Messages in this thread | | | Subject | Re: [RFC v6 06/10] PCI: Add a new PCI_BUS_FLAGS_MSI_REMAP flag | From | Yongji Xie <> | Date | Tue, 19 Apr 2016 19:13:54 +0800 |
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On 2016/4/18 19:30, David Laight wrote: > From: Yongji Xie >> Sent: 18 April 2016 11:59 >> We introduce a new pci_bus_flags, PCI_BUS_FLAGS_MSI_REMAP >> which indicates all devices on the bus are protected by the >> hardware which supports IRQ remapping(intel naming). >> >> This flag will be used to know whether it's safe to expose >> MSI-X tables of PCI BARs to userspace. Because the capability >> of IRQ remapping can guarantee the PCI device cannot trigger >> MSIs that correspond to interrupt IDs of other devices. > I'm worried that this entire series is going to break drivers > for existing hardware. > > I understand some of the reasoning for 'vm pass through' configurations, > but there will be PCIe devices out there that have the MSI-X tables > in the same BAR as other device registers. > If you are lucky nothing else is in the same 4k area, but I wouldn't > assume it.
Thanks for your comments. But I didn't get your point here. Why will exposing MSI-X table to userspace break the driver for hardware which have the MSI-X tables in the same BAR as other device registers? Could you give me more details?
The reason why we want to mmap MSI-X table is that there may be some other critical device registers in the same page as the MSI-X table. We prefer to handle the mmio access to these registers in guest rather than in QEMU. So we would like to see there is something else in the same 4k/64k area.
> In any case, if the hardware can't police the card's master transfers > there is nothing to stop a different bus master block on the card > from raising MSI-X interrupts - they are just a PCIe write. > So all you are doing is raising the bar slightly and giving a very false > sense of security.
Do you mean we can request a DMA to the target address area that raises MSI-X interrupts? But for PPC64 with IODA bridge, this invalid PCIe write will be prevented on PHB before raising MSI-X interrupt. And I think the capability of interrupt remapping or ITS can also do the same thing. If hardware didn't support this, we would not expose MSI-X table in my patch.
Thanks, Yongji
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