KVM: PPC: Bookehv: Get vcpu's last instruction for emulation

On book3e, KVM uses load external pid (lwepx) dedicated instruction to read
guest last instruction on the exit path. lwepx exceptions (DTLB_MISS, DSI
and LRAT), generated by loading a guest address, needs to be handled by KVM.
These exceptions are generated in a substituted guest translation context
(EPLC[EGS] = 1) from host context (MSR[GS] = 0).

Currently, KVM hooks only interrupts generated from guest context (MSR[GS] = 1),
doing minimal checks on the fast path to avoid host performance degradation.
lwepx exceptions originate from host state (MSR[GS] = 0) which implies
additional checks in DO_KVM macro (beside the current MSR[GS] = 1) by looking
at the Exception Syndrome Register (ESR[EPID]) and the External PID Load Context
Register (EPLC[EGS]). Doing this on each Data TLB miss exception is obvious
too intrusive for the host.

Read guest last instruction from kvmppc_load_last_inst() by searching for the
physical address and kmap it. This address the TODO for TLB eviction and
execute-but-not-read entries, and allow us to get rid of lwepx until we are
able to handle failures.

A simple stress benchmark shows a 1% sys performance degradation compared with
previous approach (lwepx without failure handling):

time for i in `seq 1 10000`; do /bin/echo > /dev/null; done

real    0m 8.85s
user    0m 4.34s
sys     0m 4.48s

vs

real    0m 8.84s
user    0m 4.36s
sys     0m 4.44s

A solution to use lwepx and to handle its exceptions in KVM would be to temporary
highjack the interrupt vector from host. This imposes additional synchronizations
for cores like FSL e6500 that shares host IVOR registers between hardware threads.
This optimized solution can be later developed on top of this patch.

Signed-off-by: Mihai Caraman <mihai.caraman@freescale.com>
Signed-off-by: Alexander Graf <agraf@suse.de>

1 files changed, 9 insertions, 28 deletions

diff --git a/arch/powerpc/kvm/bookehv_interrupts.S b/arch/powerpc/kvm/bookehv_interrupts.S
index 6ff448046301..e000b397ece3 100644
--- a/arch/powerpc/kvm/bookehv_interrupts.S
+++ b/arch/powerpc/kvm/bookehv_interrupts.S
@@ -121,38 +121,14 @@
 1:
 
 	.if	\flags & NEED_EMU
-	/*
-	 * This assumes you have external PID support.
-	 * To support a bookehv CPU without external PID, you'll
-	 * need to look up the TLB entry and create a temporary mapping.
-	 *
-	 * FIXME: we don't currently handle if the lwepx faults.  PR-mode
-	 * booke doesn't handle it either.  Since Linux doesn't use
-	 * broadcast tlbivax anymore, the only way this should happen is
-	 * if the guest maps its memory execute-but-not-read, or if we
-	 * somehow take a TLB miss in the middle of this entry code and
-	 * evict the relevant entry.  On e500mc, all kernel lowmem is
-	 * bolted into TLB1 large page mappings, and we don't use
-	 * broadcast invalidates, so we should not take a TLB miss here.
-	 *
-	 * Later we'll need to deal with faults here.  Disallowing guest
-	 * mappings that are execute-but-not-read could be an option on
-	 * e500mc, but not on chips with an LRAT if it is used.
-	 */
-
-	mfspr	r3, SPRN_EPLC	/* will already have correct ELPID and EGS */
 	PPC_STL	r15, VCPU_GPR(R15)(r4)
 	PPC_STL	r16, VCPU_GPR(R16)(r4)
 	PPC_STL	r17, VCPU_GPR(R17)(r4)
 	PPC_STL	r18, VCPU_GPR(R18)(r4)
 	PPC_STL	r19, VCPU_GPR(R19)(r4)
-	mr	r8, r3
 	PPC_STL	r20, VCPU_GPR(R20)(r4)
-	rlwimi	r8, r6, EPC_EAS_SHIFT - MSR_IR_LG, EPC_EAS
 	PPC_STL	r21, VCPU_GPR(R21)(r4)
-	rlwimi	r8, r6, EPC_EPR_SHIFT - MSR_PR_LG, EPC_EPR
 	PPC_STL	r22, VCPU_GPR(R22)(r4)
-	rlwimi	r8, r10, EPC_EPID_SHIFT, EPC_EPID
 	PPC_STL	r23, VCPU_GPR(R23)(r4)
 	PPC_STL	r24, VCPU_GPR(R24)(r4)
 	PPC_STL	r25, VCPU_GPR(R25)(r4)
@@ -162,10 +138,15 @@
 	PPC_STL	r29, VCPU_GPR(R29)(r4)
 	PPC_STL	r30, VCPU_GPR(R30)(r4)
 	PPC_STL	r31, VCPU_GPR(R31)(r4)
-	mtspr	SPRN_EPLC, r8
-	isync
-	lwepx   r9, 0, r5
-	mtspr	SPRN_EPLC, r3
+
+	/*
+	 * We don't use external PID support. lwepx faults would need to be
+	 * handled by KVM and this implies aditional code in DO_KVM (for
+	 * DTB_MISS, DSI and LRAT) to check ESR[EPID] and EPLC[EGS] which
+	 * is too intrusive for the host. Get last instuction in
+	 * kvmppc_get_last_inst().
+	 */
+	li	r9, KVM_INST_FETCH_FAILED
 	stw	r9, VCPU_LAST_INST(r4)
 	.endif