[RFC PATCH 37/43] KVM: PPC: Book3S HV P9: Comment and fix MMU context switching code

[Nicholas Piggin <npiggin@gmail.com>]

Tighten up partition switching code synchronisation and comments.

In particular, hwsync ; isync is required after the last access that is
performed in the context of a partition, before the partition is
switched away from.

-301 cycles (6319) POWER9 virt-mode NULL hcall

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
 arch/powerpc/kvm/book3s_64_mmu_radix.c |  4 +++
 arch/powerpc/kvm/book3s_hv_p9_entry.c  | 40 +++++++++++++++++++-------
 2 files changed, 33 insertions(+), 11 deletions(-)

diff --git a/arch/powerpc/kvm/book3s_64_mmu_radix.c b/arch/powerpc/kvm/book3s_64_mmu_radix.c
index d909c069363e..5a6ab0a61b68 100644
--- a/arch/powerpc/kvm/book3s_64_mmu_radix.c
+++ b/arch/powerpc/kvm/book3s_64_mmu_radix.c
@@ -53,6 +53,8 @@ unsigned long __kvmhv_copy_tofrom_guest_radix(int lpid, int pid,
 
 	preempt_disable();
 
+	asm volatile("hwsync" ::: "memory");
+	isync();
 	/* switch the lpid first to avoid running host with unallocated pid */
 	old_lpid = mfspr(SPRN_LPID);
 	if (old_lpid != lpid)
@@ -69,6 +71,8 @@ unsigned long __kvmhv_copy_tofrom_guest_radix(int lpid, int pid,
 	else
 		ret = copy_to_user_nofault((void __user *)to, from, n);
 
+	asm volatile("hwsync" ::: "memory");
+	isync();
 	/* switch the pid first to avoid running host with unallocated pid */
 	if (quadrant == 1 && pid != old_pid)
 		mtspr(SPRN_PID, old_pid);
diff --git a/arch/powerpc/kvm/book3s_hv_p9_entry.c b/arch/powerpc/kvm/book3s_hv_p9_entry.c
index 55286a8357f7..7aa72efcac6c 100644
--- a/arch/powerpc/kvm/book3s_hv_p9_entry.c
+++ b/arch/powerpc/kvm/book3s_hv_p9_entry.c
@@ -503,17 +503,19 @@ static void switch_mmu_to_guest_radix(struct kvm *kvm, struct kvm_vcpu *vcpu, u6
 	lpid = nested ? nested->shadow_lpid : kvm->arch.lpid;
 
 	/*
-	 * All the isync()s are overkill but trivially follow the ISA
-	 * requirements. Some can likely be replaced with justification
-	 * comment for why they are not needed.
+	 * Prior memory accesses to host PID Q3 must be completed before we
+	 * start switching, and stores must be drained to avoid not-my-LPAR
+	 * logic (see switch_mmu_to_host).
 	 */
+	asm volatile("hwsync" ::: "memory");
 	isync();
 	mtspr(SPRN_LPID, lpid);
-	isync();
 	mtspr(SPRN_LPCR, lpcr);
-	isync();
 	mtspr(SPRN_PID, vcpu->arch.pid);
-	isync();
+	/*
+	 * isync not required here because we are HRFID'ing to guest before
+	 * any guest context access, which is context synchronising.
+	 */
 }
 
 static void switch_mmu_to_guest_hpt(struct kvm *kvm, struct kvm_vcpu *vcpu, u64 lpcr)
@@ -523,25 +525,41 @@ static void switch_mmu_to_guest_hpt(struct kvm *kvm, struct kvm_vcpu *vcpu, u64
 
 	lpid = kvm->arch.lpid;
 
+	/*
+	 * See switch_mmu_to_guest_radix. ptesync should not be required here
+	 * even if the host is in HPT mode because speculative accesses would
+	 * not cause RC updates (we are in real mode).
+	 */
+	asm volatile("hwsync" ::: "memory");
+	isync();
 	mtspr(SPRN_LPID, lpid);
 	mtspr(SPRN_LPCR, lpcr);
 	mtspr(SPRN_PID, vcpu->arch.pid);
 
 	for (i = 0; i < vcpu->arch.slb_max; i++)
 		mtslb(vcpu->arch.slb[i].orige, vcpu->arch.slb[i].origv);
-
-	isync();
+	/*
+	 * isync not required here, see switch_mmu_to_guest_radix.
+	 */
 }
 
 static void switch_mmu_to_host(struct kvm *kvm, u32 pid)
 {
+	/*
+	 * The guest has exited, so guest MMU context is no longer being
+	 * non-speculatively accessed, but a hwsync is needed before the
+	 * mtLPIDR / mtPIDR switch, in order to ensure all stores are drained,
+	 * so the not-my-LPAR tlbie logic does not overlook them.
+	 */
+	asm volatile("hwsync" ::: "memory");
 	isync();
 	mtspr(SPRN_PID, pid);
-	isync();
 	mtspr(SPRN_LPID, kvm->arch.host_lpid);
-	isync();
 	mtspr(SPRN_LPCR, kvm->arch.host_lpcr);
-	isync();
+	/*
+	 * isync is not required after the switch, because mtmsrd with L=0
+	 * is performed after this switch, which is context synchronising.
+	 */
 
 	if (!radix_enabled())
 		slb_restore_bolted_realmode();
-- 
2.23.0