Re: [RFC, PATCHv1 15/28] x86: detect 5-level paging support

From: Boris Petkov
Date: Tue Dec 13 2016 - 18:08:07 EST

On December 13, 2016 11:44:06 PM GMT+01:00, "H. Peter Anvin" <hpa@xxxxxxxxx> wrote:
>When compiling with -fPIC gcc treats ebx as a "fixed register". A
>register can't be spilled, and so a clobber of a fixed register is a
>fatal error.
>Like it or not, it's how it works.
> -hpa

In the meantime I talked to my gcc guy and here's the deal:

There are gcc versions (4.x and earlier) which do not save/restore the PIC register around an inline asm even if it is one of the registers that the inline asm clobbers. Therefore the saving/restoring needs to be done by the inline asm itself.

5.x and later handle that fine.

Thus I was thinking of adding a build-time check for the gcc version but that might turn out to be more code in the end than those ugly ifnc clauses.
Sent from a small device: formatting sux and brevity is inevitable.