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

On Tue, Jul 21, 2020 at 12:42:02PM +0200, Laurent Dufour wrote:

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

            ^^ call directly instead of triggering..

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

		          ^^ held

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>

Reviewed-by: Ram Pai <redacted>

RP