Re: [PATCH v2 00/12] Alternate p2m: support multiple copies of host p2m

From: Ed White <edmund.h.white@intel.com>
To: "Lengyel, Tamas" <tlengyel@novetta.com>
Cc: Ravi Sahita <ravi.sahita@intel.com>,
	Wei Liu <wei.liu2@citrix.com>,
	Razvan Cojocaru <rcojocaru@bitdefender.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Ian Jackson <ian.jackson@eu.citrix.com>, Tim Deegan <tim@xen.org>,
	Jan Beulich <jbeulich@suse.com>,
	Daniel De Graaf <dgdegra@tycho.nsa.gov>,
	Xen-devel <xen-devel@lists.xen.org>
Subject: Re: [PATCH v2 00/12] Alternate p2m: support multiple copies of host p2m
Date: Thu, 25 Jun 2015 13:34:25 -0700	[thread overview]
Message-ID: <558C65D1.3070501@intel.com> (raw)
In-Reply-To: <CAD33N+44bqZ+mfLjNZGQfTvp_CMGs4w7H5PCBya1Ht2FwZgYiA@mail.gmail.com>

On 06/25/2015 10:29 AM, Lengyel, Tamas wrote:
>>
>> I plan to fix the bug, such that unshare will always occur before a copy.
>> I don't plan to make the altp2m's able to have shared pages.
>>
>> Ed
>>
> 
> For now that is of course fine, memsharing is experimental and that's what
> I meant above. I would however like to see that option in the future
> eventually. I'll be digging it into a bit more once altp2m is merged, do
> you see any obstacle why it wouldn't/couldn't work?

The general philosophy is that an altp2m is a copy of the host p2m with
the same or more restrictive access permissions. Remapping tweaks that
by allowing different permutations of the gfn->mfn mappings, but only
from the same gfn and mfn sets.

There are a number of issues to do with locking order, coherency
between multiple p2m's and code that operates on the host p2m 
implicitly that would need to be addressed to handle all
page types in altp2m's, but I think what you are asking for violates
the overarching philosophy stated above.

Ed