[Xen-devel] [PATCH RFC v1 04/12] KVM: Prepare kvm_is_reserved_pfn() for PG_reserved changes

[David Hildenbrand <david@xxxxxxxxxx>]

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. Note that ZONE_DEVICE memory is
never online (IOW, managed by the buddy).

Switching to pfn_to_online_page() keeps the existing behavior for
PFNs without a memmap and for ZONE_DEVICE memory. They are treated as
reserved and the page is not touched (e.g., to set it dirty or accessed).

Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: "Radim Krčmář" <rkrcmar@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
Cc: KarimAllah Ahmed <karahmed@xxxxxxxxx>
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
 virt/kvm/kvm_main.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 66a977472a1c..b233d4129014 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -151,9 +151,15 @@ __weak int kvm_arch_mmu_notifier_invalidate_range(struct 
kvm *kvm,
 
 bool kvm_is_reserved_pfn(kvm_pfn_t pfn)
 {
-       if (pfn_valid(pfn))
-               return PageReserved(pfn_to_page(pfn));
+       struct page *page = pfn_to_online_page(pfn);
 
+       /*
+        * We treat any pages that are not online (not managed by the buddy)
+        * as reserved - this includes ZONE_DEVICE pages and pages without
+        * a memmap (e.g., mapped via /dev/mem).
+        */
+       if (page)
+               return PageReserved(page);
        return true;
 }
 
-- 
2.21.0


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