Re: [PATCH v3 2/5] xen/common: Guard iommu symbols with CONFIG_HAS_PASSTHROUGH

[Connor Davis <connojdavis@xxxxxxxxx>]

On 5/17/21 9:42 AM, Julien Grall wrote:
> Hi Jan,
>
> On 17/05/2021 12:16, Jan Beulich wrote:
> > On 14.05.2021 20:53, Connor Davis wrote:
> > > --- a/xen/common/memory.c
> > > +++ b/xen/common/memory.c
> > > @@ -294,7 +294,9 @@ int guest_remove_page(struct domain *d, unsigned 
> > > long gmfn)
> > >       p2m_type_t p2mt;
> > >   #endif
> > >       mfn_t mfn;
> > > +#ifdef CONFIG_HAS_PASSTHROUGH
> > >       bool *dont_flush_p, dont_flush;
> > > +#endif
> > >       int rc;
> > >     #ifdef CONFIG_X86
> > > @@ -385,13 +387,17 @@ int guest_remove_page(struct domain *d, 
> > > unsigned long gmfn)
> > >        * Since we're likely to free the page below, we need to suspend
> > >        * xenmem_add_to_physmap()'s suppressing of IOMMU TLB flushes.
> > >        */
> > > +#ifdef CONFIG_HAS_PASSTHROUGH
> > >       dont_flush_p = &this_cpu(iommu_dont_flush_iotlb);
> > >       dont_flush = *dont_flush_p;
> > >       *dont_flush_p = false;
> > > +#endif
> > >         rc = guest_physmap_remove_page(d, _gfn(gmfn), mfn, 0);
> > >   +#ifdef CONFIG_HAS_PASSTHROUGH
> > >       *dont_flush_p = dont_flush;
> > > +#endif
> > >         /*
> > >        * With the lack of an IOMMU on some platforms, domains with 
> > > DMA-capable
> > > @@ -839,11 +845,13 @@ int xenmem_add_to_physmap(struct domain *d, 
> > > struct xen_add_to_physmap *xatp,
> > >       xatp->gpfn += start;
> > >       xatp->size -= start;
> > >   +#ifdef CONFIG_HAS_PASSTHROUGH
> > >       if ( is_iommu_enabled(d) )
> > >       {
> > >          this_cpu(iommu_dont_flush_iotlb) = 1;
> > >          extra.ppage = &pages[0];
> > >       }
> > > +#endif
> > >         while ( xatp->size > done )
> > >       {
> > > @@ -868,6 +876,7 @@ int xenmem_add_to_physmap(struct domain *d, 
> > > struct xen_add_to_physmap *xatp,
> > >           }
> > >       }
> > >   +#ifdef CONFIG_HAS_PASSTHROUGH
> > >       if ( is_iommu_enabled(d) )
> > >       {
> > >           int ret;
> > > @@ -894,6 +903,7 @@ int xenmem_add_to_physmap(struct domain *d, 
> > > struct xen_add_to_physmap *xatp,
> > >           if ( unlikely(ret) && rc >= 0 )
> > >               rc = ret;
> > >       }
> > > +#endif
> > >         return rc;
> > >   }
> > I wonder whether all of these wouldn't better become CONFIG_X86:
> > ISTR Julien indicating that he doesn't see the override getting used
> > on Arm. (Julien, please correct me if I'm misremembering.)
> Right, so far, I haven't been in favor to introduce it because:
>    1) The P2M code may free some memory. So you can't always ignore 
> the flush (I think this is wrong for the upper layer to know when this 
> can happen).
>    2) It is unclear what happen if the IOMMU TLBs and the PT contains 
> different mappings (I received conflicted advice).
> So it is better to always flush and as early as possible.
So keep it as is or switch to CONFIG_X86?


Thanks,

Connor