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

[Avi Kivity]

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.

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

So it seems.  Btw, is this procedure correct for sizing a BAR which is
larger than 4GB?

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