Re: [Xen-devel] [PATCH RFC 00/10] x86 passthrough code cleanup

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Mon, Feb 26, 2018 at 01:47:38AM +0100, Marek Marczykowski-Górecki wrote:
> On Fri, Feb 23, 2018 at 10:39:20PM -0600, Doug Goldstein wrote:
> > On 2/22/18 11:12 PM, Tian, Kevin wrote:
> > >> From: Wei Liu
> > >> Sent: Thursday, February 22, 2018 5:47 AM
> > >>
> > >> Hi all
> > >>
> > >> At some point I would like to make CONFIG_HVM and CONFIG_PV work.
> > >> The
> > >> passthrough code is one of the road blocks for that work.
> > > 
> > > Can you elaborate the motivation of this change? why does someone
> > > want to disable HVM or PV logic completely from hypervisor?
> > 
> > I can say I recall advocating for this at Xen Summit in Cambridge. I
> > believe I talked about it in Toronto as well. There are a number of
> > users of Xen that would certainly want to ship without all the code
> > associated with PV compiled in. Given the nature of design "compromises"
> > in many parts of x86 systems there is certainly a non-zero sum of people
> > that would likely utilize the ability to remove code that doesn't need
> > to be there. I think every individual on this list who has been involved
> > in the security has been in a room of @intel.com folks has seen features
> > vs security win out many times.
> > 
> > I don't think its a hard stretch of the imagination to see people
> > disabling PV in data centers running newer workloads on PVH and HVM
> > only.
> 
> Yes, definitely disabling PV will be useful. Right after being able to
> use PCI passthrough with PVH.
> 
> > I can see the real question being why HVM? That I would say lies
> > with the direction of discretionary access controls in Xen vs mandatory
> > access controls. To solve for the lack of functionality we've grown
> > things like "dmops" and I could certainly see a product like Qubes
> > running only PVH domains in the future.
> > 
> > Since I picked on Qubes I've CC'd Marek.
> 
> So, is it going to be an option to have CONFIG_HVM=n and CONFIG_PVH=y at
> the same time? While currently we do support Windows, so need
> CONFIG_HVM=y, but I can see in some future/alternative version we could
> have even that disabled. For example right now we do have
> CONFIG_SHADOW_PAGING disabled.
> 

Hypervisor doesn't distinguish HVM and PVH at this point. More work is
needed there. But I expect the debate of what each option covers will
take longer than actually writing the code.

Wei.

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