Re: [PATCH v1 5/5] i386: Hyper-V VMBus ACPI DSDT entry

[Jon Doron]

On 08/04/2020, Roman Kagan wrote:
> On Wed, Apr 08, 2020 at 07:16:39AM +0300, Jon Doron wrote:
> > On 07/04/2020, Maciej S. Szmigiero wrote:
> > > On 07.04.2020 20:56, Roman Kagan wrote:
> > > > On Mon, Apr 06, 2020 at 11:20:39AM +0300, Jon Doron wrote:
> > > > > Well I want it to be merged in :-)
> > > >
> > > > Hmm I'm curious why, it has little to offer over virtio.
> > > >
> > > > Anyway the series you've posted seems to be based on a fairly old
> > > > version.
> > > >
> > > > The one in openvz repo is more recent.  It's still in need for
> > > > improvement, too, but should be testable at least.
> >
> > Well I have implemented the hyperv synthetic kernel debugger interface, but
> > on Windows 10 it requires to have a working VMBus (it's not really using it,
> > but without a function vmbus that will answer to the initiate contact then
> > the kdnet will simply be stuck in a loop.
>
> I see, thanks, I've never heard of this before.
>
> > With the synthetic kernel debugger interface you can debug older OS (Win7 up
> > to latest Win10). The benefit is that its much faster than all other
> > interfaces.
>
> I guess you compare it to debugging via serial port.  I wonder where the
> difference comes from?  I thought the transport didn't require any
> significant throughput, and latency-wise the (emulated) serial port was
> just as good as any other.  Am I missing something?
>
> Thanks,
> Roman.

Well kdcom is sending out UART through the virtual serial port, this
results in very slow speeds (try it out if you get a chance), because
of that most Windows kernel developers use  VMWare with a combination
of a tool called VirtualKD which implements it's own debug transport on
the Windows part and patches the hypervisor (aka VMWare) to get the 
VMExits.
This way it can transfer more and bigger blocks faster, to the debugger.

With the synthetic debugger interface in-place (which you can use since 
Windows 7) all these tricks are not really required, you just need to 
implement it :P .

Thanks,
-- Jon.

> > In addition to that Michael Kelley from Microsoft has informed us that
> > Microsoft might be dropped the synthetic kernel debugger interface sometime
> > in the future, and it seems like the new mode is simply to use hvnet device
> > for the communication (which is again much faster).
> >
> > Cheers,
> > -- Jon.
> > >
> > > Isn't the one at
> > > 
> > https://src.openvz.org/projects/UP/repos/qemu/commits?until=refs%2Fheads%2Fvmbus
> > > the latest one?
> > >
> > > It seems to be last changed in October 2019 - is there a
> > > later one?
> > >
> > > > Thanks,
> > > > Roman.
> > >
> > > Thanks,
> > > Maciej