Re: [PATCH v2 2/2] KVM: PPC: Book3S HV: rework secure mem slot dropping

When a secure memslot is dropped, all the pages backed in the secure device
(aka really backed by secure memory by the Ultravisor) should be paged out
to a normal page. Previously, this was achieved by triggering the page
fault mechanism which is calling kvmppc_svm_page_out() on each pages.

This can't work when hot unplugging a memory slot because the memory slot
is flagged as invalid and gfn_to_pfn() is then not trying to access the
page, so the page fault mechanism is not triggered.

Since the final goal is to make a call to kvmppc_svm_page_out() it seems
simpler to directly calling it instead of triggering such a mechanism. This
way kvmppc_uvmem_drop_pages() can be called even when hot unplugging a
memslot.

Since kvmppc_uvmem_drop_pages() is already holding kvm->arch.uvmem_lock,
the call to __kvmppc_svm_page_out() is made.
As __kvmppc_svm_page_out needs the vma pointer to migrate the pages, the
VMA is fetched in a lazy way, to not trigger find_vma() all the time. In
addition, the mmap_sem is help in read mode during that time, not in write
mode since the virual memory layout is not impacted, and
kvm->arch.uvmem_lock prevents concurrent operation on the secure device.

Cc: Ram Pai <redacted>
Cc: Bharata B Rao <redacted>
Cc: Paul Mackerras <redacted>
Signed-off-by: Laurent Dufour <redacted>
---
 arch/powerpc/kvm/book3s_hv_uvmem.c | 54 ++++++++++++++++++++----------
 1 file changed, 37 insertions(+), 17 deletions(-)

diff --git a/arch/powerpc/kvm/book3s_hv_uvmem.c b/arch/powerpc/kvm/book3s_hv_uvmem.c
index 5a4b02d3f651..ba5c7c77cc3a 100644
--- a/arch/powerpc/kvm/book3s_hv_uvmem.c
+++ b/arch/powerpc/kvm/book3s_hv_uvmem.c

@@ -624,35 +624,55 @@ static inline int kvmppc_svm_page_out(struct vm_area_struct *vma,
  * fault on them, do fault time migration to replace the device PTEs in
  * QEMU page table with normal PTEs from newly allocated pages.
  */
-void kvmppc_uvmem_drop_pages(const struct kvm_memory_slot *free,
+void kvmppc_uvmem_drop_pages(const struct kvm_memory_slot *slot,
 			     struct kvm *kvm, bool skip_page_out)
 {
 	int i;
 	struct kvmppc_uvmem_page_pvt *pvt;
-	unsigned long pfn, uvmem_pfn;
-	unsigned long gfn = free->base_gfn;
+	struct page *uvmem_page;
+	struct vm_area_struct *vma = NULL;
+	unsigned long uvmem_pfn, gfn;
+	unsigned long addr, end;
+
+	mmap_read_lock(kvm->mm);
+
+	addr = slot->userspace_addr;