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

David Hildenbrand dhildenb at redhat.com
Thu Nov 7 18:22:12 UTC 2019

> Am 07.11.2019 um 16:40 schrieb Dan Williams <dan.j.williams at intel.com>:
> On Thu, Oct 24, 2019 at 5:12 AM David Hildenbrand <david at redhat.com> 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 at redhat.com>
>> Cc: Cornelia Huck <cohuck at redhat.com>
>> Signed-off-by: David Hildenbrand <david at redhat.com>
>> ---
>> 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.

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