On 7/26/23 01:17, Ard Biesheuvel wrote: > >>>> Hints welcome on where the architectural behavior is specified, and in > >>>> particular, > >>>> whether or not other 64-bit GPRs can be relied upon to preserve their > >>>> full 64-bit > >>>> length values. > >>> > >>> No idea about chapter and verse, but it has the feel of being part and > >>> parcel with the > >>> truncation of eip. While esp is always special, I suspect that none of > >>> the GPRs can be > >>> relied on carrying all bits. > >> > >> Coincidentally, I was having a gander at the newly announced APX extension > >> [1], > >> and happened across > >> > >> 3.1.4.1.2 Extended GPR Access (Direct and Indirect) > >> > >> ... Entering/leaving 64-bit mode via traditional (explicit) > >> control flow does not directly alter the content of the EGPRs > >> (EGPRs behave similar to R8-R15 in this regard). > >> > >> which suggests to me that the 8 low registers are squashed to 32-bit > >> on transition to 32-bit IA-32e mode. > >> > >> I still have not found similar language in the main architecture manual. > >> > > > > Interesting - that matches my observations on those Ice Lake cores: > > RSP will be truncated, but preserving/restoring it to/from R8 across > > the exit from long mode works fine. > > Found it: > > Volume 1 Basic Architecture > 3.4.1.1 General-Purpose Registers in 64-Bit Mode > > # Registers only available in 64-bit mode (R8-R15 and XMM8-XMM15) > # are preserved across transitions from 64-bit mode into compatibility mode > # then back into 64-bit mode. However, values of R8-R15 and XMM8-XMM15 are > # undefined after transitions from 64-bit mode through compatibility mode > # to legacy or real mode and then back through compatibility mode to 64-bit > mode. >
Thanks. Not what I was hoping though ...
