Re: [PATCH 8/9] PCI: Ignore BAR contents when firmware left decoding disabled

[Bjorn Helgaas]

On Fri, Mar 21, 2014 at 2:25 PM, Sasha Levin <sasha.levin@xxxxxxxxxx> wrote:
> On 03/21/2014 04:07 PM, Bjorn Helgaas wrote:
>>
>> I think I figured out what the problem is.  In virtio_pci__init(), we
>> allocate some address space with pci_get_io_space_block(), save its
>> address in vpci->mmio_addr, and hook that address space up to
>> virtio_pci__io_mmio_callback with kvm__register_mmio().
>>
>> But when we update the BAR value in pci__config_wr(), the address
>> space mapping is never updated.  I think this means that virtio-pci
>> can't tolerate its devices being moved by the OS.
>>
>> In my opinion, this is a bug in linux-kvm.  We've managed to avoid
>> triggering this bug by preventing Linux from moving the BAR (either by
>> me reverting my patch, or by Sasha's linux-kvm change [1]).  But it's
>> not very robust to assume that the OS will never change the BAR, so
>> it's quite possible that you'll trip over this again in the future.
>
>
> The purpose of KVM tool is to implement as much as possible of the KVM
> interface and the virtio spec so that we'll have a good development/testing
> environment with a very simple to understand codebase.
>
> The issue you've mentioned is the "evil" side of the KVM tool. It never
> tried (or claimed) to implement anything close to legacy hardware
> interfaces. This means, for example, that it doesn't run any BIOS, there's
> very lacking APIC support and the kernel is just injected into the virtual
> RAM and gets run from there.
>
> It also means that we went into the PCI spec deep enough to get the code
> to work with the kernel. The only reason we implemented MSI interrupts
> for example is because they provide improved performance with KVM, not
> because we were trying to get a complete implementation of the PCI spec.
>
> So yes, the PCI implementation in the KVM tool is lacking and what we
> have there might be broken by making the kernel conform more closely
> to the spec, but we are always happy to adapt and improve our code to
> work with any changes in the kernel.
>
> To sum it up, If you'll end up adding a change to the kernel that is
> valid according to the spec but breaks the KVM tool we'll just go ahead
> and fix the tool. You really don't need to worry about breaking it.

That makes sense, and I'm glad I had a chance to get acquainted with
the KVM tool.  If I get another problem report related to it, I'll try
to remember that I don't need to worry about breaking it :)

Bjorn
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