Re: [RFC PATCH 04/17] arm64: microoptimise exit path

[Jan Kiszka]

On 06.11.18 00:01, Ralf Ramsauer wrote:
Similar to 6b02cd08a506 ("inmates: arm64: save registers on irq entry"), use
immediate values to address the absolute offset within the stack when storing
registers.
This is a bit more efficient that the previous push-decrement stack pattern.

Can you provide a pointer why? KVM is doing the same, but it would still be 
better to document that if possible.


This patch also reverses the order of registers when being pushed. We will
later utilise this.

Signed-off-by: Ralf Ramsauer <ralf.ramsa...@oth-regensburg.de>
---
  hypervisor/arch/arm64/entry.S | 66 ++++++++++++++++++-----------------
  1 file changed, 34 insertions(+), 32 deletions(-)

diff --git a/hypervisor/arch/arm64/entry.S b/hypervisor/arch/arm64/entry.S
index d6280484..0aa8fed8 100644
--- a/hypervisor/arch/arm64/entry.S
+++ b/hypervisor/arch/arm64/entry.S
@@ -270,24 +270,25 @@ bootstrap_vectors:
  .macro handle_vmexit exit_reason
        .align  7
        /* Fill the union registers. Should comply with NUM_USR_REGS */
-       stp     x29, x30, [sp, #-16]!
-       stp     x27, x28, [sp, #-16]!
-       stp     x25, x26, [sp, #-16]!
-       stp     x23, x24, [sp, #-16]!
-       stp     x21, x22, [sp, #-16]!
-       stp     x19, x20, [sp, #-16]!
-       stp     x17, x18, [sp, #-16]!
-       stp     x15, x16, [sp, #-16]!
-       stp     x13, x14, [sp, #-16]!
-       stp     x11, x12, [sp, #-16]!
-       stp     x9, x10, [sp, #-16]!
-       stp     x7, x8, [sp, #-16]!
-       stp     x5, x6, [sp, #-16]!
-       stp     x3, x4, [sp, #-16]!
-       stp     x1, x2, [sp, #-16]!
+       sub     sp, sp, #(16 * 16)
+       stp     x1, x2, [sp, #(1 * 16)]
+       stp     x3, x4, [sp, #(2 * 16)]
+       stp     x5, x6, [sp, #(3 * 16)]
+       stp     x7, x8, [sp, #(4 * 16)]
+       stp     x9, x10, [sp, #(5 * 16)]
+       stp     x11, x12, [sp, #(6 * 16)]
+       stp     x13, x14, [sp, #(7 * 16)]
+       stp     x15, x16, [sp, #(8 * 16)]
+       stp     x17, x18, [sp, #(9 * 16)]
+       stp     x19, x20, [sp, #(10 * 16)]
+       stp     x21, x22, [sp, #(11 * 16)]
+       stp     x23, x24, [sp, #(12 * 16)]
+       stp     x25, x26, [sp, #(13 * 16)]
+       stp     x27, x28, [sp, #(14 * 16)]
+       stp     x29, x30, [sp, #(15 * 16)]
  
  	mov	x1, #\exit_reason
-       stp     x1, x0, [sp, #-16]!
+       stp     x1, x0, [sp, #(0 * 16)]
  
  	mov	x29, xzr	/* reset fp,lr */
        mov     x30, xzr
@@ -391,21 +392,22 @@ shutdown_el2:
  vmreturn:
        /* x0: union registers* */
        mov     sp, x0
-       ldp     x1, x0, [sp], #16       /* x1 is the exit_reason */
-       ldp     x1, x2, [sp], #16
-       ldp     x3, x4, [sp], #16
-       ldp     x5, x6, [sp], #16
-       ldp     x7, x8, [sp], #16
-       ldp     x9, x10, [sp], #16
-       ldp     x11, x12, [sp], #16
-       ldp     x13, x14, [sp], #16
-       ldp     x15, x16, [sp], #16
-       ldp     x17, x18, [sp], #16
-       ldp     x19, x20, [sp], #16
-       ldp     x21, x22, [sp], #16
-       ldp     x23, x24, [sp], #16
-       ldp     x25, x26, [sp], #16
-       ldp     x27, x28, [sp], #16
-       ldp     x29, x30, [sp], #16
+       ldp     x29, x30, [sp, #(15 * 16)]
+       ldp     x27, x28, [sp, #(14 * 16)]
+       ldp     x25, x26, [sp, #(13 * 16)]
+       ldp     x23, x24, [sp, #(12 * 16)]
+       ldp     x21, x22, [sp, #(11 * 16)]
+       ldp     x19, x20, [sp, #(10 * 16)]
+       ldp     x17, x18, [sp, #(9 * 16)]
+       ldp     x15, x16, [sp, #(8 * 16)]
+       ldp     x13, x14, [sp, #(7 * 16)]
+       ldp     x11, x12, [sp, #(6 * 16)]
+       ldp     x9, x10, [sp, #(5 * 16)]
+       ldp     x7, x8, [sp, #(4 * 16)]
+       ldp     x5, x6, [sp, #(3 * 16)]
+       ldp     x3, x4, [sp, #(2 * 16)]
+       ldp     x1, x0, [sp, #(0 * 16)] /* x1 is the exit_reason */

Maybe use the change make the comment clearer:
"x1 consumes orginal exit_reason" or so.

+       ldp     x1, x2, [sp, #(1 * 16)]
+       add     sp, sp, #(16 * 16)
        eret
        .popsection


Jan

--
Siemens AG, Corporate Technology, CT RDA IOT SES-DE
Corporate Competence Center Embedded Linux

--
You received this message because you are subscribed to the Google Groups 
"Jailhouse" group.
To unsubscribe from this group and stop receiving emails from it, send an email 
to jailhouse-dev+unsubscr...@googlegroups.com.
For more options, visit https://groups.google.com/d/optout.