pve-qemu/debian/patches/pve/0045-PVE-block-pbs-fast-path-reads-without-allocation-if-.patch

From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Stefan Reiter <s.reiter@proxmox.com>
Date: Wed, 9 Dec 2020 11:46:57 +0100
Subject: [PATCH] PVE: block/pbs: fast-path reads without allocation if
 possible

...and switch over to g_malloc/g_free while at it to align with other
QEMU code.

Tracing shows the fast-path is taken almost all the time, though not
100% so the slow one is still necessary.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
---
 block/pbs.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/block/pbs.c b/block/pbs.c
index 9d1f1f39d4..ce9a870885 100644
--- a/block/pbs.c
+++ b/block/pbs.c
@@ -200,7 +200,16 @@ static coroutine_fn int pbs_co_preadv(BlockDriverState *bs,
     BDRVPBSState *s = bs->opaque;
     int ret;
     char *pbs_error = NULL;
-    uint8_t *buf = malloc(bytes);
+    uint8_t *buf;
+    bool inline_buf = true;
+
+    /* for single-buffer IO vectors we can fast-path the write directly to it */
+    if (qiov->niov == 1 && qiov->iov->iov_len >= bytes) {
+        buf = qiov->iov->iov_base;
+    } else {
+        inline_buf = false;
+        buf = g_malloc(bytes);
+    }
 
     if (offset < 0 || bytes < 0) {
         fprintf(stderr, "unexpected negative 'offset' or 'bytes' value!\n");
@@ -223,8 +232,10 @@ static coroutine_fn int pbs_co_preadv(BlockDriverState *bs,
         return -EIO;
     }
 
-    qemu_iovec_from_buf(qiov, 0, buf, bytes);
-    free(buf);
+    if (!inline_buf) {
+        qemu_iovec_from_buf(qiov, 0, buf, bytes);
+        g_free(buf);
+    }
 
     return 0;
 }