Re: [Qemu-devel] [PATCH v3 1/3] IOMMU: add VTD_CAP_CM to vIOMMU capability exposed to guest

[Alex Williamson <alex.williamson@redhat.com>]

On Thu, 2 Jun 2016 16:44:39 +0800
Peter Xu <peterx@redhat.com> wrote:

> On Sat, May 21, 2016 at 06:42:03PM +0200, Jan Kiszka wrote:
> > On 2016-05-21 18:19, Aviv B.D wrote:  
> > > From: "Aviv Ben-David" <bd.aviv@gmail.com>
> > > 
> > > This flag tells the guest to invalidate tlb cache also after unmap operations.
> > > 
> > > Signed-off-by: Aviv Ben-David <bd.aviv@gmail.com>
> > > ---
> > >  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 347718f..1af8da8 100644
> > > --- a/hw/i386/intel_iommu.c
> > > +++ b/hw/i386/intel_iommu.c
> > > @@ -1949,7 +1949,8 @@ static void vtd_init(IntelIOMMUState *s)
> > >      s->iq_last_desc_type = VTD_INV_DESC_NONE;
> > >      s->next_frcd_reg = 0;
> > >      s->cap = VTD_CAP_FRO | VTD_CAP_NFR | VTD_CAP_ND | VTD_CAP_MGAW |
> > > -             VTD_CAP_SAGAW | VTD_CAP_MAMV | VTD_CAP_PSI | VTD_CAP_SLLPS;
> > > +             VTD_CAP_SAGAW | VTD_CAP_MAMV | VTD_CAP_PSI | VTD_CAP_SLLPS |
> > > +             VTD_CAP_CM;  
> > 
> > Again, needs to be optional because not all guests will support it or
> > behave differently when it's set (I've one that refuses to work).  
> 
> There should be more than one way to make it optional. Which is
> better? What I can think of:
> 
> (Assume we have Marcel's "-device intel_iommu" working already)
> 
> 1. Let the CM bit optional, or say, we need to specify something like
>    "-device intel_iommu,cmbit=on" or we will disable CM bit. If we
>    have CM disabled but with VFIO device, let QEMU raise error.
> 
> 2. We automatically detect whether we need CM bit. E.g., if we have
>    VFIO and vIOMMU both enabled, we automatically set the bit. Another
>    case is maybe we would in the future support nested vIOMMU? If so,
>    we can do the same thing for the nested feature.


Why do we need to support VT-d for guests that do not support CM=1?
The VT-d spec indicates that software should be written to handle both
caching modes (6.1).  Granted this is a *should* and not a *must*,
but can't we consider guests that do not support CM=1 incompatible with
emulated VT-d?  If CM=0 needs to be supported then we need to shadow
all of the remapping structures since vfio effectively becomes a cache
of the that would otherwise depend on the invalidation of both present
and non-present entries.  What guests do not support CM=1?  Thanks,

Alex