mirror of https://github.com/xemu-project/xemu.git
2cc9ddcceb
QEMU is not handling the global DMAR switch well, especially when from "on" to "off". Let's first take the example of system reset. Assuming that a guest has IOMMU enabled. When it reboots, we will drop all the existing DMAR mappings to handle the system reset, however we'll still keep the existing memory layouts which has the IOMMU memory region enabled. So after the reboot and before the kernel reloads again, there will be no mapping at all for the host device. That's problematic since any software (for example, SeaBIOS) that runs earlier than the kernel after the reboot will assume the IOMMU is disabled, so any DMA from the software will fail. For example, a guest that boots on an assigned NVMe device might fail to find the boot device after a system reboot/reset and we'll be able to observe SeaBIOS errors if we capture the debugging log: WARNING - Timeout at nvme_wait:144! Meanwhile, we should see DMAR errors on the host of that NVMe device. It's the DMA fault that caused a NVMe driver timeout. The correct fix should be that we do proper switching of device DMA address spaces when system resets, which will setup correct memory regions and notify the backend of the devices. This might not affect much on non-assigned devices since QEMU VT-d emulation will assume a default passthrough mapping if DMAR is not enabled in the GCMD register (please refer to vtd_iommu_translate). However that's required for an assigned devices, since that'll rebuild the correct GPA to HPA mapping that is needed for any DMA operation during guest bootstrap. Besides the system reset, we have some other places that might change the global DMAR status and we'd better do the same thing there. For example, when we change the state of GCMD register, or the DMAR root pointer. Do the same refresh for all these places. For these two places we'll also need to explicitly invalidate the context entry cache and iotlb cache. Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1625173 CC: QEMU Stable <qemu-stable@nongnu.org> Reported-by: Cong Li <coli@redhat.com> Signed-off-by: Peter Xu <peterx@redhat.com> -- v2: - do the same for GCMD write, or root pointer update [Alex] - test is carried out by me this time, by observing the vtd_switch_address_space tracepoint after system reboot v3: - rewrite commit message as suggested by Alex Signed-off-by: Peter Xu <peterx@redhat.com> Reviewed-by: Eric Auger <eric.auger@redhat.com> Reviewed-by: Jason Wang <jasowang@redhat.com> Reviewed-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com> |
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