[PATCH v5 03/40] KVM: arm64: Avoid storing the vcpu pointer on the stack

[cdall@kernel.org (Christoffer Dall)]

From: Christoffer Dall <christoffer.dall@linaro.org>

We already have the percpu area for the host cpu state, which points to
the VCPU, so there's no need to store the VCPU pointer on the stack on
every context switch.  We can be a little more clever and just use
tpidr_el2 for the percpu offset and load the VCPU pointer from the host
context.

This has the benefit of being able to retrieve the host context even
when our stack is corrupted, and it has a potential performance benefit
because we trade a store plus a load for an mrs and a load on a round
trip to the guest.

This does require us to calculate the percpu offset without including
the offset from the kernel mapping of the percpu array to the linear
mapping of the array (which is what we store in tpidr_el1), because a
PC-relative generated address in EL2 is already giving us the hyp alias
of the linear mapping of a kernel address.  We do this in
__cpu_init_hyp_mode() by using kvm_ksym_ref().

The code that accesses ESR_EL2 was previously using an alternative to
use the _EL1 accessor on VHE systems, but this was actually unnecessary
as the _EL1 accessor aliases the ESR_EL2 register on VHE, and the _EL2
accessor does the same thing on both systems.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
---

Notes:
    Changes since v4:
     - Clarified rationale in commit message.
     - Called get_host_ctxt from get_vcpu and rename get_vcpu to
       get_vcpu_ptr.
    
    Changes since v3:
     - Reworked the assembly part of the patch after rebasing on v4.16-rc1
       which created a conflict with the variant 2 mitigations.
     - Removed Marc's reviewed-by due to the rework.
     - Removed unneeded extern keyword in declaration in header file
    
    Changes since v1:
     - Use PC-relative addressing to access per-cpu variables instead of
       using a load from the literal pool.
     - Remove stale comments as pointed out by Marc
     - Reworded the commit message as suggested by Drew

 arch/arm64/include/asm/kvm_asm.h  | 15 +++++++++++++++
 arch/arm64/include/asm/kvm_host.h | 15 +++++++++++++++
 arch/arm64/kernel/asm-offsets.c   |  1 +
 arch/arm64/kvm/hyp/entry.S        |  6 +-----
 arch/arm64/kvm/hyp/hyp-entry.S    | 28 ++++++++++------------------
 arch/arm64/kvm/hyp/switch.c       |  5 +----
 arch/arm64/kvm/hyp/sysreg-sr.c    |  5 +++++
 7 files changed, 48 insertions(+), 27 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
index 24961b732e65..7149f1520382 100644
--- a/arch/arm64/include/asm/kvm_asm.h
+++ b/arch/arm64/include/asm/kvm_asm.h
@@ -33,6 +33,7 @@
 #define KVM_ARM64_DEBUG_DIRTY_SHIFT	0
 #define KVM_ARM64_DEBUG_DIRTY		(1 << KVM_ARM64_DEBUG_DIRTY_SHIFT)
 
+/* Translate a kernel address of @sym into its equivalent linear mapping */
 #define kvm_ksym_ref(sym)						\
 	({								\
 		void *val = &sym;					\
@@ -70,6 +71,20 @@ extern u32 __init_stage2_translation(void);
 
 extern void __qcom_hyp_sanitize_btac_predictors(void);
 
+#else /* __ASSEMBLY__ */
+
+.macro get_host_ctxt reg, tmp
+	adr_l	\reg, kvm_host_cpu_state
+	mrs	\tmp, tpidr_el2
+	add	\reg, \reg, \tmp
+.endm
+
+.macro get_vcpu_ptr vcpu, ctxt
+	get_host_ctxt \ctxt, \vcpu
+	ldr	\vcpu, [\ctxt, #HOST_CONTEXT_VCPU]
+	kern_hyp_va	\vcpu
+.endm
+
 #endif
 
 #endif /* __ARM_KVM_ASM_H__ */
diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 596f8e414a4c..618cfee7206a 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -358,10 +358,15 @@ int kvm_perf_teardown(void);
 
 struct kvm_vcpu *kvm_mpidr_to_vcpu(struct kvm *kvm, unsigned long mpidr);
 
