Re: [PATCH v1] intel_iommu: Expose SMPWC when SVM is enabled

On Tue, 2026-08-04 at 16:39 +0800, Yi Liu wrote:
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> 
> On 8/3/26 15:26, Duan, Zhenzhong wrote:
> 
> > Hi Clement,
> > 
> > 
> > > -----Original Message-----
> > > From: Clément MATHIEU--DRIF <[[email protected]](mailto:[email protected])>
> > > Subject: [PATCH v1] intel_iommu: Expose SMPWC when SVM is enabled
> > > 
> > > SVM in Linux requires SMPWC as of 780dfed68862. This is free in the
> > > current implementation as it is already in line with the expected
> > > behaviour.
> > 
> > 
> > Not an issue in this patch, what about 'C: Page-walk Coherency' ecap bit,
> > I think we can enable it too? Maybe in another patch?
> > 
> > 
> > > 
> > > Signed-off-by: Clement Mathieu--Drif <[[email protected]](mailto:[email protected])>
> > > ---
> > > hw/i386/intel_iommu.c          | 3 ++-
> > > hw/i386/intel_iommu_internal.h | 1 +
> > > 2 files changed, 3 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/hw/i386/intel_iommu.c b/hw/i386/intel_iommu.c
> > > index 8245452d89..9f21622a4f 100644
> > > --- a/hw/i386/intel_iommu.c
> > > +++ b/hw/i386/intel_iommu.c
> > > @@ -5108,7 +5108,8 @@ static void vtd_init(IntelIOMMUState *s)
> > >      }
> > > 
> > >      if (s->svm) {
> > > -        s->ecap |= VTD_ECAP_PRS | VTD_ECAP_PDS | VTD_ECAP_NWFS;
> > > +        s->ecap |= VTD_ECAP_PRS | VTD_ECAP_PDS | VTD_ECAP_NWFS |
> > > +                   VTD_ECAP_SMPWCS;
> > 
> > 
> > I think we need a check for passthrough device to expose SMPWCS, something like below pseudo code:
> 
> 
> thumb up. yes.  It's strange guest reports SMPWCS while host does not.

Will do,

thanks

> 
> 
> > @@ -37,6 +37,12 @@ bool vtd_check_hiod_accel(IntelIOMMUState *s, VTDHostIOMMUDevice *vtd_hiod,
> >           return false;
> >       }
> > 
> > +    if (VTD_ECAP_GET_SMPWCS(s->ecap) && !VTD_ECAP_GET_SMPWCS(vtd->cap_reg)) {
> > +        error_setg(errp,
> > +                   " Scalable-Mode Page-walk Coherency is unsupported by host IOMMU");
> > +        return false;
> > +    }
> > +
> >       if (s->fs1gp && !(vtd->cap_reg & VTD_CAP_FS1GP)) {
> >           error_setg(errp,
> >                      "First stage 1GB large page is unsupported by host IOMMU");
> > 
> > 
> > >      }
> > > 
> > >      vtd_reset_caches(s);
> > > diff --git a/hw/i386/intel_iommu_internal.h b/hw/i386/intel_iommu_internal.h
> > > index 924e91cb8a..df7a0efa6e 100644
> > > --- a/hw/i386/intel_iommu_internal.h
> > > +++ b/hw/i386/intel_iommu_internal.h
> > > @@ -202,6 +202,7 @@
> > > #define VTD_ECAP_SMTS               (1ULL << 43)
> > > #define VTD_ECAP_SSTS               (1ULL << 46)
> > > #define VTD_ECAP_FSTS               (1ULL << 47)
> > > +#define VTD_ECAP_SMPWCS             (1ULL << 48)
> > > 
> > > /* CAP_REG */
> > > /* (offset >> 4) << 24 */
> > > --
> > > 2.54.0
> >
> 
>