Re: [Xen-devel] [PATCH v1 04/10] vfio/type1: Prepare is_invalid_reserved_pfn() for PG_reserved changes

[David Hildenbrand <david@xxxxxxxxxx>]

On 07.11.19 19:22, David Hildenbrand wrote:
> > Am 07.11.2019 um 16:40 schrieb Dan Williams <dan.j.williams@xxxxxxxxx>:
> >
> > On Thu, Oct 24, 2019 at 5:12 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
> > > Right now, ZONE_DEVICE memory is always set PG_reserved. We want to
> > > change that.
> > >
> > > KVM has this weird use case that you can map anything from /dev/mem
> > > into the guest. pfn_valid() is not a reliable check whether the memmap
> > > was initialized and can be touched. pfn_to_online_page() makes sure
> > > that we have an initialized memmap (and don't have ZONE_DEVICE memory).
> > >
> > > Rewrite is_invalid_reserved_pfn() similar to kvm_is_reserved_pfn() to make
> > > sure the function produces the same result once we stop setting ZONE_DEVICE
> > > pages PG_reserved.
> > >
> > > Cc: Alex Williamson <alex.williamson@xxxxxxxxxx>
> > > Cc: Cornelia Huck <cohuck@xxxxxxxxxx>
> > > Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> > > ---
> > > drivers/vfio/vfio_iommu_type1.c | 10 ++++++++--
> > > 1 file changed, 8 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> > > index 2ada8e6cdb88..f8ce8c408ba8 100644
> > > --- a/drivers/vfio/vfio_iommu_type1.c
> > > +++ b/drivers/vfio/vfio_iommu_type1.c
> > > @@ -299,9 +299,15 @@ static int vfio_lock_acct(struct vfio_dma *dma, long 
> > > npage, bool async)
> > >   */
> > > static bool is_invalid_reserved_pfn(unsigned long pfn)
> > > {
> > > -       if (pfn_valid(pfn))
> > > -               return PageReserved(pfn_to_page(pfn));
> > > +       struct page *page = pfn_to_online_page(pfn);
> > Ugh, I just realized this is not a safe conversion until
> > pfn_to_online_page() is moved over to subsection granularity. As it
> > stands it will return true for any ZONE_DEVICE pages that share a
> > section with boot memory.
> That should not happen right now and I commented back when you introduced subsection 
> support that I don’t want to have ZONE_DEVICE mixed with online pages in a 
> section. Having memory block devices that partially span ZONE_DEVICE would be ... 
> really weird. With something like pfn_active() - as discussed - we could at least make 
> this check work - but I am not sure if we really want to go down that path. In the 
> worst case, some MB of RAM are lost ... I guess this needs more thought.
I just realized the "boot memory" part. Is that a real thing? IOW, can 
we have ZONE_DEVICE falling into a memory block (with holes)? I somewhat 
have doubts that this would work ...
--

Thanks,

David / dhildenb


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