+void __kvm_set_tpidr_el2(u64 tpidr_el2);
+DECLARE_PER_CPU(kvm_cpu_context_t, kvm_host_cpu_state);
+
 static inline void __cpu_init_hyp_mode(phys_addr_t pgd_ptr,
 				       unsigned long hyp_stack_ptr,
 				       unsigned long vector_ptr)
 {
+	u64 tpidr_el2;
+
 	/*
 	 * Call initialization code, and switch to the full blown HYP code.
 	 * If the cpucaps haven't been finalized yet, something has gone very
@@ -370,6 +375,16 @@ static inline void __cpu_init_hyp_mode(phys_addr_t pgd_ptr,
 	 */
 	BUG_ON(!static_branch_likely(&arm64_const_caps_ready));
 	__kvm_call_hyp((void *)pgd_ptr, hyp_stack_ptr, vector_ptr);
+
+	/*
+	 * Calculate the raw per-cpu offset without a translation from the
+	 * kernel's mapping to the linear mapping, and store it in tpidr_el2
+	 * so that we can use adr_l to access per-cpu variables in EL2.
+	 */
+	tpidr_el2 = (u64)this_cpu_ptr(&kvm_host_cpu_state)
+		- (u64)kvm_ksym_ref(kvm_host_cpu_state);
+
+	kvm_call_hyp(__kvm_set_tpidr_el2, tpidr_el2);
 }
 
 static inline void kvm_arch_hardware_unsetup(void) {}
diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
index 1303e04110cd..78e1b0a70aaf 100644
--- a/arch/arm64/kernel/asm-offsets.c
+++ b/arch/arm64/kernel/asm-offsets.c
@@ -138,6 +138,7 @@ int main(void)
   DEFINE(CPU_FP_REGS,		offsetof(struct kvm_regs, fp_regs));
   DEFINE(VCPU_FPEXC32_EL2,	offsetof(struct kvm_vcpu, arch.ctxt.sys_regs[FPEXC32_EL2]));
   DEFINE(VCPU_HOST_CONTEXT,	offsetof(struct kvm_vcpu, arch.host_cpu_context));
+  DEFINE(HOST_CONTEXT_VCPU,	offsetof(struct kvm_cpu_context, __hyp_running_vcpu));
 #endif
 #ifdef CONFIG_CPU_PM
   DEFINE(CPU_SUSPEND_SZ,	sizeof(struct cpu_suspend_ctx));
diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
index fdd1068ee3a5..1f458f7c3b44 100644
--- a/arch/arm64/kvm/hyp/entry.S
+++ b/arch/arm64/kvm/hyp/entry.S
@@ -62,9 +62,6 @@ ENTRY(__guest_enter)
 	// Store the host regs
 	save_callee_saved_regs x1
 
-	// Store host_ctxt and vcpu for use@exit time
-	stp	x1, x0, [sp, #-16]!
-
 	add	x18, x0, #VCPU_CONTEXT
 
 	// Restore guest regs x0-x17
@@ -118,8 +115,7 @@ ENTRY(__guest_exit)
 	// Store the guest regs x19-x29, lr
 	save_callee_saved_regs x1
 
-	// Restore the host_ctxt from the stack
-	ldr	x2, [sp], #16
+	get_host_ctxt	x2, x3
 
 	// Now restore the host regs
 	restore_callee_saved_regs x2
diff --git a/arch/arm64/kvm/hyp/hyp-entry.S b/arch/arm64/kvm/hyp/hyp-entry.S
index f36464bd57c5..82fbc368f738 100644
--- a/arch/arm64/kvm/hyp/hyp-entry.S
+++ b/arch/arm64/kvm/hyp/hyp-entry.S
@@ -57,13 +57,8 @@ ENDPROC(__vhe_hyp_call)
 el1_sync:				// Guest trapped into EL2
 	stp	x0, x1, [sp, #-16]!
 
