A optimisation is going to want to conditionally have extra data on the stack
around VMExit.

We could alternative between `mov %rsp, %rdi` and `lea 8(%rsp), %rdi`, but 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>
---
CC: Jan Beulich <JBeulich@suse.com>
CC: Roger Pau Monné <roger.pau@citrix.com>
CC: Wei Liu <wl@xen.org>
---
 xen/arch/x86/hvm/svm/entry.S     | 3 ---
 xen/arch/x86/hvm/svm/nestedsvm.c | 3 ++-
 xen/arch/x86/hvm/svm/svm.c       | 6 ++++--
 3 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/xen/arch/x86/hvm/svm/entry.S b/xen/arch/x86/hvm/svm/entry.S
index a60d759f7108..be4ce52bd81d 100644
--- a/xen/arch/x86/hvm/svm/entry.S
+++ b/xen/arch/x86/hvm/svm/entry.S
@@ -26,7 +26,6 @@ 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
 
@@ -52,7 +51,6 @@ UNLIKELY_START(ne, nsvm_hap)
         jmp  .Lsvm_do_resume
 __UNLIKELY_END(nsvm_hap)
 
-        mov  %rsp, %rdi
         call svm_vmenter_helper
 
         clgi
@@ -132,7 +130,6 @@ __UNLIKELY_END(nsvm_hap)
          */
         stgi
 GLOBAL(svm_stgi_label)
-        mov  %rsp,%rdi
         call svm_vmexit_handler
         jmp  .Lsvm_do_resume
 
diff --git a/xen/arch/x86/hvm/svm/nestedsvm.c b/xen/arch/x86/hvm/svm/nestedsvm.c
index 9f5f35f16aff..77f754736023 100644
--- a/xen/arch/x86/hvm/svm/nestedsvm.c
+++ b/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;
diff --git a/xen/arch/x86/hvm/svm/svm.c b/xen/arch/x86/hvm/svm/svm.c
index 0849a9dc5f41..81f0cf55676b 100644
--- a/xen/arch/x86/hvm/svm/svm.c
+++ b/xen/arch/x86/hvm/svm/svm.c
@@ -1040,8 +1040,9 @@ 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;
 
@@ -2570,8 +2571,9 @@ static struct hvm_function_table __initdata_cf_clobber 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;
-- 
2.11.0

On 11.08.2022 21:59, Andrew Cooper wrote:
> A optimisation is going to want to conditionally have extra data on the stack
> around VMExit.
> 
> We could alternative between `mov %rsp, %rdi` and `lea 8(%rsp), %rdi`, but 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>