Re: [Xen-devel] xen disk spec and emulation

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: 27 March 2019 14:27
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
> Subject: Re: [Xen-devel] xen disk spec and emulation
> 
> >>> On 27.03.19 at 14:54, <Paul.Durrant@xxxxxxxxxx> wrote:
> >   Currently, when specifying a virtual disk, the only way to determine the
> > emulation model used is by selecting a particular 'vdev' numbering scheme as
> > detailed in xl-disk-configuration in the docs. That document refers the
> > reader to xen-vbd-interface which says:
> >
> > xvd* -> xen virtual disk (from which one infers PV-only)
> > hd* -> IDE emulation
> > sd* -> SCSI emulation
> > d*p* -> unknown emulation
> >
> >   The first choice actually results in IDE emulation, which is not stated
> > anywhere AFAIK, and the fourth choice actually results in no emulation,
> > although that's not stated either. These two things can, of course, be fixed
> > in the documentation but I also think that the behaviour of vdev=xvd* is
> > surprising and wrong.
> 
> Wasn't this behavior intended to allow a guest to be installed
> without PV drivers, but to have them enabled once they got
> installed?
> 

I don't think so. AIUI that's what hd* and sd* were for. They seem to be the 
only vdev types that are documented to have emulated equivalents.

I also appear to have been wrong about AHCI being completely undocumented. 
There is a line in xen-vbd-interface that specifies an extra 'hdtype=ahci' 
option, to add add ich9 controller emulation to the VM.

  Paul

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