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
>
>

I have some patches that fix this, but they're very hacky since they're
dealing with the old and rotten core.  I much prefer to let this resolve
itself in my continuing rewrite.  Is this an urgent problem for you or
can you live with this for a while?

-- 
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