KVM: arm64: Drop trapping of PAuth instructions/keys

We currently insist on disabling PAuth on vcpu_load(), and get to
enable it on first guest use of an instruction or a key (ignoring
the NV case for now).

It isn't clear at all what this is trying to achieve: guests tend
to use PAuth when available, and nothing forces you to expose it
to the guest if you don't want to. This also isn't totally free:
we take a full GPR save/restore between host and guest, only to
write ten 64bit registers. The "value proposition" escapes me.

So let's forget this stuff and enable PAuth eagerly if exposed to
the guest. This results in much simpler code. Performance wise,
that's not bad either (tested on M2 Pro running a fully automated
Debian installer as the workload):

- On a non-NV guest, I can see reduction of 0.24% in the number
  of cycles (measured with perf over 10 consecutive runs)

- On a NV guest (L2), I see a 2% reduction in wall-clock time
  (measured with 'time', as M2 doesn't have a PMUv3 and NV
  doesn't support it either)

So overall, a much reduced complexity and a (small) performance
improvement.

Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20240419102935.1935571-16-maz@kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>

3 files changed, 3 insertions, 85 deletions

diff --git a/arch/arm64/kvm/hyp/include/hyp/switch.h b/arch/arm64/kvm/hyp/include/hyp/switch.h
index a0908d7a8f56..7c733decbe43 100644
--- a/arch/arm64/kvm/hyp/include/hyp/switch.h
+++ b/arch/arm64/kvm/hyp/include/hyp/switch.h
@@ -27,6 +27,7 @@
 #include <asm/kvm_hyp.h>
 #include <asm/kvm_mmu.h>
 #include <asm/kvm_nested.h>
+#include <asm/kvm_ptrauth.h>
 #include <asm/fpsimd.h>
 #include <asm/debug-monitors.h>
 #include <asm/processor.h>
@@ -447,82 +448,6 @@ static inline bool handle_tx2_tvm(struct kvm_vcpu *vcpu)
 	return true;
 }
 
-static inline bool esr_is_ptrauth_trap(u64 esr)
-{
-	switch (esr_sys64_to_sysreg(esr)) {
-	case SYS_APIAKEYLO_EL1:
-	case SYS_APIAKEYHI_EL1:
-	case SYS_APIBKEYLO_EL1:
-	case SYS_APIBKEYHI_EL1:
-	case SYS_APDAKEYLO_EL1:
-	case SYS_APDAKEYHI_EL1:
-	case SYS_APDBKEYLO_EL1:
-	case SYS_APDBKEYHI_EL1:
-	case SYS_APGAKEYLO_EL1:
-	case SYS_APGAKEYHI_EL1:
-		return true;
-	}
-
-	return false;
-}
-
-#define __ptrauth_save_key(ctxt, key)					\
-	do {								\
-	u64 __val;                                                      \
-	__val = read_sysreg_s(SYS_ ## key ## KEYLO_EL1);                \
-	ctxt_sys_reg(ctxt, key ## KEYLO_EL1) = __val;                   \
-	__val = read_sysreg_s(SYS_ ## key ## KEYHI_EL1);                \
-	ctxt_sys_reg(ctxt, key ## KEYHI_EL1) = __val;                   \
-} while(0)
-
-DECLARE_PER_CPU(struct kvm_cpu_context, kvm_hyp_ctxt);
-
-static bool kvm_hyp_handle_ptrauth(struct kvm_vcpu *vcpu, u64 *exit_code)
-{
-	struct kvm_cpu_context *ctxt;
-	u64 enable = 0;
-
-	if (!vcpu_has_ptrauth(vcpu))
-		return false;
-
-	/*
-	 * NV requires us to handle API and APK independently, just in
-	 * case the hypervisor is totally nuts. Please barf >here<.
-	 */
-	if (vcpu_has_nv(vcpu) && !is_hyp_ctxt(vcpu)) {
-		switch (ESR_ELx_EC(kvm_vcpu_get_esr(vcpu))) {
-		case ESR_ELx_EC_PAC:
-			if (!(__vcpu_sys_reg(vcpu, HCR_EL2) & HCR_API))
-				return false;
-
-			enable |= HCR_API;
-			break;
-
-		case ESR_ELx_EC_SYS64:
-			if (!(__vcpu_sys_reg(vcpu, HCR_EL2) & HCR_APK))
-				return false;
-
-			enable |= HCR_APK;
-			break;
-		}
-	} else {
-		enable = HCR_API | HCR_APK;
-	}
-
-	ctxt = this_cpu_ptr(&kvm_hyp_ctxt);
-	__ptrauth_save_key(ctxt, APIA);
-	__ptrauth_save_key(ctxt, APIB);
-	__ptrauth_save_key(ctxt, APDA);
-	__ptrauth_save_key(ctxt, APDB);
-	__ptrauth_save_key(ctxt, APGA);
-
-
-	vcpu->arch.hcr_el2 |= enable;
-	sysreg_clear_set(hcr_el2, 0, enable);
-
-	return true;
-}
-
 static bool kvm_hyp_handle_cntpct(struct kvm_vcpu *vcpu)
 {
 	struct arch_timer_context *ctxt;
@@ -610,9 +535,6 @@ static bool kvm_hyp_handle_sysreg(struct kvm_vcpu *vcpu, u64 *exit_code)
 	    __vgic_v3_perform_cpuif_access(vcpu) == 1)
 		return true;
 
