Re: Ongoing/future speculative mitigation work

From: "Jan Beulich" <JBeulich@suse.com>
To: Wei Liu <wei.liu2@citrix.com>
Cc: Martin Pohlack <mpohlack@amazon.de>,
	Julien Grall <julien.grall@arm.com>,
	Joao Martins <joao.m.martins@oracle.com>,
	Stefano Stabellini <sstabellini@kernel.org>,
	Mihai Dontu <mdontu@bitdefender.com>,
	Marek Marczykowski <marmarek@invisiblethingslab.com>,
	Anthony Liguori <aliguori@amazon.com>,
	uwed@amazon.de, Lars Kurth <lars.kurth@citrix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	Ross Philipson <ross.philipson@oracle.com>,
	george.dunlap@citrix.com, Dario Faggioli <dfaggioli@suse.com>,
	Matt Wilson <msw@amazon.com>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Juergen Gross <jgross@suse.com>,
	Sergey Dyasli <sergey.dyasli@citrix.com>,
	George Dunlap <George.Dunlap@eu.citrix.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Xen-devel List <xen-devel@lists.xen.org>,
	Daniel Kiper <daniel.kiper@oracle.com>,
	David Woodhouse <dwmw@amazon.co.uk>, Roger Pau Monne <roger.>
Subject: Re: Ongoing/future speculative mitigation work
Date: Fri, 26 Oct 2018 03:56:58 -0600	[thread overview]
Message-ID: <5BD2E4EA02000078001F4F96@prv1-mh.provo.novell.com> (raw)
In-Reply-To: <20181026092820.bernuj2cskqtwqqv@zion.uk.xensource.com>

>>> On 26.10.18 at 11:28, <wei.liu2@citrix.com> wrote:
> On Fri, Oct 26, 2018 at 03:16:15AM -0600, Jan Beulich wrote:
>> >>> On 25.10.18 at 18:29, <andrew.cooper3@citrix.com> wrote:
>> > A split xenheap model means that data pertaining to other guests isn't
>> > mapped in the context of this vcpu, so cannot be brought into the cache.
>> 
>> It was not clear to me from Wei's original mail that talk here is
>> about "split" in a sense of "per-domain"; I was assuming the
>> CONFIG_SEPARATE_XENHEAP mode instead.
> 
> The split heap was indeed referring to CONFIG_SEPARATE_XENHEAP mode, yet
> I what I wanted most is the partial direct map which reduces the amount
> of data mapped inside Xen context -- the original idea was removing
> direct map discussed during one of the calls IIRC. I thought making the
> partial direct map mode work and make it as small as possible will get
> us 90% there.
> 
> The "per-domain" heap is a different work item.

But if we mean to go that route, going (back) to the separate
Xen heap model seems just like an extra complication to me.
Yet I agree that this would remove the need for a fair chunk of
the direct map. Otoh a statically partitioned Xen heap would
bring back scalability issues which we had specifically meant to
get rid of by moving away from that model.

Jan

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