x86/svm: Remove regs param from asm-called functions

It is easier just to make the functions void and let the compiler do the (not
very) hard work.

Passing regs is a bit weird for HVM guests anyway, because the resulting
pointer is invariant (this isn't native exception handling where the regs
pointers *are* important), and all functions calculate current themselves
which is another invariant.

Finally, the compiler can merge the get_cpu_info() calculation which is common
to both current and guest_cpu_user_regs(), meaning the delta in C really is
just one LEA, and not any more expensive than MOV's in ASM anyway.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/hvm/svm/entry.S

@@ -28,7 +28,6 @@ ENTRY(svm_asm_do_resume)
         GET_CURRENT(bx)
 .Lsvm_do_resume:
         call svm_intr_assist
-        mov  %rsp,%rdi
         call nsvm_vcpu_switch
         ASSERT_NOT_IN_ATOMIC
 
@@ -54,7 +53,6 @@ UNLIKELY_START(ne, nsvm_hap)
         jmp  .Lsvm_do_resume
 __UNLIKELY_END(nsvm_hap)
 
-        mov  %rsp, %rdi
         call svm_vmenter_helper
 
         clgi
@@ -134,7 +132,6 @@ __UNLIKELY_END(nsvm_hap)
          */
         stgi
 GLOBAL(svm_stgi_label)
-        mov  %rsp,%rdi
         call svm_vmexit_handler
         jmp  .Lsvm_do_resume
 

xen/arch/x86/hvm/svm/nestedsvm.c

@@ -1460,8 +1460,9 @@ nestedsvm_vcpu_vmexit(struct vcpu *v, struct cpu_user_regs *regs,
 }
 
 /* VCPU switch */
-void nsvm_vcpu_switch(struct cpu_user_regs *regs)
+void nsvm_vcpu_switch(void)
 {
+    struct cpu_user_regs *regs = guest_cpu_user_regs();
     struct vcpu *v = current;
     struct nestedvcpu *nv;
     struct nestedsvm *svm;

xen/arch/x86/hvm/svm/svm.c

@@ -1055,8 +1055,9 @@ static void noreturn cf_check svm_do_resume(void)
     reset_stack_and_jump(svm_asm_do_resume);
 }
 
-void svm_vmenter_helper(const struct cpu_user_regs *regs)
+void svm_vmenter_helper(void)
 {
+    const struct cpu_user_regs *regs = guest_cpu_user_regs();
     struct vcpu *curr = current;
     struct vmcb_struct *vmcb = curr->arch.hvm.svm.vmcb;
 
@@ -2585,8 +2586,9 @@ const struct hvm_function_table * __init start_svm(void)
     return &svm_function_table;
 }
 
-void svm_vmexit_handler(struct cpu_user_regs *regs)
+void svm_vmexit_handler(void)
 {
+    struct cpu_user_regs *regs = guest_cpu_user_regs();
     uint64_t exit_reason;
     struct vcpu *v = current;
     struct vmcb_struct *vmcb = v->arch.hvm.svm.vmcb;