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SubjectRe: [PATCH 8/9] PCI: Ignore BAR contents when firmware left decoding disabled
On Tue, Mar 18, 2014 at 10:52 PM, Ming Lei <> wrote:
> Hi,
> Looks Sasha fixed the problem in lkvm tool[1].
> Sasha, looks we both saw the problem, but from technical
> view, I am wondering if the fix is correct, because PCI spec.
> requires that the IO/MMIO bits in COMMAND register should
> be cleared after reset, maybe there are some potential problem
> in lkvm pci emulation.

I think I'm going to revert this patch ([2], "Ignore BAR contents when
firmware left decoding disabled"). The main reason for that patch was
to try for a consistent way of figuring out whether BARs are valid
that we could use on all architectures, but I think we can do it in a
better way.

That said, this kvm change should not be necessary. We *should* be
able to take any PCI device and initialize it from power-on state
without any dependencies on what the BIOS left in the BARs or the
command register. As far as I can tell, the PCI core actually worked
fine in this case (we assigned valid addresses to the devices), but
something else blew up. If I revert that patch, it will cover up
whatever this other bug is, but it would be much better to figure out
what it is and fix is.

You said earlier that "The memory allocation failure is caused by
mistaken value read from pci address after the device is failed to
enable." Can you elaborate on that? Are you saying that something
tried to read from a region mapped by a BAR even though
pci_enable_device() failed? That would be a programming error, of
course. If you have any more details about exactly where this
happened, that would help a lot in finding the problem.



> [1], commit 6478ce1416aacf1ce35530f79ea035f89fb21e90
> Author: Sasha Levin <>
> Date: Wed Mar 5 23:08:16 2014 -0500
> kvm tools: mark our PCI card as PIO and MMIO able
> Thanks,
> --
> Ming Lei
> On Wed, Mar 19, 2014 at 11:32 AM, Ming Lei <> wrote:
>> On Tue, Mar 18, 2014 at 8:27 AM, Bjorn Helgaas <> wrote:
>>> On Fri, Mar 14, 2014 at 09:48:35AM +0800, Ming Lei wrote:
>>>> On Fri, Mar 14, 2014 at 12:08 AM, Bjorn Helgaas <> wrote:
>>>> > On Thu, Mar 13, 2014 at 2:51 AM, Ming Lei <> wrote:
>>>> >> Hi Bjorn,
>>>> >>
>>>> >> I found this patch broke virtio-pci devices.
>>>> >
>>>> > Thanks a lot for testing this.
>>>> >
>>>> >> On Thu, Feb 27, 2014 at 3:37 AM, Bjorn Helgaas <> wrote:
>>>> >>> Don't rely on BAR contents when the command register says the BAR is
>>>> >>> disabled.
>>>> >>>
>>>> >>> If we receive a PCI device from firmware (or a hot-added device that was
>>>> >>> just powered up) with the MEMORY or IO enable bits in the PCI command
>>>> >>> register cleared, there's no reason to believe the BARs contain valid
>>>> >>> addresses.
>>>> >>
>>>> >> From PCI LOCAL BUS SPECIFICATION, REV. 3.0, both
>>>> >> PCI_COMMAND_IO and PCI_COMMAND_MEM should be
>>>> >> cleared after reset, so looks the patch sets IORESOURCE_UNSET
>>>> >> too early because PCI drivers may call pci_enable_device()
>>>> >> (->pci_enable_resources()) to enable the two bits of
>>>> >> PCI_COMMAND explicitly.
>>>> >
>>>> > The point is that it's not safe to enable those two bits unless we're
>>>> > certain that the BARs they control contain valid values that don't
>>>> > conflict with anything else in the system.
>>>> >
>>>> > Maybe we should only set IORESOURCE_UNSET when we find a conflict or a
>>>> > BAR that's not contained by an upstream bridge window, and we should
>>>> > try to reallocate then. I'm pretty sure we do that at least in some
>>>> > cases, but it would probably simplify things if we did it more
>>>> > consistently, and maybe we shouldn't set it at all here in
>>>> > __pci_read_base().
>>>> I think so because __pci_read_base() is called in device emulation
>>>> path.
>>> Which path is this? I don't know anything about virtio-pci, and I only see
>>> calls to __pci_read_base() from:
>>> sriov_init()
>>> pci_sriov_resource_alignment()
>>> pci_read_bases()
>>>> > But I'd like to understand your situation better, so can you provide
>>>> > more details, please? Complete before/after dmesg logs would go a
>>>> > long way toward illustrating the problem you're seeing.
>>>> Please see the two attachment log. The memory allocation failure
>>>> is caused by mistaken value read from pci address after the device
>>>> is failed to enable.
>>> Your logs are harder than necessary to compare because one has a lot more
>>> debug turned on than the other.
>>> In the failing case, we ignore all the initial BAR values, but we do assign
>>> values to all of them later:
>>> pci 0000:00:00.0: can't claim BAR 0 [mem size 0x00000400]: no address assigned
>>> pci 0000:00:00.0: can't claim BAR 1 [io size 0x0400]: no address assigned
>>> ...
>>> pci 0000:00:00.0: BAR 0: assigned [mem 0x40000000-0x400003ff]
>>> pci 0000:00:00.0: BAR 1: assigned [io 0x1000-0x13ff]
>>> ...
>>> The newly-assigned values look valid, and as far as I can tell, they should
>>> work. Do you know why they don't? Is there an assumption somewhere that
>>> we never change BAR values?
>> I don't know the cause, maybe it is related with the hypervisor
>> implementation.
>>> Even if we don't need to ignore BAR values in as many cases as we do, it
>>> should be legal to ignore them and reassign them, so I want to understand
>>> what's going on here before reverting this.
>>> Is there an easy way I can reproduce the problem on my own box?
>> It is not quite difficult, you can build a lkvm following the README in
>> below link and test -next tree on the small kvm hypervisor:
>> Thanks,
>> --
>> Ming Lei
> --
> Ming Lei

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