-alternative_if_not ARM64_HAS_VIRT_HOST_EXTN
-	mrs	x1, esr_el2
-alternative_else
-	mrs	x1, esr_el1
-alternative_endif
-	lsr	x0, x1, #ESR_ELx_EC_SHIFT
-
+	mrs	x0, esr_el2
+	lsr	x0, x0, #ESR_ELx_EC_SHIFT
 	cmp	x0, #ESR_ELx_EC_HVC64
 	ccmp	x0, #ESR_ELx_EC_HVC32, #4, ne
 	b.ne	el1_trap
@@ -117,10 +112,14 @@ el1_hvc_guest:
 	eret
 
 el1_trap:
+	get_vcpu_ptr	x1, x0
+
+	mrs		x0, esr_el2
+	lsr		x0, x0, #ESR_ELx_EC_SHIFT
 	/*
 	 * x0: ESR_EC
+	 * x1: vcpu pointer
 	 */
-	ldr	x1, [sp, #16 + 8]	// vcpu stored by __guest_enter
 
 	/*
 	 * We trap the first access to the FP/SIMD to save the host context
@@ -138,13 +137,13 @@ alternative_else_nop_endif
 
 el1_irq:
 	stp     x0, x1, [sp, #-16]!
-	ldr	x1, [sp, #16 + 8]
+	get_vcpu_ptr	x1, x0
 	mov	x0, #ARM_EXCEPTION_IRQ
 	b	__guest_exit
 
 el1_error:
 	stp     x0, x1, [sp, #-16]!
-	ldr	x1, [sp, #16 + 8]
+	get_vcpu_ptr	x1, x0
 	mov	x0, #ARM_EXCEPTION_EL1_SERROR
 	b	__guest_exit
 
@@ -180,14 +179,7 @@ ENTRY(__hyp_do_panic)
 ENDPROC(__hyp_do_panic)
 
 ENTRY(__hyp_panic)
-	/*
-	 * '=kvm_host_cpu_state' is a host VA from the constant pool, it may
-	 * not be accessible by this address from EL2, hyp_panic() converts
-	 * it with kern_hyp_va() before use.
-	 */
-	ldr	x0, =kvm_host_cpu_state
-	mrs	x1, tpidr_el2
-	add	x0, x0, x1
+	get_host_ctxt x0, x1
 	b	hyp_panic
 ENDPROC(__hyp_panic)
 
diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
index 24f52fedfb9e..46717da75643 100644
--- a/arch/arm64/kvm/hyp/switch.c
+++ b/arch/arm64/kvm/hyp/switch.c
@@ -469,7 +469,7 @@ static hyp_alternate_select(__hyp_call_panic,
 			    __hyp_call_panic_nvhe, __hyp_call_panic_vhe,
 			    ARM64_HAS_VIRT_HOST_EXTN);
 
-void __hyp_text __noreturn hyp_panic(struct kvm_cpu_context *__host_ctxt)
+void __hyp_text __noreturn hyp_panic(struct kvm_cpu_context *host_ctxt)
 {
 	struct kvm_vcpu *vcpu = NULL;
 
@@ -478,9 +478,6 @@ void __hyp_text __noreturn hyp_panic(struct kvm_cpu_context *__host_ctxt)
 	u64 par = read_sysreg(par_el1);
 
 	if (read_sysreg(vttbr_el2)) {
-		struct kvm_cpu_context *host_ctxt;
-
-		host_ctxt = kern_hyp_va(__host_ctxt);
 		vcpu = host_ctxt->__hyp_running_vcpu;
 		__timer_disable_traps(vcpu);
 		__deactivate_traps(vcpu);
diff --git a/arch/arm64/kvm/hyp/sysreg-sr.c b/arch/arm64/kvm/hyp/sysreg-sr.c
index 2c17afd2be96..43b7dd65e3e6 100644
--- a/arch/arm64/kvm/hyp/sysreg-sr.c
+++ b/arch/arm64/kvm/hyp/sysreg-sr.c
@@ -189,3 +189,8 @@ void __hyp_text __sysreg32_restore_state(struct kvm_vcpu *vcpu)
 	if (vcpu->arch.debug_flags & KVM_ARM64_DEBUG_DIRTY)
 		write_sysreg(sysreg[DBGVCR32_EL2], dbgvcr32_el2);
 }
+
+void __hyp_text __kvm_set_tpidr_el2(u64 tpidr_el2)
+{
+	asm("msr tpidr_el2, %0": : "r" (tpidr_el2));
+}
-- 
2.14.2