Re: [Xen-devel] Ongoing/future speculative mitigation work

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 26.10.18 at 11:28, <wei.liu2@xxxxxxxxxx> wrote:
> On Fri, Oct 26, 2018 at 03:16:15AM -0600, Jan Beulich wrote:
>> >>> On 25.10.18 at 18:29, <andrew.cooper3@xxxxxxxxxx> 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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