Re: [PATCH] x86: make hypervisor build with gcc11

["Roger Pau Monné" <roger.pau@citrix.com>]

On Wed, May 19, 2021 at 05:39:50PM +0200, Jan Beulich wrote:
> Gcc 11 looks to make incorrect assumptions about valid ranges that
> pointers may be used for addressing when they are derived from e.g. a
> plain constant. See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100680.
> 
> Utilize RELOC_HIDE() to work around the issue, which for x86 manifests
> in at least
> - mpparse.c:efi_check_config(),
> - tboot.c:tboot_probe(),
> - tboot.c:tboot_gen_frametable_integrity(),
> - x86_emulate.c:x86_emulate() (at -O2 only).
> The last case is particularly odd not just because it only triggers at
> higher optimization levels, but also because it only affects one of at
> least three similar constructs. Various "note" diagnostics claim the
> valid index range to be [0, 2⁶³-1].
> 
> Signed-off-by: Jan Beulich <jbeulich@suse.com>

Acked-by: Roger Pau Monné <roger.pau@citrix.com>

This is all quite ugly, but I don't have any recommendation short of
getting gcc fixed (or being able to disable those heuristics).

> 
> --- a/tools/tests/x86_emulator/x86-emulate.c
> +++ b/tools/tests/x86_emulator/x86-emulate.c
> @@ -8,6 +8,13 @@
>  
>  #define ERR_PTR(val) NULL
>  
> +/* See gcc bug 100680, but here don't bother making this version dependent. */

Might be worth also referencing 99578 which seems to be the parent
bug? (as 100680 has been closed as a duplicate)

Thanks, Roger.