Hi Alexander,

Thanks for your help. I've been working on getting the dummy driver to load
at boot time, but having some issues. After rereading your comments,
however, I'm wondering if I'm overriding the correct driver. I've been
trying to override the pci-ata driver, but should I be PCI bus driver one
level up (pci.c)? I suspect I'm trying to override the wrong driver.

On Tue, Apr 23, 2013 at 10:48 AM, Alexander Motin <m...@freebsd.org> wrote:

> On 23.04.2013 17:07, Larry Melia wrote:
>> 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.
> I am sorry, I don't really understand what is your proposition, so I just
> describe my vision of the solution here, hoping it will be useful.
> You are right that FreeBSD has priority mechanism to select best driver
> for each device. That mechanism should be sufficient to prevent default OS
> driver from attaching to emulated ATA controller when Hyper-V environment
> is detected and paravirtual drivers are installed. To do that you should
> just create a very small dummy driver for PCI bus, implementing single
> probe() method. That probe() method should in your preferable way check
> that Hyper-V environment is active and match PCI ID of the device against
> the list of used by Hyper-V for emulated ATA controller. If both conditions
> match, the probe() method should return highest priority value to override
> any other ATA driver in system for this specific device. After that the
> dummy driver should just have empty attach() and detach() routines, not
> doing anything with the emulated device, relying on paravitrual driver to
> handle the disks.
> Since the dummy driver can be made machine-dependent, you may use there
> any VM detection mechanisms you like, even direct MSR access. Dummy driver
> can be made completely independent from the rest of the system, so it can
> even be loaded as kernel module without requiring any base system
> modifications. If you still prefer it for some reason (for example to not
> do some complicated checks multiple times in multiple drivers), you may
> move Hyper-V detection logic out of that driver to some single place in
> base system. But you don't need to touch OS-native ATA driver in any way.
> --
> Alexander Motin
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