Hi all, This old subject came up again in a discussion related to PCIe support for QEMU/KVM under Tianocore. The fact that we need to map PCI MMIO regions as cacheable is preventing us from reusing a significant slice of the PCIe support infrastructure, and so I'd like to bring this up again, perhaps just to reiterate why we're simply out of luck.
To refresh your memories, the issue is that on ARM, PCI MMIO regions for emulated devices may be backed by memory that is mapped cacheable by the host. Note that this has nothing to do with the device being DMA coherent or not: in this case, we are dealing with regions that are not memory from the POV of the guest, and it is reasonable for the guest to assume that accesses to such a region are not visible to the device before they hit the actual PCI MMIO window and are translated into cycles on the PCI bus. That means that mapping such a region cacheable is a strange thing to do, in fact, and it is unlikely that patches implementing this against the generic PCI stack in Tianocore will be accepted by the maintainers. Note that this issue not only affects framebuffers on PCI cards, it also affects emulated USB host controllers (perhaps Alex can remind us which one exactly?) and likely other emulated generic PCI devices as well. Since the issue exists only for emulated PCI devices whose MMIO regions are backed by host memory, is there any way we can already distinguish such memslots from ordinary ones? If we can, is there anything we could do to treat these specially? Perhaps something like using read-only memslots so we can at least trap guest writes instead of having main memory going out of sync with the caches unnoticed? I am just brainstorming here ... In any case, it would be good to put this to bed one way or the other (assuming it hasn't been put to bed already) Thanks, Ard. _______________________________________________ kvmarm mailing list kvmarm@lists.cs.columbia.edu https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
