[glibc] POWER: Add context-synchronizing instructions to pkey_write [BZ #25954]

[Florian Weimer <fw@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=e627106266ad8785457fadbf5bf67ed604d2a353

commit e627106266ad8785457fadbf5bf67ed604d2a353
Author: Florian Weimer <fweimer@redhat.com>
Date:   Mon May 11 11:20:02 2020 +0200

    POWER: Add context-synchronizing instructions to pkey_write [BZ #25954]
    
    Sandipan Das reported that,
    
    "The Power ISA mandates that all writes to the Authority
    Mask Register (AMR) must always be preceded as well as
    succeeded by a context-synchronizing instruction. This
    applies to both the privileged and unprivileged variants
    of the Move To AMR instruction.
    
    This [requirement] is from Table 6 of Chapter 11 in page 1134 of Power
    ISA 3.0B. The document can be found here:
    <https://ibm.ent.box.com/s/1hzcwkwf8rbju5h9iyf44wm94amnlcrv>
    "
    
    See this kernel patch submission:
    
    <https://lore.kernel.org/linuxppc-dev/5f65cf37be993760de8112a88da194e3ccbb2bf8.1588959697.git.sandipan@linux.ibm.com/>

Diff:
---
 sysdeps/unix/sysv/linux/powerpc/powerpc64/arch-pkey.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/sysdeps/unix/sysv/linux/powerpc/powerpc64/arch-pkey.h b/sysdeps/unix/sysv/linux/powerpc/powerpc64/arch-pkey.h
index 623b073d5a..25d080c9a6 100644
--- a/sysdeps/unix/sysv/linux/powerpc/powerpc64/arch-pkey.h
+++ b/sysdeps/unix/sysv/linux/powerpc/powerpc64/arch-pkey.h
@@ -37,7 +37,7 @@ pkey_read (void)
 static inline void
 pkey_write (unsigned long int value)
 {
-  __asm__ volatile ("mtspr 13, %0" : : "r" (value));
+  __asm__ volatile ("isync; mtspr 13, %0; isync" : : "r" (value));
 }
 
 /* Number of the largest supported key.  This depends on the width of