Re: [PATCH 00/31] Skip automatic zero-init of large arrays / structs in I/O paths

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

On 10/6/25 14:36, Daniel P. Berrangé wrote:
> This series is an extension of Stefan's proposal:
> 
>    https://lists.nongnu.org/archive/html/qemu-devel/2025-06/msg00736.html
> 
> It used '-Wframe-larger-than=4096' to identify all code locations
> with more than 4k on the stack. Any locations in the I/O paths
> were chosen to avoid automatic zero-init, to eliminate the performance
> overhead of the automatic initialization.

Should we eventually add it to our default CFLAGS?

> Note, although all the changed locations are I/O paths, this does not
> imply that the changes make a measurement performance difference in
> every case.
> 
> This is because many of the emulated devices are likely limited by the
> emulation impl, rather than any implicit memory zero'ing overhead of
> stack data.
> 
> None the less the memory zero'ing is still a redundant CPU burn in
> all these cases, so I felt it worth setting the general precedent
> that any data over 4k on a stack in a device I/O path should be
> skipping zero-init.
> 
> I did reasonable review in each case to identify that the data was
> indeed initialized explicitly later in the method.