Re: [PATCH v1 2/9] KVM: x86/mmu: Add support for prewrite page tracking

[Sean Christopherson <seanjc@google.com>]

On Fri, May 05, 2023, Micka�l Sala�n wrote:
> diff --git a/arch/x86/include/asm/kvm_page_track.h b/arch/x86/include/asm/kvm_page_track.h
> index eb186bc57f6a..a7fb4ff888e6 100644
> --- a/arch/x86/include/asm/kvm_page_track.h
> +++ b/arch/x86/include/asm/kvm_page_track.h
> @@ -3,6 +3,7 @@
>  #define _ASM_X86_KVM_PAGE_TRACK_H
>  
>  enum kvm_page_track_mode {
> +	KVM_PAGE_TRACK_PREWRITE,

Heh, just when I decide to finally kill off support for multiple modes[1] :-)

My assessment from that changelog still holds true for this case:

  Drop "support" for multiple page-track modes, as there is no evidence
  that array-based and refcounted metadata is the optimal solution for
  other modes, nor is there any evidence that other use cases, e.g. for
  access-tracking, will be a good fit for the page-track machinery in
  general.
  
  E.g. one potential use case of access-tracking would be to prevent guest
  access to poisoned memory (from the guest's perspective).  In that case,
  the number of poisoned pages is likely to be a very small percentage of
  the guest memory, and there is no need to reference count the number of
  access-tracking users, i.e. expanding gfn_track[] for a new mode would be
  grossly inefficient.  And for poisoned memory, host userspace would also
  likely want to trap accesses, e.g. to inject #MC into the guest, and that
  isn't currently supported by the page-track framework.
  
  A better alternative for that poisoned page use case is likely a
  variation of the proposed per-gfn attributes overlay (linked), which
  would allow efficiently tracking the sparse set of poisoned pages, and by
  default would exit to userspace on access.

Of particular relevance:

  - Using the page-track machinery is inefficient because the guest is likely
    going to write-protect a minority of its memory.  And this

      select KVM_EXTERNAL_WRITE_TRACKING if KVM

    is particularly nasty because simply enabling HEKI in the Kconfig will cause
    KVM to allocate rmaps and gfn tracking.

  - There's no need to reference count the protection, i.e. 15 of the 16 bits of
    gfn_track are dead weight.

  - As proposed, adding a second "mode" would double the cost of gfn tracking.

  - Tying the protections to the memslots will create an impossible-to-maintain
    ABI because the protections will be lost if the owning memslot is deleted and
    recreated.

  - The page-track framework provides incomplete protection and will lead to an
    ongoing game of whack-a-mole, e.g. this patch catches the obvious cases by
    adding calls to kvm_page_track_prewrite(), but misses things like kvm_vcpu_map().

  - The scaling and maintenance issues will only get worse if/when someone tries
    to support dropping read and/or execute permissions, e.g. for execute-only.

  - The code is x86-only, and is likely to stay that way for the foreseeable
    future.

The proposed alternative is to piggyback the memory attributes implementation[2]
that is being added (if all goes according to plan) for confidential VMs.  This
use case (dropping permissions) came up not too long ago[3], which is why I have
a ready-made answer).

I have no doubt that we'll need to solve performance and scaling issues with the
memory attributes implementation, e.g. to utilize xarray multi-range support
instead of storing information on a per-4KiB-page basis, but AFAICT, the core
idea is sound.  And a very big positive from a maintenance perspective is that
any optimizations, fixes, etc. for one use case (CoCo vs. hardening) should also
benefit the other use case.

[1] https://lore.kernel.org/all/20230311002258.852397-22-seanjc@google.com
[2] https://lore.kernel.org/all/Y2WB48kD0J4VGynX@google.com
[3] https://lore.kernel.org/all/Y1a1i9vbJ%2FpVmV9r@google.com