Re: [PATCH 03/16] SUPPORT.md: Add some x86 features

From: George Dunlap <George.Dunlap@citrix.com>
To: Jan Beulich <JBeulich@suse.com>
Cc: StefanoStabellini <sstabellini@kernel.org>,
	Wei Liu <wei.liu2@citrix.com>,
	Konrad Wilk <konrad.wilk@oracle.com>,
	Andrew Cooper <Andrew.Cooper3@citrix.com>,
	"Tim (Xen.org)" <tim@xen.org>,
	Ian Jackson <Ian.Jackson@citrix.com>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	Roger Pau Monne <roger.pau@citrix.com>
Subject: Re: [PATCH 03/16] SUPPORT.md: Add some x86 features
Date: Tue, 21 Nov 2017 12:24:24 +0000	[thread overview]
Message-ID: <D99A3A2B-DB38-4F80-9DAB-45F679F835D5@citrix.com> (raw)
In-Reply-To: <5A141D990200007800190776@prv-mh.provo.novell.com>

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On Nov 21, 2017, at 11:35 AM, Jan Beulich <JBeulich@suse.com<mailto:JBeulich@suse.com>> wrote:

On 21.11.17 at 11:42, <george.dunlap@citrix.com<mailto:george.dunlap@citrix.com>> wrote:
On 11/21/2017 08:09 AM, Jan Beulich wrote:
On 13.11.17 at 16:41, <george.dunlap@citrix.com<mailto:george.dunlap@citrix.com>> wrote:
+### x86/PVH guest
+
+    Status: Supported
+
+PVH is a next-generation paravirtualized mode
+designed to take advantage of hardware virtualization support when possible.
+During development this was sometimes called HVMLite or PVHv2.
+
+Requires hardware virtualisation support (Intel VMX / AMD SVM)

I think it needs to be said that only DomU is considered supported.
Dom0 is perhaps not even experimental at this point, considering
the panic() in dom0_construct_pvh().

Indeed, that's why dom0 PVH isn't in the list, and why this says 'PVH
guest', and is in the 'Guest Type' section.  We generally don't say,
"Oh, and we don't have this feature at all".

If you think it's important we could add a sentence here explicitly
stating that dom0 PVH isn't supported, but I sort of feel like it isn't
necessary.

Much depends on whether you think "guest" == "DomU". To me
Dom0 is a guest, too.

That’s not how I’ve ever understood those terms.

A guest at a hotel is someone who is served, and who does not have (legal) access to the internals of the system.  The maids who clean the room and the janitors who sweep the floors are hosts, because they have (to various degrees) extra access designed to help them serve the guests.

A “guest” is a virtual machine that does not have access to the internals of the system; that is the “target” of virtualization.  As such, the dom0 kernel and all the toolstack / emulation code running in domain 0 are part of the “host”.

Domain 0 is a domain and a VM, but only domUs are guests.

Any other opinions on this?  Do we need to add these to the terms defined at the bottom?

 -George

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