Re: [PATCH 00/24] KVM: x86/mmu: Introduce MMU_PRESENT and fix bugs

[Paolo Bonzini]

On 25/02/21 21:47, Sean Christopherson wrote:
> This series adds the simple idea of tagging shadow-present SPTEs with
> a single bit, instead of looking for non-zero SPTEs that aren't MMIO and
> aren't REMOVED.  Doing so reduces KVM's code footprint by 2k bytes on
> x86-64, and presumably adds a tiny performance boost in related paths.
>
> But, actually adding MMU_PRESENT without breaking one flow or another is
> a bit of a debacle.  The main issue is that EPT doesn't have many low
> available bits, and PAE doesn't have any high available bits.  And, the
> existing MMU_WRITABLE and HOST_WRITABLE flags aren't optional, i.e. are
> needed for all flavors of paging.  The solution I settled on is to let
> make the *_WRITABLE bit configurable so that EPT can use high available
> bits.
>
> Of course, I forgot the above PAE restriction multiple times, and
> journeyed down several dead ends.  The most notable failed idea was
> using the AD_* masks in bits 52 and 53 to denote shadow-present SPTEs.
> That would have been quite clever as it would provide the same benefits
> without burning another available bit.
>
> Along the many failed attempts, I collected a variety of bug fixes and
> cleanups, mostly things found by inspection after doing a deep dive to
> figure out what I broke.
>
> Sean Christopherson (24):
>    KVM: x86/mmu: Set SPTE_AD_WRPROT_ONLY_MASK if and only if PML is
>      enabled
>    KVM: x86/mmu: Check for shadow-present SPTE before querying A/D status
>    KVM: x86/mmu: Bail from fast_page_fault() if SPTE is not
>      shadow-present
>    KVM: x86/mmu: Disable MMIO caching if MMIO value collides with L1TF
>    KVM: x86/mmu: Retry page faults that hit an invalid memslot
>    KVM: x86/mmu: Don't install bogus MMIO SPTEs if MMIO caching is
>      disabled
>    KVM: x86/mmu: Handle MMIO SPTEs directly in mmu_set_spte()
>    KVM: x86/mmu: Drop redundant trace_kvm_mmu_set_spte() in the TDP MMU
>    KVM: x86/mmu: Rename 'mask' to 'spte' in MMIO SPTE helpers
>    KVM: x86/mmu: Stop using software available bits to denote MMIO SPTEs
>    KVM: x86/mmu: Add module param to disable MMIO caching (for testing)
>    KVM: x86/mmu: Rename and document A/D scheme for TDP SPTEs
>    KVM: x86/mmu: Use MMIO SPTE bits 53 and 52 for the MMIO generation
>    KVM: x86/mmu: Document dependency bewteen TDP A/D type and saved bits
>    KVM: x86/mmu: Move initial kvm_mmu_set_mask_ptes() call into MMU
>      proper
>    KVM: x86/mmu: Co-locate code for setting various SPTE masks
>    KVM: x86/mmu: Move logic for setting SPTE masks for EPT into the MMU
>      proper
>    KVM: x86/mmu: Make Host-writable and MMU-writable bit locations
>      dynamic
>    KVM: x86/mmu: Use high bits for host/mmu writable masks for EPT SPTEs
>    KVM: x86/mmu: Use a dedicated bit to track shadow/MMU-present SPTEs
>    KVM: x86/mmu: Tweak auditing WARN for A/D bits to !PRESENT (was MMIO)
>    KVM: x86/mmu: Use is_removed_spte() instead of open coded equivalents
>    KVM: x86/mmu: Use low available bits for removed SPTEs
>    KVM: x86/mmu: Dump reserved bits if they're detected on non-MMIO SPTE
>
>   Documentation/virt/kvm/locking.rst |  49 +++++----
>   arch/x86/include/asm/kvm_host.h    |   3 -
>   arch/x86/kvm/mmu.h                 |  15 +--
>   arch/x86/kvm/mmu/mmu.c             |  87 +++++++---------
>   arch/x86/kvm/mmu/mmu_internal.h    |  16 +--
>   arch/x86/kvm/mmu/paging_tmpl.h     |   2 +-
>   arch/x86/kvm/mmu/spte.c            | 157 ++++++++++++++++++++---------
>   arch/x86/kvm/mmu/spte.h            | 135 +++++++++++++++++--------
>   arch/x86/kvm/mmu/tdp_mmu.c         |  22 ++--
>   arch/x86/kvm/svm/svm.c             |   2 +-
>   arch/x86/kvm/vmx/vmx.c             |  24 +----
>   arch/x86/kvm/x86.c                 |   3 -
>   12 files changed, 290 insertions(+), 225 deletions(-)

Queued (patch 1 for 5.12, the rest for 5.13).

Paolo