Re: [Qemu-devel] [PATCH 0/6] virtio: handle non contigious s/g entries

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Oct 27, 2015 at 10:47:54AM +0200, Michael S. Tsirkin wrote:
> TL;DR:
> This fixes virtio in a way transparent to guest.
> We should now be able to revert commits aa8580cd and df0acded19ec which worked
> around it in a way that's not transparent.

Note: I'm clean out of time so this is compiled only.
Igor said he can test. Thanks Igor!

> -----
> 
> commit aa8580cd "pc: memhp: force gaps between DIMM's GPA"
> introduced gaps in GPA space to work around a bug in virtio,
> originally reported by Igor, and I quote:
> 
> ------
>     QEMU aborts during guest reboot with following backtrace:
> 
>     Breakpoint 1, virtqueue_map_sg (sg=0x555557cbc6c0, addr=0x555557cb86c0, 
>     num_sg=0x12, is_write=0x1) at hw/virtio/virtio.c:453
>     453                 error_report("virtio: error trying to map MMIO memory");
>     (gdb) bt
>     #0  virtqueue_map_sg (sg=0x555557cbc6c0, addr=0x555557cb86c0, num_sg=0x12, 
>     is_write=0x1) at hw/virtio/virtio.c:453
>     #1  0x000055555569b3ef in virtqueue_pop (vq=0x555558a3fab0, 
>     elem=0x555557cb86b0) at hw/virtio/virtio.c:520
>     #2  0x0000555555666611 in virtio_blk_get_request (s=0x5555588c7a00) at 
>     hw/block/virtio-blk.c:194
>     #3  0x00005555556676ec in virtio_blk_handle_output (vdev=0x5555588c7a00, 
>     vq=0x555558a3fab0) at hw/block/virtio-blk.c:603
>     #4  0x000055555569c5c8 in virtio_queue_notify_vq (vq=0x555558a3fab0) at 
>     hw/virtio/virtio.c:921
>     #5  0x000055555569e009 in virtio_queue_host_notifier_read (n=0x555558a3faf8) at 
>     hw/virtio/virtio.c:1480
>     #6  0x000055555591b062 in qemu_iohandler_poll (pollfds=0x555556363800, ret=0x1) 
>     at iohandler.c:126
>     #7  0x000055555591ad33 in main_loop_wait (nonblocking=0x0) at main-loop.c:503
>     #8  0x00005555557466b5 in main_loop () at vl.c:1902
>     #9  0x000055555574e69b in main (argc=0x4d, argv=0x7fffffffda18, 
>     envp=0x7fffffffdc88) at vl.c:4653
> 
>     could be reproduced with following options:
> 
>     -enable-kvm  -m 1G,slots=250,maxmem=32G  -drive if=virtio,file=rhel72 -netdev 
>     tap,id=foo,ifname=tap0,script=./qemu-ifup -device 
>     virtio-net-pci,id=n1,netdev=foo `for i in $(seq 0 15); do echo -n "-object 
>     memory-backend-ram,id=m$i,size=10M -device pc-dimm,id=dimm$i,memdev=m$i "; 
>     done` -snapshot -monitor unix:/tmp/m,server,nowait
> 
>     boot and login to guest shell and execute 'reboot' command
>     on the reboot when guest kernel boots, QEMU will abort in virtio.
>     Reproducible in about 80% cases. If QEMU doesn't crash on reboot
>     then try again whit freshly started QEMU.
> 
> 
>     Reason for crashing is that guest allocates buffer that crosses
>     boundary between 2 different memory regions and as result
>     cpu_physical_memory_map() maps GPA to HVA for only head of buffer
>     that belongs to the first region which makes conditon
>      len != sg[i].iov_len
>     true since declared buffer size (sg[i].iov_len) isn't what
>     cpu_physical_memory_map() has been able to map (len),
>     which leads to abort:
> 
>     virtqueue_map_sg() {
>       ...
>       sg[i].iov_base = cpu_physical_memory_map(addr[i], &len, is_write);
>       if (sg[i].iov_base == NULL || len != sg[i].iov_len) {
>            abort()
> ------
> 
> However, the work-around regressed memory hot-unplug for linux guests
> triggering the following within the guest:
> 
> ------
>  =====
>  kernel BUG at mm/memory_hotplug.c:703!
>  ...
>  [<ffffffff81385fa7>] acpi_memory_device_remove+0x79/0xa5
>  [<ffffffff81357818>] acpi_bus_trim+0x5a/0x8d
>  [<ffffffff81359026>] acpi_device_hotplug+0x1b7/0x418
>  ===
>     BUG_ON(phys_start_pfn & ~PAGE_SECTION_MASK);
>  ===
> 
> ------
> 
> The reason for the crash is that x86-64 linux guest supports memory hotplug in
> chunks of 128Mb and assumes that memory sections are also 128Mb aligned.
> However gaps forced between 128Mb DIMMs with backend's natural alignment of 2Mb
> make the 2nd and following DIMMs not being aligned on 128Mb boundary.
> 
> ------
> 
> Michael S. Tsirkin (6):
>   virtio: introduce virtio_map
>   virtio: switch to virtio_map
>   virtio-blk: convert to virtqueue_map
>   virtio-serial: convert to virtio_map
>   virtio-scsi: convert to virtqueue_map
>   virtio: drop virtqueue_map_sg
> 
>  include/hw/virtio/virtio.h  |  3 +--
>  hw/block/virtio-blk.c       |  5 +----
>  hw/char/virtio-serial-bus.c |  5 +----
>  hw/scsi/virtio-scsi.c       | 16 ++------------
>  hw/virtio/virtio.c          | 52 +++++++++++++++++++++++++++++++++++----------
>  5 files changed, 46 insertions(+), 35 deletions(-)
> 
> -- 
> MST
>