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

Gustavo A. R. Silva
2024-01-04T19:02:46Z

On 1/4/24 12:31, Vegard Nossum wrote:
>
> 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).

Yep, I don't think this is a problem.

I look forward to reviewing v2 of this patch.

Thanks
--
Gustavo

>
> 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
>