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

[Avi Kivity]

On 01/27/2012 06:42 AM, Alexey Korolev wrote:
> 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.
>

In fact qemu segfaults for me, since registering a ram region not on a
page boundary is broken.  This happens when the ivshmem bar is split by
the hpet region, which is less than page long.

-- 
error compiling committee.c: too many arguments to function