KVM: PPC: Book3S HV: Snapshot timebase offset on guest entry

Currently, the HV KVM guest entry/exit code adds the timebase offset
from the vcore struct to the timebase on guest entry, and subtracts
it on guest exit.  Which is fine, except that it is possible for
userspace to change the offset using the SET_ONE_REG interface while
the vcore is running, as there is only one timebase offset per vcore
but potentially multiple VCPUs in the vcore.  If that were to happen,
KVM would subtract a different offset on guest exit from that which
it had added on guest entry, leading to the timebase being out of sync
between cores in the host, which then leads to bad things happening
such as hangs and spurious watchdog timeouts.

To fix this, we add a new field 'tb_offset_applied' to the vcore struct
which stores the offset that is currently applied to the timebase.
This value is set from the vcore tb_offset field on guest entry, and
is what is subtracted from the timebase on guest exit.  Since it is
zero when the timebase offset is not applied, we can simplify the
logic in kvmhv_start_timing and kvmhv_accumulate_time.

In addition, we had secondary threads reading the timebase while
running concurrently with code on the primary thread which would
eventually add or subtract the timebase offset from the timebase.
This occurred while saving or restoring the DEC register value on
the secondary threads.  Although no specific incorrect behaviour has
been observed, this is a race which should be fixed.  To fix it, we
move the DEC saving code to just before we call kvmhv_commence_exit,
and the DEC restoring code to after the point where we have waited
for the primary thread to switch the MMU context and add the timebase
offset.  That way we are sure that the timebase contains the guest
timebase value in both cases.
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

arch/powerpc/include/asm/kvm_book3s.h

@@ -96,6 +96,7 @@ struct kvmppc_vcore {
 	struct kvm_vcpu *runner;
 	struct kvm *kvm;
 	u64 tb_offset;		/* guest timebase - host timebase */
+	u64 tb_offset_applied;	/* timebase offset currently in force */
 	ulong lpcr;
 	u32 arch_compat;
 	ulong pcr;

arch/powerpc/kernel/asm-offsets.c

@@ -562,6 +562,7 @@ int main(void)
 	OFFSET(VCORE_NAPPING_THREADS, kvmppc_vcore, napping_threads);
 	OFFSET(VCORE_KVM, kvmppc_vcore, kvm);
 	OFFSET(VCORE_TB_OFFSET, kvmppc_vcore, tb_offset);
+	OFFSET(VCORE_TB_OFFSET_APPL, kvmppc_vcore, tb_offset_applied);
 	OFFSET(VCORE_LPCR, kvmppc_vcore, lpcr);
 	OFFSET(VCORE_PCR, kvmppc_vcore, pcr);
 	OFFSET(VCORE_DPDES, kvmppc_vcore, dpdes);

arch/powerpc/kvm/book3s_hv.c

@@ -2441,6 +2441,7 @@ static void init_vcore_to_run(struct kvmppc_vcore *vc)
 	vc->in_guest = 0;
 	vc->napping_threads = 0;
 	vc->conferring_threads = 0;
+	vc->tb_offset_applied = 0;
 }
 
 static bool can_dynamic_split(struct kvmppc_vcore *vc, struct core_info *cip)

arch/powerpc/kvm/book3s_hv_rmhandlers.S

@@ -692,6 +692,7 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_300)
 22:	ld	r8,VCORE_TB_OFFSET(r5)
 	cmpdi	r8,0
 	beq	37f
+	std	r8, VCORE_TB_OFFSET_APPL(r5)
 	mftb	r6		/* current host timebase */
 	add	r8,r8,r6
 	mtspr	SPRN_TBU40,r8	/* update upper 40 bits */
@@ -940,18 +941,6 @@ FTR_SECTION_ELSE
 ALT_FTR_SECTION_END_IFCLR(CPU_FTR_ARCH_300)
 8:
 
-	/*
-	 * 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
-
 	ld	r5, VCPU_SPRG0(r4)
 	ld	r6, VCPU_SPRG1(r4)
 	ld	r7, VCPU_SPRG2(r4)
@@ -1005,6 +994,18 @@ ALT_FTR_SECTION_END_IFCLR(CPU_FTR_ARCH_300)
 	mtspr	SPRN_LPCR,r8
 	isync
 
+	/*
+	 * 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_APPL(r5)
+	add	r8,r8,r6
+	mftb	r7
+	subf	r3,r7,r8
+	mtspr	SPRN_DEC,r3
+
 	/* Check if HDEC expires soon */
 	mfspr	r3, SPRN_HDEC
 	EXTEND_HDEC(r3)
@@ -1597,8 +1598,27 @@ END_MMU_FTR_SECTION_IFSET(MMU_FTR_TYPE_RADIX)
 
 guest_bypass:
 	stw	r12, STACK_SLOT_TRAP(r1)
-	mr 	r3, r12
+
+	/* Save DEC */
+	/* Do this before kvmhv_commence_exit so we know TB is guest TB */
+	ld	r3, HSTATE_KVM_VCORE(r13)
+	mfspr	r5,SPRN_DEC
+	mftb	r6
+	/* On P9, if the guest has large decr enabled, don't sign extend */
+BEGIN_FTR_SECTION
+	ld	r4, VCORE_LPCR(r3)
+	andis.	r4, r4, LPCR_LD@h
+	bne	16f
+END_FTR_SECTION_IFSET(CPU_FTR_ARCH_300)
+	extsw	r5,r5
+16:	add	r5,r5,r6
+	/* r5 is a guest timebase value here, convert to host TB */
+	ld	r4,VCORE_TB_OFFSET_APPL(r3)
+	subf	r5,r4,r5
+	std	r5,VCPU_DEC_EXPIRES(r9)
+
 	/* Increment exit count, poke other threads to exit */
+	mr 	r3, r12
 	bl	kvmhv_commence_exit
 	nop
 	ld	r9, HSTATE_KVM_VCPU(r13)
@@ -1639,23 +1659,6 @@ guest_bypass:
 	mtspr	SPRN_PURR,r3
 	mtspr	SPRN_SPURR,r4
 
