Re: [RFC PATCH 0/8] KVM: x86/mmu: Introduce pinned SPTEs framework

From: Eric van Tassell
Date: Sun Aug 02 2020 - 23:00:26 EST

What commit did you base your series on?

-evt(Eric van Tassell)

On 7/31/20 4:23 PM, Sean Christopherson wrote:
SEV currently needs to pin guest memory as it doesn't support migrating
encrypted pages. Introduce a framework in KVM's MMU to support pinning
pages on demand without requiring additional memory allocations, and with
(somewhat hazy) line of sight toward supporting more advanced features for
encrypted guest memory, e.g. host page migration.

The idea is to use a software available bit in the SPTE to track that a
page has been pinned. The decision to pin a page and the actual pinning
managment is handled by vendor code via kvm_x86_ops hooks. There are
intentionally two hooks (zap and unzap) introduced that are not needed for
SEV. I included them to again show how the flag (probably renamed?) could
be used for more than just pin/unpin.

Bugs in the core implementation are pretty much guaranteed. The basic
concept has been tested, but in a fairly different incarnation. Most
notably, tagging PRESENT SPTEs as PINNED has not been tested, although
using the PINNED flag to track zapped (and known to be pinned) SPTEs has
been tested. I cobbled this variation together fairly quickly to get the
code out there for discussion.

The last patch to pin SEV pages during sev_launch_update_data() is
incomplete; it's there to show how we might leverage MMU-based pinning to
support pinning pages before the guest is live.

Sean Christopherson (8):
KVM: x86/mmu: Return old SPTE from mmu_spte_clear_track_bits()
KVM: x86/mmu: Use bits 2:0 to check for present SPTEs
KVM: x86/mmu: Refactor handling of not-present SPTEs in mmu_set_spte()
KVM: x86/mmu: Add infrastructure for pinning PFNs on demand
KVM: SVM: Use the KVM MMU SPTE pinning hooks to pin pages on demand
KVM: x86/mmu: Move 'pfn' variable to caller of direct_page_fault()
KVM: x86/mmu: Introduce kvm_mmu_map_tdp_page() for use by SEV
KVM: SVM: Pin SEV pages in MMU during sev_launch_update_data()

arch/x86/include/asm/kvm_host.h | 7 ++
arch/x86/kvm/mmu.h | 3 +
arch/x86/kvm/mmu/mmu.c | 186 +++++++++++++++++++++++++-------
arch/x86/kvm/mmu/paging_tmpl.h | 3 +-
arch/x86/kvm/svm/sev.c | 141 +++++++++++++++++++++++-
arch/x86/kvm/svm/svm.c | 3 +
arch/x86/kvm/svm/svm.h | 3 +
7 files changed, 302 insertions(+), 44 deletions(-)