Re: [PATCH] x86/kexec: Fix crash on transition to a 32bit kernel on AMD hardware

[Ian Jackson <iwj@xxxxxxxxxxxxxx>]

Andrew Cooper writes ("[PATCH] x86/kexec: Fix crash on transition to a 32bit 
kernel on AMD hardware"):
> 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.
> 
> Switch to using lretq, which is compatible between Intel and AMD, as well as
> being less logic overall.
> 
> Fixes: 5a82d5cf352d ("kexec: extend hypercall with improved load/unload ops")
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> CC: Wei Liu <wl@xxxxxxx>
> CC: Ian Jackson <iwj@xxxxxxxxxxxxxx>
> 
> For 4.16.  This is a bugfix for rare (so rare it has probably never been
> exercised) but plain-broken usecase.
> 
> One argument against taking it says that this has been broken for 8 years
> already, so what's a few extra weeks.  Another is that this patch is only
> compile tested because I don't have a suitable setup to repro, nor the time to
> try organising one.

Thanks for being frank about testing.

The bug is a ?race? ?  Which hardly ever happens ?  Or it only affects
some strange configurations ?  Or ... ?

> On the other hand, I specifically used the point of binary incompatibility to
> persuade Intel to drop Call Gates out of the architecture in the forthcoming
> FRED spec.

I'm afraid I can't make head or tail of this.  What are the
implications ?

> The lretq pattern used here matches x86_32_switch() in
> xen/arch/x86/boot/head.S, and this codepath is executed on every MB2+EFI
> xen.gz boot, which from XenServer alone is a very wide set of testing.

AIUI this is an argument saying that the basic principle of this
change is good.  Good.

However: is there some risk of a non-catastrophic breakage here, for
example, if there was a slip in the actual implementation ?
(Catastrophic breakage would break all our tests, I think.)

Thanks,
Ian.