Re: [PATCH] powerpc: Fix definition of SIAR register

[Paul Mackerras <paulus@ozlabs.org>]

On Fri, Apr 08, 2016 at 05:54:11PM +0200, Thomas Huth wrote:
> The SIAR register is available twice, one time as SPR 780 (unprivileged,
> but read-only), and one time as SPR 796 (privileged, but read and write).
> The Linux kernel code currently uses SPR 780 - and while this is OK for
> reading, writing to that register of course does not work.
> Since the KVM code tries to write to this register, too (see the mtspr
> in book3s_hv_rmhandlers.S), the contents of this register sometimes get
> lost for the guests, e.g. during migration of a VM.
> To fix this issue, simply switch to the other SPR numer 796 instead.
> 
> Signed-off-by: Thomas Huth <thuth@redhat.com>
> ---
>  Note: The perf code in core-book3s.c also seems to write to the SIAR
>        SPR, so that might be affected by this issue, too - but I did
>        not test the perf code, so I'm not sure about that part.

EBBs mean we need to context-switch the SIAR between user tasks (among
other registers).

I notice that SDAR is also wrong, and the MMCR2 definition is also
using the user-accessible number (though it at least is writable as
well as readable).

Paul.