KVM: PPC: Book3S HV: Fix decrementer timeouts with non-zero TB offset

Commit c7699822bc21 ("KVM: PPC: Book3S HV: Make physical thread 0 do
the MMU switching") reordered the guest entry/exit code so that most
of the guest register save/restore code happened in guest MMU context.
A side effect of that is that the timebase still contains the guest
timebase value at the point where we compute and use vcpu->arch.dec_expires,
and therefore that is now a guest timebase value rather than a host
timebase value.  That in turn means that the timeouts computed in
kvmppc_set_timer() are wrong if the timebase offset for the guest is
non-zero.  The consequence of that is things such as "sleep 1" in a
guest after migration may sleep for much longer than they should.

This fixes the problem by converting between guest and host timebase
values as necessary, by adding or subtracting the timebase offset.
This also fixes an incorrect comment.
Signed-off-by: NPaul Mackerras <paulus@samba.org>
Acked-by: NScott Wood <scottwood@freescale.com>

arch/powerpc/kvm/book3s_hv_rmhandlers.S

@@ -841,6 +841,10 @@ END_FTR_SECTION_IFCLR(CPU_FTR_ARCH_207S)
 	 * Set the decrementer to the guest decrementer.
 	 */
 	ld	r8,VCPU_DEC_EXPIRES(r4)
+	/* r8 is a host timebase value here, convert to guest TB */
+	ld	r5,HSTATE_KVM_VCORE(r13)
+	ld	r6,VCORE_TB_OFFSET(r5)
+	add	r8,r8,r6
 	mftb	r7
 	subf	r3,r7,r8
 	mtspr	SPRN_DEC,r3
@@ -1204,6 +1208,10 @@ END_FTR_SECTION_IFCLR(CPU_FTR_ARCH_201)
 	mftb	r6
 	extsw	r5,r5
 	add	r5,r5,r6
+	/* r5 is a guest timebase value here, convert to host TB */
+	ld	r3,HSTATE_KVM_VCORE(r13)
+	ld	r4,VCORE_TB_OFFSET(r3)
+	subf	r5,r4,r5
 	std	r5,VCPU_DEC_EXPIRES(r9)
 
 BEGIN_FTR_SECTION
@@ -1479,7 +1487,7 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S)
 	ld	r8,VCORE_TB_OFFSET(r5)
 	cmpdi	r8,0
 	beq	17f
-	mftb	r6			/* current host timebase */
+	mftb	r6			/* current guest timebase */
 	subf	r8,r8,r6
 	mtspr	SPRN_TBU40,r8		/* update upper 40 bits */
 	mftb	r7			/* check if lower 24 bits overflowed */