Re: [PATCH v2 1/3] x86: remove PVHv1 code

[Ian Jackson <ian.jackson@eu.citrix.com>]

Roger Pau Monne writes ("Re: [PATCH v2 1/3] x86: remove PVHv1 code"):
> On Wed, Mar 01, 2017 at 01:53:29PM +0000, Ian Jackson wrote:
> > Well, PVHv2 is in the process of becoming properly supported, so now
> > is the time to decide the "official" way.
> 
> I prefer builder="hvm" device_model_version="none" because I think
> it's clearer from a user PoV that a HVM guest it being
> created.

Err, but a PVH guest is not an HVM guest in the sense that the user
will expect.  Whenever I explain to anyone the difference between PV
and HVM, the explanation is that "HVM provides a complete emulated PC"
and "PV needs a guest operating systemn modified to work under Xen".
By both these measures, a PVH guest is more like PV than HVM.

The use of the CPU extensions which originally only enabled support
for HVM is a detail which most people will not be so interested in.
The details of API, ABI and so on are not of interest to the writer of
the xl domain config file.

> OTOH, using pvh=1 it's more obscure, and it isn't clear
> which kind of guest you are creating, and which options apply to
> it. Although all that can be fixed in the man page, I think it's
> less intuitive.

The explanation we have been giving to ordinary users is that there
are going to be three kinds of guest: PV, HVM, and the new PVH.

> TBH, I'm not even sure we should keep the "pvh" option, the same kernel that
> previously worked with pvh=1 might not work anymore when this patch is applied.

PVHv1 was never supported so there is no need to worry about this.

> > When you say "it will basically fill the PV side", what is "it" ?
> > Do you mean xl_parse.c ?
> 
> Yes, parse_config_data. With the current code in parse_config_data
> domain type (c_info->type) is set to PV when pvh=1 is set in the
> config file. Then in the same function, further below, options like
> nestedhvm are simply ignored.
> 
> I don't any other way to solve this rather than forcing domain type to HVM in
> parse_config_data when pvh=1 is set.
> 
> > Isn't this what libxl_domain_build_info_init_type is for ?
> 
> libxl_domain_build_info_init_type is not be able to re-parse the config file.

libxl_domain_build_info_init_type is called in xl_parse.c before any
of the type-specific fields are set.  That's it's whole purpose.

So I think having libxl_domain_build_info_init_type set type to HVM if
pvh=1 would solve the problem.

(Really I think we should extend the domain type enum but that may be
too controversial, especially as we'd have to arrange for the .hvm and
.pvh versions of the union to be the same type, so it would involve
an idl compiler extension.)

Ian.

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