slow start of Pod HVM domU with qemu 9.1

[Olaf Hering <olaf@aepfle.de>]

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Hello,

starting with qemu 9.1 a PoD HVM domU takes a long time to start.
Depending on the domU kernel, it may trigger a warning, which prompted me
to notice this change in behavior:

[    0.000000] Linux version 4.12.14-120-default (geeko@buildhost) (gcc version 4.8.5 (SUSE Linux) ) #1 SMP Thu Nov 7 16:39:09 UTC 2019 (fd9dc36)
...
[    1.096432] HPET: 3 timers in total, 0 timers will be used for per-cpu timer
[    1.101636] hpet0: at MMIO 0xfed00000, IRQs 2, 8, 0
[    1.104051] hpet0: 3 comparators, 64-bit 62.500000 MHz counter
[   16.136086] random: crng init done
[   31.712052] BUG: workqueue lockup - pool cpus=1 node=0 flags=0x0 nice=0 stuck for 30s!
[   31.716029] Showing busy workqueues and worker pools:
[   31.721164] workqueue events: flags=0x0
[   31.724054]   pwq 2: cpus=1 node=0 flags=0x0 nice=0 active=2/256
[   31.728000]     in-flight: 17:balloon_process
[   31.728000]     pending: hpet_work
[   31.728023] workqueue mm_percpu_wq: flags=0x8
[   31.732987]   pwq 2: cpus=1 node=0 flags=0x0 nice=0 active=1/256
[   31.736000]     pending: vmstat_update
[   31.736024] pool 2: cpus=1 node=0 flags=0x0 nice=0 hung=30s workers=2 idle: 34
[   50.400102] clocksource: Switched to clocksource xen
[   50.441153] VFS: Disk quotas dquot_6.6.0
...

With qemu 9.0 and older, this domU will start the /init task after 8 seconds.

The change which caused this commit is qemu.git commit 9ecdd4bf08dfe4a37e16b8a8b228575aff641468
Author:     Edgar E. Iglesias <edgar.iglesias@amd.com>
AuthorDate: Tue Apr 30 10:26:45 2024 +0200
Commit:     Edgar E. Iglesias <edgar.iglesias@amd.com>
CommitDate: Sun Jun 9 20:16:14 2024 +0200

    xen: mapcache: Add support for grant mappings

As you can see, v4 instead of v5 was apparently applied.
This was probably unintentional, but would probably not change the result.

With this change the domU starts fast again:

--- a/hw/xen/xen-mapcache.c
+++ b/hw/xen/xen-mapcache.c
@@ -522,6 +522,7 @@ ram_addr_t xen_ram_addr_from_mapcache(void *ptr)
     ram_addr_t addr;
 
     addr = xen_ram_addr_from_mapcache_single(mapcache, ptr);
+    if (1)
     if (addr == RAM_ADDR_INVALID) {
         addr = xen_ram_addr_from_mapcache_single(mapcache_grants, ptr);
     }
@@ -626,6 +627,7 @@ static void xen_invalidate_map_cache_entry_single(MapCache *mc, uint8_t *buffer)
 static void xen_invalidate_map_cache_entry_all(uint8_t *buffer)
 {
     xen_invalidate_map_cache_entry_single(mapcache, buffer);
+    if (1)
     xen_invalidate_map_cache_entry_single(mapcache_grants, buffer);
 }
 
@@ -700,6 +702,7 @@ void xen_invalidate_map_cache(void)
     bdrv_drain_all();
 
     xen_invalidate_map_cache_single(mapcache);
+    if (0)
     xen_invalidate_map_cache_single(mapcache_grants);
 }
 
I did the testing with libvirt, the domU.cfg equivalent looks like this:
maxmem = 4096
memory = 2048
maxvcpus = 4
vcpus = 2
pae = 1
acpi = 1
apic = 1
viridian = 0
rtc_timeoffset = 0
localtime = 0
on_poweroff = "destroy"
on_reboot = "destroy"
on_crash = "destroy"
device_model_override = "/usr/lib64/qemu-9.1/bin/qemu-system-i386"
sdl = 0
vnc = 1
vncunused = 1
vnclisten = "127.0.0.1"
vif = [ "mac=52:54:01:23:63:29,bridge=br0,script=vif-bridge" ]
parallel = "none"
serial = "pty"
builder = "hvm"
kernel = "/bug1236329/linux"
ramdisk = "/bug1236329/initrd"
cmdline = "console=ttyS0,115200n8 quiet ignore_loglevel""
boot = "c" 
disk = [ "format=qcow2,vdev=hda,access=rw,backendtype=qdisk,target=/bug1236329/sles12sp5.qcow2" ]
usb = 1
usbdevice = "tablet"

Any idea what can be done to restore boot times?


Olaf

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