Re: [Qemu-devel] [RFC/PATCH] Fix guest OS panic when 64bit BAR is present

[Alexey Korolev]

On 27/01/12 04:12, Avi Kivity wrote:
> On 01/26/2012 04:36 PM, Michael S. Tsirkin wrote:
>> On Thu, Jan 26, 2012 at 03:52:27PM +0200, Avi Kivity wrote:
>>> On 01/26/2012 11:14 AM, Michael S. Tsirkin wrote:
>>>> On Wed, Jan 25, 2012 at 06:46:03PM +1300, Alexey Korolev wrote:
>>>>> Hi, 
>>>>> In this post
>>>>> http://lists.gnu.org/archive/html/qemu-devel/2011-12/msg03171.html I've
>>>>> mentioned about the issues when 64Bit PCI BAR is present and 32bit
>>>>> address range is selected for it.
>>>>> The issue affects all recent qemu releases and all
>>>>> old and recent guest Linux kernel versions.
>>>>>
>>>>> We've done some investigations. Let me explain what happens.
>>>>> Assume we have 64bit BAR with size 32MB mapped at [0xF0000000 -
>>>>> 0xF2000000]
>>>>>
>>>>> When Linux guest starts it does PCI bus enumeration.
>>>>> The OS enumerates 64BIT bars using the following procedure.
>>>>> 1. Write all FF's to lower half of 64bit BAR
>>>>> 2. Write address back to lower half of 64bit BAR
>>>>> 3. Write all FF's to higher half of 64bit BAR
>>>>> 4. Write address back to higher half of 64bit BAR
>>>>>
>>>>> Linux code is here: 
>>>>> http://lxr.linux.no/#linux+v3.2.1/drivers/pci/probe.c#L149
>>>>>
>>>>> What does it mean for qemu?
>>>>>
>>>>> At step 1. qemu pci_default_write_config() recevies all FFs for lower
>>>>> part of the 64bit BAR. Then it applies the mask and converts the value
>>>>> to "All FF's - size + 1" (FE000000 if size is 32MB).
>>>>> Then pci_bar_address() checks if BAR address is valid. Since it is a
>>>>> 64bit bar it reads 0x00000000FE000000 - this address is valid. So qemu
>>>>> updates topology and sends request to update mappings in KVM with new
>>>>> range for the 64bit BAR FE000000 - 0xFFFFFFFF. This usually means kernel
>>>>> panic on boot, if there is another mapping in the FE000000 - 0xFFFFFFFF
>>>>> range, which is quite common.
>>>> Do you know why does it panic? As far as I can see
>>>> from code at
>>>> http://lxr.linux.no/#linux+v2.6.35.9/drivers/pci/probe.c#L162
>>>>
>>>>  171        pci_read_config_dword(dev, pos, &l);
>>>>  172        pci_write_config_dword(dev, pos, l | mask);
>>>>  173        pci_read_config_dword(dev, pos, &sz);
>>>>  174        pci_write_config_dword(dev, pos, l);
>>>>
>>>> BAR is restored: what triggers an access between lines 172 and 174?
>>> Random interrupt reading the time, likely.
>> Weird, what the backtrace shows is init, unrelated
>> to interrupts.
>>
> It's a bug then.  qemu doesn't undo the mapping correctly.
>
> If you have clear instructions, I'll try to reproduce it.
>
Well the easiest way to reproduce this is:


1. Get kernel bzImage (version < 2.6.36)
2. Apply patch to ivshmem.c

---
diff --git a/hw/ivshmem.c b/hw/ivshmem.c
index 1aa9e3b..71f8c21 100644
--- a/hw/ivshmem.c
+++ b/hw/ivshmem.c
@@ -341,7 +341,7 @@ static void create_shared_memory_BAR(IVShmemState *s, int 
fd) {
     memory_region_add_subregion(&s->bar, 0, &s->ivshmem);
 
     /* region for shared memory */
-    pci_register_bar(&s->dev, 2, PCI_BASE_ADDRESS_SPACE_MEMORY, &s->bar);
+    pci_register_bar(&s->dev, 2, 
PCI_BASE_ADDRESS_SPACE_MEMORY|PCI_BASE_ADDRESS_MEM_TYPE_64, &s->bar)
 }
 
 static void close_guest_eventfds(IVShmemState *s, int posn)
---

3. Launch qemu with a command like that

/usr/bin/qemu-system-x86_64 -M pc-0.14 -enable-kvm -m 2048 -smp 
1,socket=1,cores=1,threads=1 -name centos54 -uuid
d37daefd-75bd-4387-cee1-7f0b153ee2af -nodefconfig -nodefaults -chardev
socket,id=charmonitor,path=/var/lib/libvirt/qemu/centos54.monitor,server,nowait 
-mon chardev=charmonitor,id=monitor,mode=readline -rtc
base=utc -drive 
file=/dev/dock200-1/centos54,if=none,id=drive-ide0-0-0,format=raw -device
ide-drive,bus=ide.0,unit=0,drive=drive-ide0-0-0,id=ide0-0-0,bootindex=1 -drive
file=/data/CentOS-5.4-x86_64-bin-DVD.iso,if=none,media=cdrom,id=drive-ide0-1-0,readonly=on,format=raw
 -device
ide-drive,bus=ide.1,unit=0,drive=drive-ide0-1-0,id=ide0-1-0 -chardev 
file,id=charserial0,path=/home/alexey/cent54.log -device
isa-serial,chardev=charserial0,id=serial0 -usb -vnc 127.0.0.1:0 -k en-us -vga 
cirrus -device
virtio-balloon-pci,id=balloon0,bus=pci.0,multifunction=on,addr=0x4.0x0 --device 
ivshmem,size=32,shm="shm" -kernel bzImage -append
"root=/dev/hda1 console=ttyS0,115200n8 console=tty0"

in other words add: --device ivshmem,size=32,shm="shm"

That is all.

Note: it won't necessary cause panic message on some kernels it just hangs or 
reboots.