Re: [PATCH 00/16] Remove the direct map

[Wei Liu <wl@xxxxxxx>]

On Fri, May 01, 2020 at 02:53:08PM +0100, Hongyan Xia wrote:
> On Fri, 2020-05-01 at 12:07 +0000, Wei Liu wrote:
> > On Thu, Apr 30, 2020 at 09:44:09PM +0100, Hongyan Xia wrote:
> > > From: Hongyan Xia <hongyxia@xxxxxxxxxx>
> > > 
> > > This series depends on Xen page table domheap conversion:
> > > 
> https://lists.xenproject.org/archives/html/xen-devel/2020-04/msg01374.html
> > > .
> > > 
> > > After breaking the reliance on the direct map to manipulate Xen
> > > page
> > > tables, we can now finally remove the direct map altogether.
> > > 
> > > This series:
> > > - fixes many places that use the direct map incorrectly or assume
> > > the
> > >   presence of an always-mapped direct map in a wrong way.
> > > - includes the early vmap patches for global mappings.
> > > - initialises the mapcache for all domains, disables the fast path
> > > that
> > >   uses the direct map for mappings.
> > > - maps and unmaps xenheap on-demand.
> > > - adds a boot command line switch to enable or disable the direct
> > > map.
> > > 
> > > This previous version was in RFC state and can be found here:
> > > 
> https://lists.xenproject.org/archives/html/xen-devel/2019-09/msg02647.html
> > > ,
> > > which has since been broken into small series.
> > 
> > OOI have you done any performance measurements?
> > 
> > Seeing that now even guest table needs mapping / unmapping during
> > restore, I'm curious to know how that would impact performance.
> 
> I actually have a lot of performance numbers but unfortunately on an
> older version of Xen, not staging. I need to evaluate it again before
> coming back to you. As you suspected, one strong signal from the
> performance results is definitely the impact of walking guest tables.
> For EPT, mapping and unmapping 20 times is no fun. This shows up in
> micro-benchmarks, although larger benchmarks tend to be fine.
> 
> My question is, do we care about hiding EPT? I think it is fine to just
> use xenheap pages (or any other form which does the job) so that we go
> down from 20 mappings to only 4. I have done this hack with EPT and saw
> significantly reduced impact for HVM guests in micro-benchmarks.

Not sure about hiding EPT. I will leave this question to Jan and
Andrew...

Wei.

> 
> Hongyan
>