Re: [PATCH v2] virtio: don't zero out memory region cache for indirect descriptors

[Stefan Hajnoczi]

On Fri, Aug 11, 2023 at 04:34:23PM +0200, Ilya Maximets wrote:
> Lots of virtio functions that are on a hot path in data transmission
> are initializing indirect descriptor cache at the point of stack
> allocation.  It's a 112 byte structure that is getting zeroed out on
> each call adding unnecessary overhead.  It's going to be correctly
> initialized later via special init function.  The only reason to
> actually initialize right away is the ability to safely destruct it.
> Replacing a designated initializer with a function to only initialize
> what is necessary.
> 
> Removal of the unnecessary stack initializations improves throughput
> of virtio-net devices in terms of 64B packets per second by 6-14 %
> depending on the case.  Tested with a proposed af-xdp network backend
> and a dpdk testpmd application in the guest, but should be beneficial
> for other virtio devices as well.
> 
> Signed-off-by: Ilya Maximets <i.maximets@ovn.org>
> ---
> 
> Version 2:
> 
>   * Introduced an initialization function, so we don't need to compare
>     pointers in the end. [Stefan]
>   * Removed the now unused macro. [Jason]
> 
>  hw/virtio/virtio.c    | 20 +++++++++++++++-----
>  include/exec/memory.h | 16 +++++++++++++---
>  2 files changed, 28 insertions(+), 8 deletions(-)

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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