Re: [Xen-devel] [PATCH v2] x86/domain_page: implement pure per-vCPU mapping infrastructure

[Wei Liu <wl@xxxxxxx>]

On Fri, Feb 21, 2020 at 03:55:28PM +0100, Jan Beulich wrote:
> On 21.02.2020 15:36, Wei Liu wrote:
> > On Fri, Feb 21, 2020 at 02:31:08PM +0100, Jan Beulich wrote:
> >> On 21.02.2020 13:52, Xia, Hongyan wrote:
> >>> On Fri, 2020-02-21 at 11:50 +0000, Wei Liu wrote:
> >>>> On Thu, Feb 06, 2020 at 06:58:23PM +0000, Hongyan Xia wrote:
> >>>>> +    if ( hashmfn != mfn && !vcache->refcnt[idx] )
> >>>>> +        __clear_bit(idx, vcache->inuse);
> >>>>
> >>>> Also, please flush the linear address here and the other __clear_bit
> >>>> location.
> >>>
> >>> I flush when a new entry is taking a slot. Yeah, it's probably better
> >>> to flush earlier whenever a slot is no longer in use.
> >>
> >> Question is whether such individual flushes aren't actually
> >> more overhead than a single flush covering all previously
> >> torn down entries, done at suitable points (see the present
> >> implementation).
> > 
> > I asked to flush because I was paranoid about leaving stale entry after
> > the slot is reclaimed. I think the address will be flushed when a new
> > entry is inserted.
> > 
> > So the question would be whether we care about leaving a stale entry in
> > place until a new one is inserted.
> 
> Well, we apparently don't have an issue with such today, so
> unless explicitly stated to the contrary I think any replacement
> implementation can and should make the same assumptions /
> guarantees.

In that case, Hongyan's current implementation should be fine. Flushing
is deferred to the last possible moment -- right before next use.

Wei.

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