Skip automatic zero-init of large arrays / structs in I/O paths

[PATCH 11/31] hw/audio/gus: skip automatic zero-init of large array

Daniel P. Berrangé posted 31 patches 8 months ago

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Maintainers: Kevin Wolf <kwolf@redhat.com>, Hanna Reitz <hreitz@redhat.com>, "Marc-André Lureau" <marcandre.lureau@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>, Gerd Hoffmann <kraxel@redhat.com>, Halil Pasic <pasic@linux.ibm.com>, Christian Borntraeger <borntraeger@linux.ibm.com>, Thomas Huth <thuth@redhat.com>, Dmitry Fleytman <dmitry.fleytman@gmail.com>, Alistair Francis <alistair@alistair23.me>, "Edgar E. Iglesias" <edgar.iglesias@gmail.com>, Peter Maydell <peter.maydell@linaro.org>, "Cédric Le Goater" <clg@kaod.org>, Steven Lee <steven_lee@aspeedtech.com>, Troy Lee <leetroy@gmail.com>, Jamin Lin <jamin_lin@aspeedtech.com>, Andrew Jeffery <andrew@codeconstruct.com.au>, Joel Stanley <joel@jms.id.au>, Jason Wang <jasowang@redhat.com>, Sven Schnelle <svens@stackframe.org>, "Michael S. Tsirkin" <mst@redhat.com>, Rob Herring <robh@kernel.org>, Keith Busch <kbusch@kernel.org>, Klaus Jensen <its@irrelevant.dk>, Jesper Devantier <foss@defmacro.it>, Nicholas Piggin <npiggin@gmail.com>, "Frédéric Barrat" <fbarrat@linux.ibm.com>, Daniel Henrique Barboza <danielhb413@gmail.com>, Harsh Prateek Bora <harshpb@linux.ibm.com>, Fam Zheng <fam@euphon.net>, Hannes Reinecke <hare@suse.com>, Jeuk Kim <jeuk20.kim@samsung.com>

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[PATCH 11/31] hw/audio/gus: skip automatic zero-init of large array

Posted by Daniel P. Berrangé 8 months ago

The 'mv88w8618_audio_callback' method has a 4k byte array used for
copying data between the audio backend and device. Skip the automatic
zero-init of this array to eliminate the performance overhead in
the I/O hot path.

The 'buf' array will be fully initialized when reading data from
device memory.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
---
 hw/audio/marvell_88w8618.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/hw/audio/marvell_88w8618.c b/hw/audio/marvell_88w8618.c
index 6d3ebbb0c8..c5c79d083a 100644
--- a/hw/audio/marvell_88w8618.c
+++ b/hw/audio/marvell_88w8618.c
@@ -66,7 +66,7 @@ static void mv88w8618_audio_callback(void *opaque, int free_out, int free_in)
 {
     mv88w8618_audio_state *s = opaque;
     int16_t *codec_buffer;
-    int8_t buf[4096];
+    QEMU_UNINITIALIZED int8_t buf[4096];
     int8_t *mem_buffer;
     int pos, block_size;
 
-- 
2.49.0

Re: [PATCH 11/31] hw/audio/gus: skip automatic zero-init of large array

Posted by Daniel P. Berrangé 8 months ago

Mistake in $SUBJECT - should be 'hw/audio/marvell_88w8618:' prefix

On Tue, Jun 10, 2025 at 01:36:49PM +0100, Daniel P. Berrangé wrote:
> The 'mv88w8618_audio_callback' method has a 4k byte array used for
> copying data between the audio backend and device. Skip the automatic
> zero-init of this array to eliminate the performance overhead in
> the I/O hot path.
> 
> The 'buf' array will be fully initialized when reading data from
> device memory.
> 
> Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
> ---
>  hw/audio/marvell_88w8618.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/hw/audio/marvell_88w8618.c b/hw/audio/marvell_88w8618.c
> index 6d3ebbb0c8..c5c79d083a 100644
> --- a/hw/audio/marvell_88w8618.c
> +++ b/hw/audio/marvell_88w8618.c
> @@ -66,7 +66,7 @@ static void mv88w8618_audio_callback(void *opaque, int free_out, int free_in)
>  {
>      mv88w8618_audio_state *s = opaque;
>      int16_t *codec_buffer;
> -    int8_t buf[4096];
> +    QEMU_UNINITIALIZED int8_t buf[4096];
>      int8_t *mem_buffer;
>      int pos, block_size;
>  
> -- 
> 2.49.0
> 

With regards,
Daniel
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