Re: [Xen-devel] [PATCH v6 04/18] vt-d: VT-d Posted-Interrupts feature detection

["Wu, Feng" <feng.wu@xxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: Monday, September 07, 2015 6:43 PM
> To: Wu, Feng
> Cc: Tian, Kevin; Zhang, Yang Z; xen-devel@xxxxxxxxxxxxx
> Subject: RE: [Xen-devel] [PATCH v6 04/18] vt-d: VT-d Posted-Interrupts feature
> detection
> 
> >>> On 06.09.15 at 03:49, <feng.wu@xxxxxxxxx> wrote:
> >> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> >> Sent: Friday, September 04, 2015 10:31 PM
> >> >>> On 25.08.15 at 03:57, <feng.wu@xxxxxxxxx> wrote:
> >> > --- a/xen/drivers/passthrough/vtd/iommu.c
> >> > +++ b/xen/drivers/passthrough/vtd/iommu.c
> >> > @@ -2079,6 +2079,9 @@ static int init_vtd_hw(void)
> >> >                  disable_intremap(drhd->iommu);
> >> >      }
> >> > @@ -2176,6 +2179,14 @@ int __init intel_vtd_setup(void)
> >> >          if ( iommu_intremap && !ecap_intr_remap(iommu->ecap) )
> >> >              iommu_intremap = 0;
> >> >
> >> > +        /*
> >> > +         * We cannot use posted interrupt if X86_FEATURE_CX16 is
> >> > +         * not supported, since we count on this feature to
> >> > +         * atomically update 16-byte IRTE in posted format.
> >> > +         */
> >> > +        if ( !iommu_intremap || !cap_intr_post(iommu->cap)
> >> || !cpu_has_cx16 )
> >> > +            iommu_intpost = 0;
> >>
> >> You should check iommu_intpost instead of iommu_intremap to
> >> match the other checks above here.
> >
> > Here I followed your advice:
> > http://lists.xenproject.org/archives/html/xen-devel/2015-06/msg01261.html
> >
> > I think the following code should be ok here though, if you like it, I can
> > change back.
> >
> > +        if ( !iommu_intpost || !cap_intr_post(iommu->cap)
> > +            || !cpu_has_cx16 )
> > +            iommu_intpost = 0;
> 
> The first part is neither in line with the earlier adjustments (as _still_
> see in the patch context above) not does it make much sense to set
> a variable to zero when it already is zero.

So, what about this?

 +        if ( !cap_intr_post(iommu->cap) || !cpu_has_cx16 )
 +            iommu_intpost = 0;

Thanks,
Feng

> 
> Jan


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