[xen staging-4.18] x86/hvm: Simplify stdvga_mem_accept() further

[patchbot@xxxxxxx]

commit 20d34c1e82402061b4a0be1b9e504ae55abdc5b6
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Tue Nov 12 13:53:40 2024 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Nov 12 13:53:40 2024 +0100

    x86/hvm: Simplify stdvga_mem_accept() further
    
    stdvga_mem_accept() is called on almost all IO emulations, and the
    overwhelming likely answer is to reject the ioreq.  Simply rearranging the
    expression yields an improvement:
    
      add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-57 (-57)
      Function                                     old     new   delta
      stdvga_mem_accept                            109      52     -57
    
    which is best explained looking at the disassembly:
    
      Before:                                                    After:
      f3 0f 1e fa           endbr64                              f3 0f 1e fa    
       endbr64
      0f b6 4e 1e           movzbl 0x1e(%rsi),%ecx            |  0f b6 46 1e    
       movzbl 0x1e(%rsi),%eax
      48 8b 16              mov    (%rsi),%rdx                |  31 d2          
       xor    %edx,%edx
      f6 c1 40              test   $0x40,%cl                  |  a8 30          
       test   $0x30,%al
      75 38                 jne    <stdvga_mem_accept+0x48>   |  75 23          
       jne    <stdvga_mem_accept+0x31>
      31 c0                 xor    %eax,%eax                  <
      48 81 fa ff ff 09 00  cmp    $0x9ffff,%rdx              <
      76 26                 jbe    <stdvga_mem_accept+0x41>   <
      8b 46 14              mov    0x14(%rsi),%eax            <
      8b 7e 10              mov    0x10(%rsi),%edi            <
      48 0f af c7           imul   %rdi,%rax                  <
      48 8d 54 02 ff        lea    -0x1(%rdx,%rax,1),%rdx     <
      31 c0                 xor    %eax,%eax                  <
      48 81 fa ff ff 0b 00  cmp    $0xbffff,%rdx              <
      77 0c                 ja     <stdvga_mem_accept+0x41>   <
      83 e1 30              and    $0x30,%ecx                 <
      75 07                 jne    <stdvga_mem_accept+0x41>   <
      83 7e 10 01           cmpl   $0x1,0x10(%rsi)               83 7e 10 01    
       cmpl   $0x1,0x10(%rsi)
      0f 94 c0              sete   %al                        |  75 1d          
       jne    <stdvga_mem_accept+0x31>
      c3                    ret                               |  48 8b 0e       
       mov    (%rsi),%rcx
      66 0f 1f 44 00 00     nopw   0x0(%rax,%rax,1)           |  48 81 f9 ff ff 
09 00  cmp    $0x9ffff,%rcx
      8b 46 10              mov    0x10(%rsi),%eax            |  76 11          
       jbe    <stdvga_mem_accept+0x31>
      8b 7e 14              mov    0x14(%rsi),%edi            |  8b 46 14       
       mov    0x14(%rsi),%eax
      49 89 d0              mov    %rdx,%r8                   |  48 8d 44 01 ff 
       lea    -0x1(%rcx,%rax,1),%rax
      48 83 e8 01           sub    $0x1,%rax                  |  48 3d ff ff 0b 
00     cmp    $0xbffff,%rax
      48 8d 54 3a ff        lea    -0x1(%rdx,%rdi,1),%rdx     |  0f 96 c2       
       setbe  %dl
      48 0f af c7           imul   %rdi,%rax                  |  89 d0          
       mov    %edx,%eax
      49 29 c0              sub    %rax,%r8                   <
      31 c0                 xor    %eax,%eax                  <
      49 81 f8 ff ff 09 00  cmp    $0x9ffff,%r8               <
      77 be                 ja     <stdvga_mem_accept+0x2a>   <
      c3                    ret                                  c3             
       ret
    
    By moving the "p->count != 1" check ahead of the
    ioreq_mmio_{first,last}_byte() calls, both multiplies disappear along with a
    lot of surrounding logic.
    
    No functional change.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    (cherry picked from commit 08ffd8705d36c7c445df3ecee8ad9b8f8d65fbe0)
---
 xen/arch/x86/hvm/stdvga.c | 16 ++++++----------
 1 file changed, 6 insertions(+), 10 deletions(-)

diff --git a/xen/arch/x86/hvm/stdvga.c b/xen/arch/x86/hvm/stdvga.c
index d38d30affb..c3c43f59ee 100644
--- a/xen/arch/x86/hvm/stdvga.c
+++ b/xen/arch/x86/hvm/stdvga.c
@@ -69,18 +69,14 @@ static int cf_check stdvga_mem_write(
 static bool cf_check stdvga_mem_accept(
     const struct hvm_io_handler *handler, const ioreq_t *p)
 {
-    if ( (ioreq_mmio_first_byte(p) < VGA_MEM_BASE) ||
+    /*
+     * Only accept single direct writes, as that's the only thing we can
+     * accelerate using buffered ioreq handling.
+     */
+    if ( p->dir != IOREQ_WRITE || p->data_is_ptr || p->count != 1 ||
+         (ioreq_mmio_first_byte(p) < VGA_MEM_BASE) ||
          (ioreq_mmio_last_byte(p) >= (VGA_MEM_BASE + VGA_MEM_SIZE)) )
-        return 0;
-
-    if ( p->dir != IOREQ_WRITE || p->data_is_ptr || p->count != 1 )
-    {
-        /*
-         * Only accept single direct writes, as that's the only thing we can
-         * accelerate using buffered ioreq handling.
-         */
         return false;
-    }
 
     return true;
 }
--
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