[PATCH] powerpc/kvm: Save and restore AMR instead of zeroing

[Russell Currey]

Using the hash MMU on P7+, the AMR is used for pkeys.  It's important
that the host and guest never end up with each other's AMR value, since
this could disrupt operations and break things.

The AMR gets correctly restored on context switch, however before this
happens (i.e. in a program like qemu) having the host value of the AMR
be zero would interfere with that program using pkeys.

In addition, the AMR on Radix can control kernel access to userspace
data, which you wouldn't want to be zeroed.

So, just save and restore it like the other registers that get saved and
restored.

Fixes: cf43d3b26452 ("powerpc: Enable pkey subsystem")
Cc: <stable at vger.kernel.org> # v4.16+
Signed-off-by: Russell Currey <ruscur at russell.cc>
---
I'm not entirely sure the stack frame numbers are correct, I've tested it
and it works but it'd be good if someone could double check this.

 arch/powerpc/kvm/book3s_hv_rmhandlers.S | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/arch/powerpc/kvm/book3s_hv_rmhandlers.S b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
index 9b8d50a7cbaf..6291751c4ad9 100644
--- a/arch/powerpc/kvm/book3s_hv_rmhandlers.S
+++ b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
@@ -47,7 +47,7 @@ END_FTR_SECTION_IFCLR(CPU_FTR_ARCH_300)
 #define NAPPING_NOVCPU	2
 
 /* Stack frame offsets for kvmppc_hv_entry */
-#define SFS			208
+#define SFS			224		/* must be divisible by 16 */
 #define STACK_SLOT_TRAP		(SFS-4)
 #define STACK_SLOT_SHORT_PATH	(SFS-8)
 #define STACK_SLOT_TID		(SFS-16)
@@ -58,8 +58,9 @@ END_FTR_SECTION_IFCLR(CPU_FTR_ARCH_300)
 #define STACK_SLOT_DAWR		(SFS-56)
 #define STACK_SLOT_DAWRX	(SFS-64)
 #define STACK_SLOT_HFSCR	(SFS-72)
+#define STACK_SLOT_AMR		(SFS-80)
 /* the following is used by the P9 short path */
-#define STACK_SLOT_NVGPRS	(SFS-152)	/* 18 gprs */
+#define STACK_SLOT_NVGPRS	(SFS-160)	/* 18 gprs */
 
 /*
  * Call kvmppc_hv_entry in real mode.
@@ -743,6 +744,9 @@ BEGIN_FTR_SECTION
 	std	r7, STACK_SLOT_DAWRX(r1)
 END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S)
 
+	mfspr	r5, SPRN_AMR
+	std	r5, STACK_SLOT_AMR(r1)
+
 BEGIN_FTR_SECTION
 	/* Set partition DABR */
 	/* Do this before re-enabling PMU to avoid P7 DABR corruption bug */
@@ -1640,13 +1644,14 @@ BEGIN_FTR_SECTION
 END_FTR_SECTION_IFCLR(CPU_FTR_ARCH_300)
 8:
 
-	/* Save and reset AMR and UAMOR before turning on the MMU */
+	/* Save and restore/reset AMR and UAMOR before turning on the MMU */
 	mfspr	r5,SPRN_AMR
 	mfspr	r6,SPRN_UAMOR
 	std	r5,VCPU_AMR(r9)
 	std	r6,VCPU_UAMOR(r9)
+	ld	r5,STACK_SLOT_AMR(r1)
 	li	r6,0
-	mtspr	SPRN_AMR,r6
+	mtspr	SPRN_AMR, r5
 	mtspr	SPRN_UAMOR, r6
 
 	/* Switch DSCR back to host value */
-- 
2.20.1