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

[Michael S. Tsirkin]

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?


Also, what you describe happens on a 32 bit BAR in the same way, no?

-- 
MST