Re: [Xen-devel] Is: qemu-xen mishandling upper 64-bit BAR compared to qemu-tradWas:Re: Dom0 linux 4.0 + devel/for-linus-4.1 branch: p2m.c:884:d0v0 gfn_to_mfn failed! gfn=ffffffff001ed type:4

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On June 10, 2015 8:02:52 AM EDT, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>>> On 10.06.15 at 13:13, <stefano.stabellini@xxxxxxxxxxxxx> wrote:
>> On Wed, 10 Jun 2015, Jan Beulich wrote:
>>> >>> On 10.06.15 at 03:02, <konrad.wilk@xxxxxxxxxx> wrote:
>>> > The problem is that the XSA120-addendm patch (which does not clear
>>> > the PCI_COMMAND register anymore), causes an missing functionality
>in 
>>> > qemu-xen to be exposed. This missing functionality is implemented
>in
>>> > qemu-traditional which is why it works there.
>>> > 
>>> > The problem is that qemu-xen for any write to the BAR regions
>>> > updates them to the hypervisor - but only if the real hardware has
>>> > them enabled (see pci_update_mappings in pci_default_write_config
>which
>>> > is called by xen_pt_pci_write_config). Specifically
>pci_bar_address
>>> > checks PCI_COMMAND register for PCI_COMMAND_MEMORY. If it is
>disabled
>>> > (so no XSA-120 addendum patch), it returns -1 (default value
>resulting
>>> > in no changes in the internal structures). If it is enabled, then
>>> > it updates the d->config space with the value written by the
>guest.
>>> > Once xen_pt_pci_write_config is done it syncs up the changes (if
>there
>>> > are any) which results in the QEMU calling hypervisor to update
>the P2M 
>>> > mapping.
>>> 
>>> There's one fundamental aspect I'm not understanding here:
>>> pci_update_mappings() / pci_bar_address() look at the guest view
>>> (or at least they ought to be), and the virtual command register
>>> starts out as zero. Is the bug perhaps that xen_pt_initfn(), after
>>> having initialized d->config[] via xen_host_pci_get_block(), leaves
>>> the command register at its host view value (rather than updating
>>> it alongside reg_entry->data in xen_pt_config_reg_init(), called
>>> via xen_pt_config_init()), which would have happened to be zero
>>> without that XSA-120 addendum?
>> 
>> It seems to me that Jan is right: setting the PCI_COMMAND register to
>> ~PCI_COMMAND_MEMORY could be done at initialization time. Would that
>> fix the bug?
>
>Why ~PCI_COMMAND_MEMORY? Just like in the Xen specific data,
>this field should start out as zero.

It should fix it. I will send out a patch later tonight.
>
>>> It is of course concerning that
>>> there are two (now clearly mismatching) guest views within qemu
>>> (and along those lines I also wonder whether the apparent
>>> duplicate maintenance of MSI and MSI-X state, due to
>>> pci_default_write_config() calling msi{,x}_write_config(), can do
>>> any good, or why the code uses pci_default_write_config() but
>>> not pci_default_read_config()).
>>> 
>>> It looks to me as if there was a halfhearted attempt to utilize
>>> infrastructure already available in qemu when these Xen pieces
>>> got added, leading to hard to understand issues like the one here.
>>> I.e. even if we addressed the initialization value issue above,
>>> there would still be two competing emulation layers potentially
>>> (and I suppose quite likely) leading to differing register state
>>> later on. Hence I wonder how many more issues there are (to
>>> come)...
>> 
>> The integration between the existing qemu-traditional code and the
>> upstream QEMU code was hard. I am ready to believe there are more
>than
>> just a few gaps and I would be happy to take further patches to
>improve
>> the situation.
>> 
>> In this specific instance, are you referring to d->config, part of
>> PCIDevice, and all the XenPTRegInfo instances? If so, I think the
>reason
>> for having the latter, is that we need more flexibility, we need
>> individual masks and read and write functions.  At the same time we
>> cannot really get rid of d->config.
>
>I guessed as much, but in that case we should keep the two in sync
>(i.e. where we apply custom logic we should sync back what we do
>do d->config[], and at init time we should merge host and emulated
>state according to ->emu_mask; or maybe XenPTReg shouldn't even
>have a data field, and instead modifications should go straight to
>d->config[]). Perhaps a first step ought to be to log all cases where
>they differ?
>

Let me cobble a patch to this effect and see what falls out.

Thank you for the feedback!

>Jan



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