Re: [PATCH] xen/x86: allow Dom0 PVH to call XENMEM_exchange

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 28 Apr 2025, Teddy Astie wrote:
> Hello Stefano,
> 
> Le 25/04/2025 à 22:21, Stefano Stabellini a écrit :
> > From: Xenia Ragiadakou <Xenia.Ragiadakou@xxxxxxx>
> >
> > Dom0 PVH might need XENMEM_exchange when passing contiguous memory
> > addresses to firmware or co-processors not behind an IOMMU.
> >
> > XENMEM_exchange was blocked for HVM/PVH DomUs, and accidentally it
> > impacted Dom0 PVH as well.
> >
> > Permit Dom0 PVH to call XENMEM_exchange while leaving it blocked for
> > HVM/PVH DomUs.
> >
> 
> In addition to Jan's remarks, I think it wants some additional
> clarifications on the hypercall interface. public/memory.h indicates
> "only PV guests can use this operation", so the interface is actually
> unspecified about HVM guests (e.g what are MFN/GMFN in this case ?).

That is a new addition from c08a11ab98c. If you see fae7d5be8bb, there
is a statement that "we permitted this operation".


> > Signed-off-by: Xenia Ragiadakou <Xenia.Ragiadakou@xxxxxxx>
> > Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxx>
> >
> > diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
> > index 1cf2365167..e995944333 100644
> > --- a/xen/arch/x86/mm.c
> > +++ b/xen/arch/x86/mm.c
> > @@ -4401,7 +4401,7 @@ int steal_page(
> >       const struct domain *owner;
> >       int rc;
> >
> > -    if ( paging_mode_external(d) )
> > +    if ( paging_mode_external(d) && !is_hardware_domain(d) )
> >           return -EOPNOTSUPP;
> >
> >       /* Grab a reference to make sure the page doesn't change under our 
> > feet */
> > diff --git a/xen/common/memory.c b/xen/common/memory.c
> > index 8ca4e1a842..796eec081b 100644
> > --- a/xen/common/memory.c
> > +++ b/xen/common/memory.c
> > @@ -794,7 +794,7 @@ static long 
> > memory_exchange(XEN_GUEST_HANDLE_PARAM(xen_memory_exchange_t) arg)
> >               rc = guest_physmap_add_page(d, _gfn(gpfn), mfn,
> >                                           exch.out.extent_order) ?: rc;
> >
> > -            if ( !paging_mode_translate(d) &&
> > +            if ( (!paging_mode_translate(d) || is_hardware_domain(d)) &&
> >                    __copy_mfn_to_guest_offset(exch.out.extent_start,
> >                                               (i << out_chunk_order) + j,
> >                                               mfn) )