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Re: [Qemu-devel] [RFC/PATCH] Fix guest OS panic when 64bit BAR is presen
From: |
Avi Kivity |
Subject: |
Re: [Qemu-devel] [RFC/PATCH] Fix guest OS panic when 64bit BAR is present |
Date: |
Thu, 26 Jan 2012 17:12:14 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:9.0) Gecko/20111222 Thunderbird/9.0 |
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.
--
error compiling committee.c: too many arguments to function
- Re: [Qemu-devel] [RFC/PATCH] Fix guest OS panic when 64bit BAR is present, (continued)
Re: [Qemu-devel] [RFC/PATCH] Fix guest OS panic when 64bit BAR is present, Michael S. Tsirkin, 2012/01/26
Re: [Qemu-devel] [RFC/PATCH] Fix guest OS panic when 64bit BAR is present, Alexey Korolev, 2012/01/26