[Xen-devel] [PATCH v3 5/4] x86: reduce code size of struct cpu_info member accesses

["Jan Beulich" <JBeulich@xxxxxxxx>]

Instead of addressing these fields via the base of the stack (which
uniformly requires 4-byte displacements), address them from the end
(which for everything other than guest_cpu_user_regs requires just
1-byte ones). This yields a code size reduction somewhere between 8k
and 12k in my builds.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Note that just like patch 4 of the series this also isn't directly
related to the SMEP/SMAP issue, but is again just a result of things
realized while doing that work, and again depends on the earlier
patches to apply cleanly.

--- a/xen/arch/x86/hvm/svm/entry.S
+++ b/xen/arch/x86/hvm/svm/entry.S
@@ -31,7 +31,7 @@
 #define CLGI   .byte 0x0F,0x01,0xDD
 
 ENTRY(svm_asm_do_resume)
-        GET_CURRENT(%rbx)
+        GET_CURRENT(bx)
 .Lsvm_do_resume:
         call svm_intr_assist
         mov  %rsp,%rdi
@@ -97,7 +97,7 @@ UNLIKELY_END(svm_trace)
 
         VMRUN
 
-        GET_CURRENT(%rax)
+        GET_CURRENT(ax)
         push %rdi
         push %rsi
         push %rdx
--- a/xen/arch/x86/hvm/vmx/entry.S
+++ b/xen/arch/x86/hvm/vmx/entry.S
@@ -40,7 +40,7 @@ ENTRY(vmx_asm_vmexit_handler)
         push %r10
         push %r11
         push %rbx
-        GET_CURRENT(%rbx)
+        GET_CURRENT(bx)
         push %rbp
         push %r12
         push %r13
@@ -113,7 +113,7 @@ UNLIKELY_END(realmode)
         BUG  /* vmx_vmentry_failure() shouldn't return. */
 
 ENTRY(vmx_asm_do_vmentry)
-        GET_CURRENT(%rbx)
+        GET_CURRENT(bx)
         jmp  .Lvmx_do_vmentry
 
 .Lvmx_goto_emulator:
--- a/xen/arch/x86/x86_64/compat/entry.S
+++ b/xen/arch/x86/x86_64/compat/entry.S
@@ -26,7 +26,7 @@ UNLIKELY_START(ne, msi_check)
 UNLIKELY_END(msi_check)
 
         movl  UREGS_rax(%rsp),%eax
-        GET_CURRENT(%rbx)
+        GET_CURRENT(bx)
 
         cmpl  $NR_hypercalls,%eax
         jae   compat_bad_hypercall
@@ -202,7 +202,7 @@ ENTRY(compat_restore_all_guest)
 /* This mustn't modify registers other than %rax. */
 ENTRY(cr4_pv32_restore)
         push  %rdx
-        GET_CPUINFO_FIELD(cr4, %rdx)
+        GET_CPUINFO_FIELD(cr4, dx)
         mov   (%rdx), %rax
         test  $X86_CR4_SMEP|X86_CR4_SMAP,%eax
         jnz   0f
@@ -245,7 +245,7 @@ ENTRY(cstar_enter)
         pushq %rcx
         pushq $0
         SAVE_VOLATILE TRAP_syscall
-        GET_CURRENT(%rbx)
+        GET_CURRENT(bx)
         movq  VCPU_domain(%rbx),%rcx
         cmpb  $0,DOMAIN_is_32bit_pv(%rcx)
         je    switch_to_kernel
--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -97,7 +97,7 @@ ENTRY(lstar_enter)
         pushq %rcx
         pushq $0
         SAVE_VOLATILE TRAP_syscall
-        GET_CURRENT(%rbx)
+        GET_CURRENT(bx)
         testb $TF_kernel_mode,VCPU_thread_flags(%rbx)
         jz    switch_to_kernel
 
@@ -246,7 +246,7 @@ GLOBAL(sysenter_eflags_saved)
         pushq $0 /* null rip */
         pushq $0
         SAVE_VOLATILE TRAP_syscall
-        GET_CURRENT(%rbx)
+        GET_CURRENT(bx)
         cmpb  $0,VCPU_sysenter_disables_events(%rbx)
         movq  VCPU_sysenter_addr(%rbx),%rax
         setne %cl
@@ -288,7 +288,7 @@ UNLIKELY_START(ne, msi_check)
         call  check_for_unexpected_msi
 UNLIKELY_END(msi_check)
 
-        GET_CURRENT(%rbx)
+        GET_CURRENT(bx)
 
