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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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 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?

Jan


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