Re: [RFC v8 4/5] intel_iommu: Do not notify regular iotlb to device-iotlb notifiers

[Eugenio Perez Martin <eperezma@redhat.com>]

On Tue, Sep 1, 2020 at 11:06 PM Peter Xu <peterx@redhat.com> wrote:
>
> On Tue, Sep 01, 2020 at 04:26:07PM +0200, Eugenio Pérez wrote:
> > This improves performance in case of netperf with vhost-net:
> > * TCP_STREAM: From 1923.6Mbit/s to 2175.13Mbit/s (13%)
> > * TCP_RR: From 8464.73 trans/s to 8932.703333 trans/s (5.5%)
> > * UDP_RR: From 8562.08 trans/s to 9005.62/s (5.1%)
> > * UDP_STREAM: No change observed (insignificant 0.1% improvement)
>
> Just to confirm: are these numbers about applying this patch before/after, or
> applying the whole series before/after?
>
> Asked since we actually optimized two parts:
>
> Firstly we avoid sending two invalidations for vhost.  That's done by the
> previous patch, afaict.
>
> Secondly, this patch avoids the page walk for vhost since not needed.
>
> Am I right?
>

Right! The numbers are comparing v5.1.0 tag to this commit. Avoiding
sending invalidations for vhost did improve performance but in a very
small number, I thought it would improve more.

It also depends a lot on using rhel8 (better numbers, less but
appreciable improvements) or rhel7 guest (worse performance compared
to rhel8 but patches provide more improvements).


> >
> > Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> > ---
> >  hw/i386/intel_iommu.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> >
> > diff --git a/hw/i386/intel_iommu.c b/hw/i386/intel_iommu.c
> > index cdddb089e7..fe82391b73 100644
> > --- a/hw/i386/intel_iommu.c
> > +++ b/hw/i386/intel_iommu.c
> > @@ -1964,6 +1964,12 @@ static void vtd_iotlb_domain_invalidate(IntelIOMMUState *s, uint16_t domain_id)
> >      vtd_iommu_unlock(s);
> >
> >      QLIST_FOREACH(vtd_as, &s->vtd_as_with_notifiers, next) {
> > +        if (vtd_as->iommu.iommu_notify_flags & IOMMU_NOTIFIER_DEVIOTLB) {
> > +            /* If IOMMU memory region is DEVICE IOTLB type, it does not make
> > +             * sense to send regular IOMMU notifications. */
> > +            continue;
> > +        }
> > +
>
> We want to avoid vtd_sync_shadow_page_table() for vhost, however IMHO a better
> expression would be:
>
>     if (!(vtd_as->iommu.iommu_notify_flags &
>         (IOMMU_NOTIFIER_MAP | IOMMU_NOTIFIER_UNMAP))) {
>         continue;
>     }
>
> The thing is we can't avoid the page sync if e.g. we're registered with
> MAP|UNMAP|DEVIOTLB.  The important thing here, imho, is MAP|UNMAP because these
> two messages are used for shadow page synchronizations, so we can skip that if
> neither of the message is registered.
>
> Besides, we can add this at the entry of vtd_sync_shadow_page_table() so that
> all callers of vtd_sync_shadow_page_table() can benefit.
>

Agree on both. Testing the new changes.

Thanks!



> >          if (!vtd_dev_to_context_entry(s, pci_bus_num(vtd_as->bus),
> >                                        vtd_as->devfn, &ce) &&
> >              domain_id == vtd_get_domain_id(s, &ce)) {
> > --
> > 2.18.1
> >
>
> --
> Peter Xu
>
>