KVM: PPC: Book3S HV: Synthesize segment fault if SLB lookup fails

When handling a hypervisor data or instruction storage interrupt (HDSI
or HISI), we look up the SLB entry for the address being accessed in
order to translate the effective address to a virtual address which can
be looked up in the guest HPT.  This lookup can occasionally fail due
to the guest replacing an SLB entry without invalidating the evicted
SLB entry.  In this situation an ERAT (effective to real address
translation cache) entry can persist and be used by the hardware even
though there is no longer a corresponding SLB entry.

Previously we would just deliver a data or instruction storage interrupt
(DSI or ISI) to the guest in this case.  However, this is not correct
and has been observed to cause guests to crash, typically with a
data storage protection interrupt on a store to the vmemmap area.

Instead, what we do now is to synthesize a data or instruction segment
interrupt.  That should cause the guest to reload an appropriate entry
into the SLB and retry the faulting instruction.  If it still faults,
we should find an appropriate SLB entry next time and be able to handle
the fault.

Tested-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kvm/book3s_hv_rmhandlers.S

@@ -1749,7 +1749,8 @@ kvmppc_hdsi:
 	beq	3f
 	clrrdi	r0, r4, 28
 	PPC_SLBFEE_DOT(R5, R0)		/* if so, look up SLB */
-	bne	1f			/* if no SLB entry found */
+	li	r0, BOOK3S_INTERRUPT_DATA_SEGMENT
+	bne	7f			/* if no SLB entry found */
 4:	std	r4, VCPU_FAULT_DAR(r9)
 	stw	r6, VCPU_FAULT_DSISR(r9)
 
@@ -1768,14 +1769,15 @@ kvmppc_hdsi:
 	cmpdi	r3, -2			/* MMIO emulation; need instr word */
 	beq	2f
 
-	/* Synthesize a DSI for the guest */
+	/* Synthesize a DSI (or DSegI) for the guest */
 	ld	r4, VCPU_FAULT_DAR(r9)
 	mr	r6, r3
-1:	mtspr	SPRN_DAR, r4
+1:	li	r0, BOOK3S_INTERRUPT_DATA_STORAGE
 	mtspr	SPRN_DSISR, r6
+7:	mtspr	SPRN_DAR, r4
 	mtspr	SPRN_SRR0, r10
 	mtspr	SPRN_SRR1, r11
-	li	r10, BOOK3S_INTERRUPT_DATA_STORAGE
+	mr	r10, r0
 	bl	kvmppc_msr_interrupt
 fast_interrupt_c_return:
 6:	ld	r7, VCPU_CTR(r9)
@@ -1823,7 +1825,8 @@ kvmppc_hisi:
 	beq	3f
 	clrrdi	r0, r10, 28
 	PPC_SLBFEE_DOT(R5, R0)		/* if so, look up SLB */
-	bne	1f			/* if no SLB entry found */
+	li	r0, BOOK3S_INTERRUPT_INST_SEGMENT
+	bne	7f			/* if no SLB entry found */
 4:
 	/* Search the hash table. */
 	mr	r3, r9			/* vcpu pointer */
@@ -1840,11 +1843,12 @@ kvmppc_hisi:
 	cmpdi	r3, -1			/* handle in kernel mode */
 	beq	guest_exit_cont
 
-	/* Synthesize an ISI for the guest */
+	/* Synthesize an ISI (or ISegI) for the guest */
 	mr	r11, r3
-1:	mtspr	SPRN_SRR0, r10
+1:	li	r0, BOOK3S_INTERRUPT_INST_STORAGE
+7:	mtspr	SPRN_SRR0, r10
 	mtspr	SPRN_SRR1, r11
-	li	r10, BOOK3S_INTERRUPT_INST_STORAGE
+	mr	r10, r0
 	bl	kvmppc_msr_interrupt
 	b	fast_interrupt_c_return