KVM: PPC: Book3S PR: Fix invalidation of SLB entry 0 on guest entry

On entering a PR KVM guest, we invalidate the whole SLB before loading
up the guest entries.  We do this using an slbia instruction, which
invalidates all entries except entry 0, followed by an slbie to
invalidate entry 0.  However, the slbie turns out to be ineffective
in some circumstances (specifically when the host linear mapping uses
64k pages) because of errors in computing the parameter to the slbie.
The result is that the guest kernel hangs very early in boot because
it takes a DSI the first time it tries to access kernel data using
a linear mapping address in real mode.

Currently we construct bits 36 - 43 (big-endian numbering) of the slbie
parameter by taking bits 56 - 63 of the SLB VSID doubleword.  These bits
for the tlbie are C (class, 1 bit), B (segment size, 2 bits) and 5
reserved bits.  For the SLB VSID doubleword these are C (class, 1 bit),
reserved (1 bit), LP (large page size, 2 bits), and 4 reserved bits.
Thus we are not setting the B field correctly, and when LP = 01 as
it is for 64k pages, we are setting a reserved bit.

Rather than add more instructions to calculate the slbie parameter
correctly, this takes a simpler approach, which is to set entry 0 to
zeroes explicitly.  Normally slbmte should not be used to invalidate
an entry, since it doesn't invalidate the ERATs, but it is OK to use
it to invalidate an entry if it is immediately followed by slbia,
which does invalidate the ERATs.  (This has been confirmed with the
Power architects.)  This approach takes fewer instructions and will
work whatever the contents of entry 0.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Alexander Graf <agraf@suse.de>

arch/powerpc/kvm/book3s_64_slb.S

@@ -66,10 +66,6 @@ slb_exit_skip_ ## num:
 
 	ld	r12, PACA_SLBSHADOWPTR(r13)
 
-	/* Save off the first entry so we can slbie it later */
-	ld	r10, SHADOW_SLB_ESID(0)(r12)
-	ld	r11, SHADOW_SLB_VSID(0)(r12)
-
 	/* Remove bolted entries */
 	UNBOLT_SLB_ENTRY(0)
 	UNBOLT_SLB_ENTRY(1)
@@ -81,15 +77,10 @@ slb_exit_skip_ ## num:
 
 	/* Flush SLB */
 
+	li	r10, 0
+	slbmte	r10, r10
 	slbia
 
-	/* r0 = esid & ESID_MASK */
-	rldicr  r10, r10, 0, 35
-	/* r0 |= CLASS_BIT(VSID) */
-	rldic   r12, r11, 56 - 36, 36
-	or      r10, r10, r12
-	slbie	r10
-
 	/* Fill SLB with our shadow */
 
 	lbz	r12, SVCPU_SLB_MAX(r3)