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

[Madhavan Srinivasan <maddy@linux.vnet.ibm.com>]

On Friday 08 April 2016 09:24 PM, 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.

IIUC, SIAR and SDAR are updated by hardware when we take
a pmu exception with sampling mode enabled (based on instr).
And these register contents are mainly for OS consumption.
So, we dont need to restore these register values at all,
kindly correct me if I missing something here.

Maddy

>
> 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.
>
>  arch/powerpc/include/asm/reg.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/powerpc/include/asm/reg.h b/arch/powerpc/include/asm/reg.h
> index f5f4c66..6630420 100644
> --- a/arch/powerpc/include/asm/reg.h
> +++ b/arch/powerpc/include/asm/reg.h
> @@ -752,13 +752,13 @@
>  #define SPRN_PMC6	792
>  #define SPRN_PMC7	793
>  #define SPRN_PMC8	794
> -#define SPRN_SIAR	780
>  #define SPRN_SDAR	781
>  #define SPRN_SIER	784
>  #define   SIER_SIPR		0x2000000	/* Sampled MSR_PR */
>  #define   SIER_SIHV		0x1000000	/* Sampled MSR_HV */
>  #define   SIER_SIAR_VALID	0x0400000	/* SIAR contents valid */
>  #define   SIER_SDAR_VALID	0x0200000	/* SDAR contents valid */
> +#define SPRN_SIAR	796
>  #define SPRN_TACR	888
>  #define SPRN_TCSCR	889
>  #define SPRN_CSIGR	890