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Re: [PATCH] x86/kexec: Fix crash on transition to a 32bit kernel on AMD hardware
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 2 Nov 2021 13:54:18 +0100
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Ian Jackson <iwj@xxxxxxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Tue, 02 Nov 2021 12:54:50 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 29.10.2021 01:26, Andrew Cooper wrote:
> The `ljmp *mem` instruction is (famously?) not binary compatible between Intel
> and AMD CPUS. The AMD-compatible version would require .long to be .quad in
> the second hunk.
>From all sources I have the incompatibility is only with REX.W: Intel
honors it (allowing a mem64:16) operand, while AMD ignores it (using
the same mem32:16 operand form as without REX.W). All the same as for
L{F,G,S}S. Hence I do not see why the present form of (32-bit) LJMP
would be a problem anywhere.
> Switch to using lretq, which is compatible between Intel and AMD, as well as
> being less logic overall.
I certainly don't mind the switch to LRETQ, but then the reasoning
will imo need to change.
Jan
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