I’m on a team of developers working on synthetic device drivers for FreeBSD on Hyper-V. In late March, I briefly mentioned (on this list) that we're trying to get our drivers included with the FreeBSD distribution. Also noted were some necessary patches to the ATA driver. The changes are necessary to achieve significant performance gains by replacing the native ATA driver with our synthetic storage driver when when Hyper-V is detected. Alexander Motin, the maintainer of the ATA code-base, however, expressed some concerns about making these modifications that are so specific to Hyper-V and the AMD64 build. We understand his concerns and have subsequently removed these patches from our vendor branch. Our drivers are now completely independent and require no changes to the kernel or other drivers. The only touch-point is the common makefile for modules/drivers.
Removing our ATA patches, on the other hand, results in a huge performance loss. This is because the root file system is managed by the ATA driver, which is emulated under Hyper-V. Furthermore, there may be other native drivers that we wish to replace with synthetic ones at boot time (e.g., mouse driver, video driver), but, there appears to be no easy way to do this.Therefore, we would like to work with the developer community to come up with better solution that would improve support for synthetic drivers at boot-time. One possible solution: (1) Move the call to init_param1() (in sys/kern/subr_parm.c), which is used for hypervisor detection, to an earlier point in the boot process. Presently, it appears to be called after the ATA driver is selected, which is too late in the boot process. (This was discovered after some testing with the ATA driver.) Therefore, before the bus drivers and native controllers are detected and selected, discovery of a host hypervisor should be done first. (2) Extend the boot process to dynamically insert and prioritize synthetic drivers over native ones. Presently, FreeBSD supports the selection of generic drivers for a given device, but these can be overridden when a more specific driver is available. This priority scheme could be extended by first detecting the presence of a hypervisor, then allowing this priority mechanism to override the native drivers with their synthetic cousins (i.e., we only want to override a native driver when a specific hypervisor is detected and synthetic drivers for it have been configured). This could be arranged in a separate, table-driven, input file or perhaps, by extending the existing driver/module configuration file with additional mappings of native to synthetic drivers. (3) Upgrade the init_param1() function (in sys/kern/subr_parm.c) to use the more recent approach to hypervisor detection. This approach uses the CPU-identify functions to retrieve a unique signature consisting of a fixed string of ASCII characters. This was done on Linux about five years. For backward compatibility, however, the existing logic would be retained, but augmented with this new approach. It would also be conditionally added only for x86/AMD64 builds. When reviewing the changes we're not looking at a lot of lines of code.The difficultly lies, however, in where the changes are made. Obviously, we need to work closely with the committers responsible for the kernel modules we're looking to touch. Retrospectively, these upgrades only bring FreeBSD up to the same level of hypervisor support already present in Linux.There are other issues that we would also like to discuss, but the improvements described here are on the top of the list. We look forward to working more closely with the FreeBSD community and welcome your remarks and feedback on this proposal. Larry Melia _______________________________________________ freebsd-virtualization@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-virtualization To unsubscribe, send any mail to "freebsd-virtualization-unsubscr...@freebsd.org"
