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.