hw/nvme/ctrl: skip automatic zero-init of large arrays

The 'nvme_map_sgl' method has a 256 element array 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 'segment' array will be fully initialized when reading data from
the device.

The 'nme_changed_nslist' method has a 4k byte array that is 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 array is 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>
Message-id: 20250610123709.835102-24-berrange@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

diff --git a/hw/nvme/ctrl.c b/hw/nvme/ctrl.c
index fd935507bc0280c1f49822f9e3cb035df596ae47..220002830ded1f0f89a15ae8dca1b3bd5a619e40 100644 (file)
--- a/hw/nvme/ctrl.c
+++ b/hw/nvme/ctrl.c
@@ -1057,7 +1057,8 @@ static uint16_t nvme_map_sgl(NvmeCtrl *n, NvmeSg *sg, NvmeSglDescriptor sgl,
      */
 #define SEG_CHUNK_SIZE 256
 
-    NvmeSglDescriptor segment[SEG_CHUNK_SIZE], *sgld, *last_sgld;
+    QEMU_UNINITIALIZED NvmeSglDescriptor segment[SEG_CHUNK_SIZE];
+    NvmeSglDescriptor *sgld, *last_sgld;
     uint64_t nsgld;
     uint32_t seg_len;
     uint16_t status;
@@ -5128,7 +5129,7 @@ static uint16_t nvme_error_info(NvmeCtrl *n, uint8_t rae, uint32_t buf_len,
 static uint16_t nvme_changed_nslist(NvmeCtrl *n, uint8_t rae, uint32_t buf_len,
                                     uint64_t off, NvmeRequest *req)
 {
-    uint32_t nslist[1024];
+    uint32_t nslist[1024] = {};
     uint32_t trans_len;
     int i = 0;
     uint32_t nsid;
@@ -5138,7 +5139,6 @@ static uint16_t nvme_changed_nslist(NvmeCtrl *n, uint8_t rae, uint32_t buf_len,
         return NVME_INVALID_FIELD | NVME_DNR;
     }
 
-    memset(nslist, 0x0, sizeof(nslist));
     trans_len = MIN(sizeof(nslist) - off, buf_len);
 
     while ((nsid = find_first_bit(n->changed_nsids, NVME_CHANGED_NSID_SIZE)) !=