On Thu, 5 Jul 2018 20:11:48 +0200 Cédric Le Goater <c...@kaod.org> wrote:
> PCI devices needing a ROM allocate an optional MemoryRegion with > pci_add_option_rom(). pci_del_option_rom() does the cleanup when the > device is destroyed. The only action taken by this routine is to call > vmstate_unregister_ram() which clears the id string of the optional > ROM RAMBlock and now, also flags the RAMBlock as non-migratable. This > was recently added by commit b895de502717 ("migration: discard > non-migratable RAMBlocks"), . > > VFIO devices do their own allocation of the PCI ROM region. It is > initialized in vfio_pci_size_rom() in which the PCI attribute > 'has_rom' is set to true but the RAMBlock of the ROM region is not > allocated. When the associated PCI device is deleted, > pci_del_option_rom() calls vmstate_unregister_ram() which tries to > flag a NULL RAMBlock because 'has_rom' is set, leading to a SEGV . > > The use of vmstate_unregister_ram() in the PCI device was added in > commit b0e56e0b63f350691b52d3e75e89bb64143fbeff ("unset RAMBlock idstr > when unregister MemoryRegion") and from the archive in > http://lists.gnu.org/archive/html/qemu-devel/2014-04/msg00282.html, it > seems that it was trying to fix a reference count issue. > > vmstate_unregister_ram() being a work around, let's remove it to fix > the current SEGV issue and let's try to find a fix for the initial ref > count issue if we can reproduce. > > Signed-off-by: Cédric Le Goater <c...@kaod.org> > --- > hw/pci/pci.c | 11 ----------- > 1 file changed, 11 deletions(-) Looking back through git history, I think vfio sets PCIDevice.has_rom because we needed that to have memory_region_destroy() called, but that changed with Paolo's: 469b046ead06 ("memory: remove memory_region_destroy") Now the MemoryRegion gets freed automagically, so maybe the better option is that vfio-pci should not set has_rom to keep it out of this path. I don't see that has_rom serves any other purpose. Thanks, Alex