[RFC PATCH 00/12] KVM: x86/mmu: TDX post-populate cleanups

[Sean Christopherson <seanjc@google.com>]

This is a largely untested series to do most of what was discussed in the
thread regarding locking issues between gmem and TDX's post-populate hook[*],
with more than a few side quests thrown in as I was navigating through the
code to try to figure out how best to eliminate the copy_from_user() from
sev_gmem_post_populate(), which has the same locking problem (copying from
a userspace address can fault and in theory trigger the same problematic
path, I think).

Notably absent is the extraction of copy_from_user() from
sev_gmem_post_populate() to kvm_gmem_populate().  I've had this on my todo
list for a few weeks now, and haven't been able to focus on it for long
enough to get something hammered out, and with KVM Forum on the horizon, I
don't anticipate getting 'round to it within the next month (if not much
longer).

The thing that stymied me is what to do if snp_launch_update() is passed in
a huge batch of pages.  I waffled between doing a slow one-at-a-time approach
and a batched approached, and got especially stuck when trying to remember
and/or figure out how that handling would interact with hugepage support in
SNP in particular.

If anyone wants to tackle that project, the one thing change I definitely
think we should do is change the post_populate() callback to operate on
exactly one page.  KVM_SEV_SNP_LAUNCH_UPDATE allows for partial progress,
i.e. KVM's ABI doesn't require it to unwind a batch if adding a page fails.
If we take advantage of that, then sev_gmem_post_populate() will be a bit
simpler (though I wouldn't go so far as to call it "simple").

RFC as this is compile tested only (mostly due to lack of access to a TDX
capable system, but also due to lack of cycles).

[*] http://lore.kernel.org/all/aG_pLUlHdYIZ2luh@google.com

Sean Christopherson (12):
  KVM: TDX: Drop PROVE_MMU=y sanity check on to-be-populated mappings
  KVM: x86/mmu: Add dedicated API to map guest_memfd pfn into TDP MMU
  Revert "KVM: x86/tdp_mmu: Add a helper function to walk down the TDP
    MMU"
  KVM: x86/mmu: Rename kvm_tdp_map_page() to kvm_tdp_prefault_page()
  KVM: TDX: Drop superfluous page pinning in S-EPT management
  KVM: TDX: Return -EIO, not -EINVAL, on a KVM_BUG_ON() condition
  KVM: TDX: Avoid a double-KVM_BUG_ON() in tdx_sept_zap_private_spte()
  KVM: TDX: Use atomic64_dec_return() instead of a poor equivalent
  KVM: TDX: Fold tdx_mem_page_record_premap_cnt() into its sole caller
  KVM: TDX: Assert that slots_lock is held when nr_premapped is accessed
  KVM: TDX: Track nr_premapped as an "unsigned long", not an
    "atomic64_t"
  KVM: TDX: Rename nr_premapped to nr_pending_tdh_mem_page_adds

 arch/x86/kvm/mmu.h         |   3 +-
 arch/x86/kvm/mmu/mmu.c     |  66 ++++++++++++++++++--
 arch/x86/kvm/mmu/tdp_mmu.c |  37 ++---------
 arch/x86/kvm/vmx/tdx.c     | 123 +++++++++++++------------------------
 arch/x86/kvm/vmx/tdx.h     |   9 ++-
 5 files changed, 117 insertions(+), 121 deletions(-)


base-commit: 196d9e72c4b0bd68b74a4ec7f52d248f37d0f030
-- 
2.51.0.268.g9569e192d0-goog