         /* Check that the callback is non-null. */
         leaq  VCPU_int80_bounce(%rbx),%rdx
@@ -420,10 +420,10 @@ domain_crash_page_fault:
         call  show_page_walk
 ENTRY(dom_crash_sync_extable)
         # Get out of the guest-save area of the stack.
-        GET_STACK_BASE(%rax)
+        GET_STACK_END(ax)
         leaq  STACK_CPUINFO_FIELD(guest_cpu_user_regs)(%rax),%rsp
         # create_bounce_frame() temporarily clobbers CS.RPL. Fix up.
-        __GET_CURRENT(%rax)
+        __GET_CURRENT(ax)
         movq  VCPU_domain(%rax),%rax
         testb $1,DOMAIN_is_32bit_pv(%rax)
         setz  %al
@@ -441,7 +441,7 @@ ENTRY(common_interrupt)
 
 /* No special register assumptions. */
 ENTRY(ret_from_intr)
-        GET_CURRENT(%rbx)
+        GET_CURRENT(bx)
         testb $3,UREGS_cs(%rsp)
         jz    restore_all_xen
         movq  VCPU_domain(%rbx),%rax
@@ -455,7 +455,7 @@ ENTRY(page_fault)
 GLOBAL(handle_exception)
         SAVE_ALL CLAC
 handle_exception_saved:
-        GET_CURRENT(%rbx)
+        GET_CURRENT(bx)
         testb $X86_EFLAGS_IF>>8,UREGS_eflags+1(%rsp)
         jz    exception_with_ints_disabled
 
@@ -649,7 +649,7 @@ handle_ist_exception:
         testb $3,UREGS_cs(%rsp)
         jz    1f
         /* Interrupted guest context. Copy the context to stack bottom. */
-        GET_CPUINFO_FIELD(guest_cpu_user_regs,%rdi)
+        GET_CPUINFO_FIELD(guest_cpu_user_regs,di)
         movq  %rsp,%rsi
         movl  $UREGS_kernel_sizeof/8,%ecx
         movq  %rdi,%rsp
@@ -664,7 +664,7 @@ handle_ist_exception:
         /* We want to get straight to the IRET on the NMI exit path. */
         testb $3,UREGS_cs(%rsp)
         jz    restore_all_xen
-        GET_CURRENT(%rbx)
+        GET_CURRENT(bx)
         /* Send an IPI to ourselves to cover for the lack of event checking. */
         movl  VCPU_processor(%rbx),%eax
         shll  $IRQSTAT_shift,%eax
--- a/xen/include/asm-x86/asm_defns.h
+++ b/xen/include/asm-x86/asm_defns.h
@@ -127,19 +127,19 @@ void ret_from_intr(void);
         UNLIKELY_DONE(mp, tag);   \
         __UNLIKELY_END(tag)
 
-#define STACK_CPUINFO_FIELD(field) (STACK_SIZE-CPUINFO_sizeof+CPUINFO_##field)
-#define GET_STACK_BASE(reg)                       \
-        movq $~(STACK_SIZE-1),reg;                \
-        andq %rsp,reg
+#define STACK_CPUINFO_FIELD(field) (1 - CPUINFO_sizeof + CPUINFO_##field)
+#define GET_STACK_END(reg)                        \
+        movl $STACK_SIZE-1, %e##reg;              \
+        orq  %rsp, %r##reg
 
 #define GET_CPUINFO_FIELD(field, reg)             \
-        GET_STACK_BASE(reg);                      \
-        addq $STACK_CPUINFO_FIELD(field),reg
+        GET_STACK_END(reg);                       \
+        addq $STACK_CPUINFO_FIELD(field), %r##reg
 
 #define __GET_CURRENT(reg)                        \
-        movq STACK_CPUINFO_FIELD(current_vcpu)(reg),reg
+        movq STACK_CPUINFO_FIELD(current_vcpu)(%r##reg), %r##reg
 #define GET_CURRENT(reg)                          \
-        GET_STACK_BASE(reg);                      \
+        GET_STACK_END(reg);                       \
         __GET_CURRENT(reg)
 
 #ifndef NDEBUG
--- a/xen/include/asm-x86/current.h
+++ b/xen/include/asm-x86/current.h
@@ -55,7 +55,7 @@ static inline struct cpu_info *get_cpu_i
     register unsigned long sp asm("rsp");
 #endif
 
-    return (struct cpu_info *)((sp & ~(STACK_SIZE-1)) + STACK_SIZE) - 1;
+    return (struct cpu_info *)((sp | (STACK_SIZE - 1)) + 1) - 1;
 }
 
 #define get_current()         (get_cpu_info()->current_vcpu)

Attachment: x86-gci-use-or.patch
Description: Text document

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