[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] 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
On Wed, 10 Jun 2015, Jan Beulich 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 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? Yes, I think we could substitute the data field in XenPTReg with a pointer to the right place in d->config. Or we could just get rid of it and introduce a macro to access d->config + reg->offset. _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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