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

David Hildenbrand david at redhat.com
Fri Nov 8 09:07:10 AEDT 2019


On 07.11.19 19:22, David Hildenbrand wrote:
> 
> 
>> 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.
> 

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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