-	/* Save DEC */
-	ld	r3, HSTATE_KVM_VCORE(r13)
-	mfspr	r5,SPRN_DEC
-	mftb	r6
-	/* On P9, if the guest has large decr enabled, don't sign extend */
-BEGIN_FTR_SECTION
-	ld	r4, VCORE_LPCR(r3)
-	andis.	r4, r4, LPCR_LD@h
-	bne	16f
-END_FTR_SECTION_IFSET(CPU_FTR_ARCH_300)
-	extsw	r5,r5
-16:	add	r5,r5,r6
-	/* r5 is a guest timebase value here, convert to host TB */
-	ld	r4,VCORE_TB_OFFSET(r3)
-	subf	r5,r4,r5
-	std	r5,VCPU_DEC_EXPIRES(r9)
-
 BEGIN_FTR_SECTION
 	b	8f
 END_FTR_SECTION_IFCLR(CPU_FTR_ARCH_207S)
@@ -2017,9 +2020,11 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_207S)
 
 27:
 	/* Subtract timebase offset from timebase */
-	ld	r8,VCORE_TB_OFFSET(r5)
+	ld	r8, VCORE_TB_OFFSET_APPL(r5)
 	cmpdi	r8,0
 	beq	17f
+	li	r0, 0
+	std	r0, VCORE_TB_OFFSET_APPL(r5)
 	mftb	r6			/* current guest timebase */
 	subf	r8,r8,r6
 	mtspr	SPRN_TBU40,r8		/* update upper 40 bits */
@@ -2700,7 +2705,7 @@ END_FTR_SECTION_IFSET(CPU_FTR_ARCH_300)
 	add	r3, r3, r5
 	ld	r4, HSTATE_KVM_VCPU(r13)
 	ld	r5, HSTATE_KVM_VCORE(r13)
-	ld	r6, VCORE_TB_OFFSET(r5)
+	ld	r6, VCORE_TB_OFFSET_APPL(r5)
 	subf	r3, r6, r3	/* convert to host TB value */
 	std	r3, VCPU_DEC_EXPIRES(r4)
 
@@ -2799,7 +2804,7 @@ END_FTR_SECTION(CPU_FTR_TM | CPU_FTR_P9_TM_HV_ASSIST, 0)
 	/* Restore guest decrementer */
 	ld	r3, VCPU_DEC_EXPIRES(r4)
 	ld	r5, HSTATE_KVM_VCORE(r13)
-	ld	r6, VCORE_TB_OFFSET(r5)
+	ld	r6, VCORE_TB_OFFSET_APPL(r5)
 	add	r3, r3, r6	/* convert host TB to guest TB value */
 	mftb	r7
 	subf	r3, r7, r3
@@ -3606,12 +3611,9 @@ kvmppc_fix_pmao:
  */
 kvmhv_start_timing:
 	ld	r5, HSTATE_KVM_VCORE(r13)
-	lbz	r6, VCORE_IN_GUEST(r5)
-	cmpwi	r6, 0
-	beq	5f				/* if in guest, need to */
-	ld	r6, VCORE_TB_OFFSET(r5)		/* subtract timebase offset */
-5:	mftb	r5
-	subf	r5, r6, r5
+	ld	r6, VCORE_TB_OFFSET_APPL(r5)
+	mftb	r5
+	subf	r5, r6, r5	/* subtract current timebase offset */
 	std	r3, VCPU_CUR_ACTIVITY(r4)
 	std	r5, VCPU_ACTIVITY_START(r4)
 	blr
@@ -3622,15 +3624,12 @@ kvmhv_start_timing:
  */
 kvmhv_accumulate_time:
 	ld	r5, HSTATE_KVM_VCORE(r13)
-	lbz	r8, VCORE_IN_GUEST(r5)
-	cmpwi	r8, 0
-	beq	4f				/* if in guest, need to */
-	ld	r8, VCORE_TB_OFFSET(r5)		/* subtract timebase offset */
-4:	ld	r5, VCPU_CUR_ACTIVITY(r4)
+	ld	r8, VCORE_TB_OFFSET_APPL(r5)
+	ld	r5, VCPU_CUR_ACTIVITY(r4)
 	ld	r6, VCPU_ACTIVITY_START(r4)
 	std	r3, VCPU_CUR_ACTIVITY(r4)
 	mftb	r7
-	subf	r7, r8, r7
+	subf	r7, r8, r7	/* subtract current timebase offset */
 	std	r7, VCPU_ACTIVITY_START(r4)
 	cmpdi	r5, 0
 	beqlr