Re: [RFC PATCH] VMCI: Silence memcpy() run-time false positive warning

[Vegard Nossum]

On 01/01/2024 14:55, Greg Kroah-Hartman wrote:
> On Mon, Jan 01, 2024 at 05:08:28AM -0800, Harshit Mogalapalli wrote:
> > One possible way to silence the warning is to split the memcpy() into
> > two parts -- one -- copying the msg and second taking care of payload.
>
> And what are the performance impacts of this?

I did a disasssembly diff for the version of the patch that uses
dg->payload_size directly in the second memcpy and I get this as the
only change:

@@ -419,11 +419,16 @@
        mov    %rax,%rbx
        test   %rax,%rax
        je
+       mov    0x0(%rbp),%rdx
        mov    %r14,(%rax)
-       mov    %r13,%rdx
-       mov    %rbp,%rsi
-       lea    0x30(%rax),%rdi
+       lea    0x18(%rbp),%rsi
+       lea    0x48(%rax),%rdi
        movb   $0x1,0x28(%rax)
+       mov    %rdx,0x30(%rax)
+       mov    0x8(%rbp),%rdx
+       mov    %rdx,0x38(%rax)
+       mov    0x10(%rbp),%rdx
+       mov    %rdx,0x40(%rax)
        call
        mov    0x0(%rip),%rsi        #
        lea    0x8(%rbx),%rdx

Basically, I believe it's inlining the first constant-size memcpy and
keeping the second one as a call.

Overall, the number of memory accesses should be the same.

The biggest impact that I can see is therefore the code size (which
isn't much).

There is also a kmalloc() on the same code path that I assume would
dwarf any performance impact from this patch -- but happy to be corrected.


Vegard