Re: [PATCH] powerpc/64s: Clear PCR on boot

[Guenter Roeck <linux@roeck-us.net>]

Hi Michael,

On Sun, May 27, 2018 at 12:22:23PM +1000, Michael Ellerman wrote:
> 
> 
> On 25 May 2018 11:33:08 pm AEST, Guenter Roeck <linux@roeck-us.net> wrote:
> >On Fri, May 18, 2018 at 11:37:42AM +1000, Michael Neuling wrote:
> >> Clear the PCR (Processor Compatibility Register) on boot to ensure we
> >> are not running in a compatibility mode.
> >> 
> >> We've seen this cause problems when a crash (and kdump) occurs while
> >> running compat mode guests. The kdump kernel then runs with the PCR
> >> set and causes problems. The symptom in the kdump kernel (also seen
> >in
> >> petitboot after fast-reboot) is early userspace programs taking
> >> sigills on newer instructions (seen in libc).
> >> 
> >
> >Hi folks,
> >
> >this patch causes qemu to bail out with
> >
> >Trying to write privileged spr 338 (0x152) at c000000000033454
> >
> >when running it with "-M powernv -cpu POWER8" and powernv_defconfig.
> >
> >Can you confirm that this is a bug in qemu ?
> 
> Hi Guenter,
> 
> Thanks for the report. As far as I'm concerned this is a qemu bug, that register should exist on a bare metal Power8.
> 
> Will try and get you a qemu patch to fix it.
> 
I already have a patch, or at least one that does the trick for me.
Getting qemu patched was not the problem. I just want to be sure that
the problem is indeed a qemu problem.

Thanks,
Guenter

---
>From 1617bac264b4c49d817b6947611affa9b73318f6 Mon Sep 17 00:00:00 2001
From: Guenter Roeck <linux@roeck-us.net>
Date: Fri, 25 May 2018 06:38:40 -0700
Subject: [PATCH] PowerPC: Permit privileged access to SPR_PCR for POWER7+

Without this access, Linux mainline bails out.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
---
 target/ppc/translate_init.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/target/ppc/translate_init.c b/target/ppc/translate_init.c
index 391b94b..3b09c49 100644
--- a/target/ppc/translate_init.c
+++ b/target/ppc/translate_init.c
@@ -7953,11 +7953,12 @@ static void gen_spr_power6_common(CPUPPCState *env)
 #endif
     /*
      * Register PCR to report POWERPC_EXCP_PRIV_REG instead of
-     * POWERPC_EXCP_INVAL_SPR.
+     * POWERPC_EXCP_INVAL_SPR in userspace. Permit privileged
+     * access.
      */
     spr_register(env, SPR_PCR, "PCR",
                  SPR_NOACCESS, SPR_NOACCESS,
-                 SPR_NOACCESS, SPR_NOACCESS,
+                 &spr_read_generic, &spr_write_generic,
                  0x00000000);
 }
 
-- 
2.7.4