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

On Wed, Jul 08, 2020 at 02:16:36PM +0200, Laurent Dufour wrote:

Le 08/07/2020 à 13:25, Bharata B Rao a écrit :

quoted

On Fri, Jul 03, 2020 at 05:59:14PM +0200, Laurent Dufour wrote:

quoted

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.

Yes, this appears much simpler.

Thanks Bharata for reviewing this.

quoted

quoted

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 852cc9ae6a0b..479ddf16d18c 100644
--- a/arch/powerpc/kvm/book3s_hv_uvmem.c
+++ b/arch/powerpc/kvm/book3s_hv_uvmem.c

@@ -533,35 +533,55 @@ 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;
+
+	down_read(&kvm->mm->mmap_sem);

You should be using mmap_read_lock(kvm->mm) with recent kernels.

Absolutely, shame on me, I reviewed Michel's series about that!

Paul, Michael, could you fix that when pulling this patch or should I sent a
whole new series?

Given that Ram has reworked his series, I think it would be best if
you rebase on top of his new series and re-send your series.

Thanks,
Paul.