From: Daniel P. Berrangé <berrange@redhat.com>
Date: Tue, 10 Jun 2025 12:37:06 +0000 (+0100)
Subject: hw/scsi/megasas: skip automatic zero-init of large arrays
X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=ca0559e2350c618048f7caf80cb79c1259e7cfd2;p=thirdparty%2Fqemu.git

hw/scsi/megasas: skip automatic zero-init of large arrays

The 'megasas_dcmd_pd_get_list' and 'megasas_dcmd_get_properties'
methods have 4k structs used for copying data from the device.
Skip the automatic zero-init of this array to eliminate the
performance overhead in the I/O hot path.

The 'info' structs are manually initialized with memset(). The
compiler ought to be intelligent enough to turn the memset()
into a static initialization operation, and thus not duplicate
the automatic zero-init. Replacing memset() with '{}' makes it
unambiguous that the arrays are statically initialized.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Klaus Jensen <k.jensen@samsung.com>
Reviewed-by: Harsh Prateek Bora <harshpb@linux.ibm.com>
Message-id: 20250610123709.835102-29-berrange@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---

diff --git a/hw/scsi/megasas.c b/hw/scsi/megasas.c
index 55cd188bd5..844643d916 100644
--- a/hw/scsi/megasas.c
+++ b/hw/scsi/megasas.c
@@ -981,13 +981,11 @@ static int megasas_event_wait(MegasasState *s, MegasasCmd *cmd)
 
 static int megasas_dcmd_pd_get_list(MegasasState *s, MegasasCmd *cmd)
 {
-    struct mfi_pd_list info;
-    size_t dcmd_size = sizeof(info);
+    struct mfi_pd_list info = {};
     BusChild *kid;
     uint32_t offset, dcmd_limit, num_pd_disks = 0, max_pd_disks;
     dma_addr_t residual;
 
-    memset(&info, 0, dcmd_size);
     offset = 8;
     dcmd_limit = offset + sizeof(struct mfi_pd_address);
     if (cmd->iov_size < dcmd_limit) {
@@ -1429,11 +1427,10 @@ static int megasas_dcmd_cfg_read(MegasasState *s, MegasasCmd *cmd)
 
 static int megasas_dcmd_get_properties(MegasasState *s, MegasasCmd *cmd)
 {
-    struct mfi_ctrl_props info;
+    struct mfi_ctrl_props info = {};
     size_t dcmd_size = sizeof(info);
     dma_addr_t residual;
 
-    memset(&info, 0x0, dcmd_size);
     if (cmd->iov_size < dcmd_size) {
         trace_megasas_dcmd_invalid_xfer_len(cmd->index, cmd->iov_size,
                                             dcmd_size);