-	if (esr_is_ptrauth_trap(kvm_vcpu_get_esr(vcpu)))
-		return kvm_hyp_handle_ptrauth(vcpu, exit_code);
-
 	if (kvm_hyp_handle_cntpct(vcpu))
 		return true;
 
diff --git a/arch/arm64/kvm/hyp/nvhe/switch.c b/arch/arm64/kvm/hyp/nvhe/switch.c
index 4103625e46c5..9dfe704bdb69 100644
--- a/arch/arm64/kvm/hyp/nvhe/switch.c
+++ b/arch/arm64/kvm/hyp/nvhe/switch.c
@@ -191,7 +191,6 @@ static const exit_handler_fn hyp_exit_handlers[] = {
 	[ESR_ELx_EC_IABT_LOW]		= kvm_hyp_handle_iabt_low,
 	[ESR_ELx_EC_DABT_LOW]		= kvm_hyp_handle_dabt_low,
 	[ESR_ELx_EC_WATCHPT_LOW]	= kvm_hyp_handle_watchpt_low,
-	[ESR_ELx_EC_PAC]		= kvm_hyp_handle_ptrauth,
 	[ESR_ELx_EC_MOPS]		= kvm_hyp_handle_mops,
 };
 
@@ -203,7 +202,6 @@ static const exit_handler_fn pvm_exit_handlers[] = {
 	[ESR_ELx_EC_IABT_LOW]		= kvm_hyp_handle_iabt_low,
 	[ESR_ELx_EC_DABT_LOW]		= kvm_hyp_handle_dabt_low,
 	[ESR_ELx_EC_WATCHPT_LOW]	= kvm_hyp_handle_watchpt_low,
-	[ESR_ELx_EC_PAC]		= kvm_hyp_handle_ptrauth,
 	[ESR_ELx_EC_MOPS]		= kvm_hyp_handle_mops,
 };
 
diff --git a/arch/arm64/kvm/hyp/vhe/switch.c b/arch/arm64/kvm/hyp/vhe/switch.c
index 8e1d98b691c1..f374bcdab4d4 100644
--- a/arch/arm64/kvm/hyp/vhe/switch.c
+++ b/arch/arm64/kvm/hyp/vhe/switch.c
@@ -41,9 +41,8 @@ DEFINE_PER_CPU(unsigned long, kvm_hyp_vector);
  * - TGE: we want the guest to use EL1, which is incompatible with
  *   this bit being set
  *
- * - API/APK: for hysterical raisins, we enable PAuth lazily, which
- *   means that the guest's bits cannot be directly applied (we really
- *   want to see the traps). Revisit this at some point.
+ * - API/APK: they are already accounted for by vcpu_load(), and can
+ *   only take effect across a load/put cycle (such as ERET)
  */
 #define NV_HCR_GUEST_EXCLUDE	(HCR_TGE | HCR_API | HCR_APK)
 
@@ -268,7 +267,6 @@ static const exit_handler_fn hyp_exit_handlers[] = {
 	[ESR_ELx_EC_IABT_LOW]		= kvm_hyp_handle_iabt_low,
 	[ESR_ELx_EC_DABT_LOW]		= kvm_hyp_handle_dabt_low,
 	[ESR_ELx_EC_WATCHPT_LOW]	= kvm_hyp_handle_watchpt_low,
-	[ESR_ELx_EC_PAC]		= kvm_hyp_handle_ptrauth,
 	[ESR_ELx_EC_ERET]		= kvm_hyp_handle_eret,
 	[ESR_ELx_EC_MOPS]		= kvm_hyp_handle_mops,
 };