Re: [PATCH 2/4] intel_iommu: Support IR-only mode without DMA translation

[Michael S. Tsirkin]

On Mon, Mar 14, 2022 at 03:45:47PM +0000, David Woodhouse wrote:
> On Mon, 2022-03-14 at 11:24 -0400, Michael S. Tsirkin wrote:
> > On Mon, Mar 14, 2022 at 02:25:42PM +0000, David Woodhouse wrote:
> > > From: David Woodhouse <dwmw@amazon.co.uk>
> > > 
> > > By setting none of the SAGAW bits we can indicate to a guest that DMA
> > > translation isn't supported. Tested by booting Windows 10, as well as
> > > Linux guests with the fix at https://git.kernel.org/torvalds/c/c40aaaac10
> > > 
> > > 
> > > Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
> > > Reviewed-by: Peter Xu <peterx@redhat.com>
> > > Acked-by: Jason Wang <jasowang@redhat.com>
> > 
> > this is borderline like a feature, but ...
> 
> It's the opposite of a feature — it's turning the feature *off* ;)

Right. Still - do you believe it's appropriate in soft freeze
and if yes why?

> > > 
> > > @@ -3627,12 +3628,17 @@ static void vtd_init(IntelIOMMUState *s)
> > >      s->next_frcd_reg = 0;
> > >      s->cap = VTD_CAP_FRO | VTD_CAP_NFR | VTD_CAP_ND |
> > >               VTD_CAP_MAMV | VTD_CAP_PSI | VTD_CAP_SLLPS |
> > > -             VTD_CAP_SAGAW_39bit | VTD_CAP_MGAW(s->aw_bits);
> > > +             VTD_CAP_MGAW(s->aw_bits);
> > >      if (s->dma_drain) {
> > >          s->cap |= VTD_CAP_DRAIN;
> > >      }
> > > -    if (s->aw_bits == VTD_HOST_AW_48BIT) {
> > > -        s->cap |= VTD_CAP_SAGAW_48bit;
> > > +    if (s->dma_translation) {
> > > +            if (s->aw_bits >= VTD_HOST_AW_39BIT) {
> > > +                    s->cap |= VTD_CAP_SAGAW_39bit;
> > > +            }
> > > +            if (s->aw_bits >= VTD_HOST_AW_48BIT) {
> > > +                    s->cap |= VTD_CAP_SAGAW_48bit;
> > > +            }
> > >      }
> > >      s->ecap = VTD_ECAP_QI | VTD_ECAP_IRO;
> > > 
> > 
> > 
> > ... this looks like you are actually fixing aw_bits < VTD_HOST_AW_39BIT,
> > right? So maybe this patch is ok like this since it also fixes a
> > bug. Pls add this to commit log though.
> 
> Nah, aw_bits cannot be < VTD_HOST_AW_39BIT. We explicitly check in
> vtd_decide_config(), and only 39 or 48 bits are supported,
> corresponding to 3-level or 4-level page tables.

Oh right. So not a bugfix then.

> The only time we'd want to *not* advertise 39-bit support is if we
> aren't advertising DMA translation at all. Which is the (anti-)feature
> introduced by this patch.
>