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This permits I/O devices on the host to directly access wired memory dedicated to guests using passthru devices. Note that wired memory belonging to guests that do not use passthru devices has always been accessible by I/O devices on the host. bhyve maps guest physical addresses into the user address space of the bhyve process by mmap'ing /dev/vmm/<vmname>. Device models pass pointers derived from this mapping directly to system calls such as preadv() to minimize copies when emulating DMA. If the backing store for a device model is a raw host device (e.g. when exporting a raw disk device such as /dev/ada<n> as a drive in the guest), the host device driver (e.g. ahci for /dev/ada<n>) can itself use DMA on the host directly to the guest's memory. However, if the guest's memory is not present in the host IOMMU domain, these DMA requests by the host device will fail without raising an error visible to the host device driver or to the guest resulting in non-working I/O in the guest. It is unclear why guest addresses were removed from the IOMMU host domain initially, especially only for VM's with a passthru device as the host IOMMU domain does not affect the permissions of passthru devices, only devices on the host. A considered alternative was using bounce buffers instead (D34535 is a proof of concept), but that adds additional overhead for unclear benefit. This solves a long-standing problem when using passthru devices and physical disks in the same VM. Thanks to: grehan (patience and help) Thanks to: jhb (for improving the commit message) PR: 260178 Reviewed by: grehan, jhb MFC after: 3 days Differential Revision: https://reviews.freebsd.org/D34607 |
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| conf | ||
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| include | ||
| linux | ||
| linux32 | ||
| pci | ||
| sgx | ||
| vmm | ||
| Makefile | ||