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

[PATCH 24/31] hw/ppc/pnv_occ: skip automatic zero-init of large struct

Daniel P. Berrangé posted 31 patches 5 months, 1 week 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 24/31] hw/ppc/pnv_occ: skip automatic zero-init of large struct

Posted by Daniel P. Berrangé 5 months, 1 week ago

The 'occ_model_tick' method has a 12k struct used for copying
data between guest and host. Skip the automatic zero-init of this
struct to eliminate the performance overhead in the I/O hot path.

The 'dynamic_data' buffer will be fully initialized when reading
data from the guest.

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

diff --git a/hw/ppc/pnv_occ.c b/hw/ppc/pnv_occ.c
index fa6f31cb8d..24b789c191 100644
--- a/hw/ppc/pnv_occ.c
+++ b/hw/ppc/pnv_occ.c
@@ -789,7 +789,7 @@ static bool occ_opal_process_command(PnvOCC *occ,
 
 static bool occ_model_tick(PnvOCC *occ)
 {
-    struct occ_dynamic_data dynamic_data;
+    QEMU_UNINITIALIZED struct occ_dynamic_data dynamic_data;
 
     if (!occ_read_dynamic_data(occ, &dynamic_data, NULL)) {
         /* Can't move OCC state field to safe because we can't map it! */
-- 
2.49.0

Re: [PATCH 24/31] hw/ppc/pnv_occ: skip automatic zero-init of large struct

Posted by Harsh Prateek Bora 5 months, 1 week ago


On 6/10/25 18:07, Daniel P. Berrangé wrote:
> The 'occ_model_tick' method has a 12k struct used for copying
> data between guest and host. Skip the automatic zero-init of this
> struct to eliminate the performance overhead in the I/O hot path.
> 
> The 'dynamic_data' buffer will be fully initialized when reading
> data from the guest.
> 
> Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
> ---
>   hw/ppc/pnv_occ.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/hw/ppc/pnv_occ.c b/hw/ppc/pnv_occ.c
> index fa6f31cb8d..24b789c191 100644
> --- a/hw/ppc/pnv_occ.c
> +++ b/hw/ppc/pnv_occ.c
> @@ -789,7 +789,7 @@ static bool occ_opal_process_command(PnvOCC *occ,
>   
>   static bool occ_model_tick(PnvOCC *occ)
>   {
> -    struct occ_dynamic_data dynamic_data;
> +    QEMU_UNINITIALIZED struct occ_dynamic_data dynamic_data;

Reviewed-by: Harsh Prateek Bora <harshpb@linux.ibm.com>

>   
>       if (!occ_read_dynamic_data(occ, &dynamic_data, NULL)) {
>           /* Can't move OCC state field to safe because we can't map it! */