MIPS: KVM: Restore host EBase from ebase variable

The host kernel's exception vector base address is currently saved in
the VCPU structure at creation time, and restored on a guest exit.
However it doesn't change and can already be easily accessed from the
'ebase' variable (arch/mips/kernel/traps.c), so drop the host_ebase
member of kvm_vcpu_arch, export the 'ebase' variable to modules and load
from there instead.

This does result in a single extra instruction (lui) on the guest exit
path, but simplifies the code a bit and removes the redundant storage of
the host exception base address.

Credit for the idea goes to Cavium's VZ KVM implementation.
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: kvm@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/mips/include/asm/kvm_host.h

@@ -334,7 +334,7 @@ struct kvm_mips_tlb {
 
 #define KVM_MIPS_GUEST_TLB_SIZE	64
 struct kvm_vcpu_arch {
-	void *host_ebase, *guest_ebase;
+	void *guest_ebase;
 	int (*vcpu_run)(struct kvm_run *run, struct kvm_vcpu *vcpu);
 	unsigned long host_stack;
 	unsigned long host_gp;

arch/mips/kernel/asm-offsets.c

@@ -355,7 +355,6 @@ void output_kvm_defines(void)
 	OFFSET(VCPU_RUN, kvm_vcpu, run);
 	OFFSET(VCPU_HOST_ARCH, kvm_vcpu, arch);
 
-	OFFSET(VCPU_HOST_EBASE, kvm_vcpu_arch, host_ebase);
 	OFFSET(VCPU_GUEST_EBASE, kvm_vcpu_arch, guest_ebase);
 
 	OFFSET(VCPU_HOST_STACK, kvm_vcpu_arch, host_stack);

arch/mips/kernel/traps.c

@@ -1859,6 +1859,7 @@ void __noreturn nmi_exception_handler(struct pt_regs *regs)
 #define VECTORSPACING 0x100	/* for EI/VI mode */
 
 unsigned long ebase;
+EXPORT_SYMBOL_GPL(ebase);
 unsigned long exception_handlers[32];
 unsigned long vi_handlers[64];
 

arch/mips/kvm/locore.S

@@ -319,7 +319,7 @@ NESTED (MIPSX(GuestException), CALLFRAME_SIZ, ra)
 	mtc0	k0, CP0_STATUS
 	ehb
 
-	LONG_L	k0, VCPU_HOST_EBASE(k1)
+	LONG_L	k0, ebase
 	mtc0	k0,CP0_EBASE
 
 	/*

arch/mips/kvm/mips.c

@@ -273,9 +273,6 @@ struct kvm_vcpu *kvm_arch_vcpu_create(struct kvm *kvm, unsigned int id)
 	else
 		size = 0x4000;
 
-	/* Save Linux EBASE */
-	vcpu->arch.host_ebase = (void *)read_c0_ebase();
-
 	gebase = kzalloc(ALIGN(size, PAGE_SIZE), GFP_KERNEL);
 
 	if (!gebase) {