KVM: x86/tdp_mmu: Propagate building mirror page tables

Integrate hooks for mirroring page table operations for cases where TDX
will set PTEs or link page tables.

Like other Coco technologies, TDX has the concept of private and shared
memory. For TDX the private and shared mappings are managed on separate
EPT roots. The private half is managed indirectly through calls into a
protected runtime environment called the TDX module, where the shared half
is managed within KVM in normal page tables.

Since calls into the TDX module are relatively slow, walking private page
tables by making calls into the TDX module would not be efficient. Because
of this, previous changes have taught the TDP MMU to keep a mirror root,
which is separate, unmapped TDP root that private operations can be
directed to. Currently this root is disconnected from any actual guest
mapping. Now add plumbing to propagate changes to the "external" page
tables being mirrored. Just create the x86_ops for now, leave plumbing the
operations into the TDX module for future patches.

Add two operations for setting up external page tables, one for linking
new page tables and one for setting leaf PTEs. Don't add any op for
configuring the root PFN, as TDX handles this itself. Don't provide a
way to set permissions on the PTEs also, as TDX doesn't support it.

This results in MMU "mirroring" support that is very targeted towards TDX.
Since it is likely there will be no other user, the main benefit of making
the support generic is to keep TDX specific *looking* code outside of the
MMU. As a generic feature it will make enough sense from TDX's
perspective. For developers unfamiliar with TDX arch it can express the
general concepts such that they can continue to work in the code.

TDX MMU support will exclude certain MMU operations, so only plug in the
mirroring x86 ops where they will be needed. For setting/linking, only
hook tdp_mmu_set_spte_atomic() which is used for mapping and linking
PTs. Don't bother hooking tdp_mmu_iter_set_spte() as it is only used for
setting PTEs in operations unsupported by TDX: splitting huge pages and
write protecting. Sprinkle KVM_BUG_ON()s to document as code that these
paths are not supported for mirrored page tables. For zapping operations,
leave those for near future changes.

Many operations in the TDP MMU depend on atomicity of the PTE update.
While the mirror PTE on KVM's side can be updated atomically, the update
that happens inside the external operations (S-EPT updates via TDX module
call) can't happen atomically with the mirror update. The following race
could result during two vCPU's populating private memory:

* vcpu 1: atomically update 2M level mirror EPT entry to be present
* vcpu 2: read 2M level EPT entry that is present
* vcpu 2: walk down into 4K level EPT
* vcpu 2: atomically update 4K level mirror EPT entry to be present
* vcpu 2: set_exterma;_spte() to update 4K secure EPT entry => error
          because 2M secure EPT entry is not populated yet
* vcpu 1: link_external_spt() to update 2M secure EPT entry

Prevent this by setting the mirror PTE to FROZEN_SPTE while the reflect
operations are performed. Only write the actual mirror PTE value once the
reflect operations have completed. When trying to set a PTE to present and
encountering a frozen SPTE, retry the fault.

By doing this the race is prevented as follows:
* vcpu 1: atomically update 2M level EPT entry to be FROZEN_SPTE
* vcpu 2: read 2M level EPT entry that is FROZEN_SPTE
* vcpu 2: find that the EPT entry is frozen
          abandon page table walk to resume guest execution
* vcpu 1: link_external_spt() to update 2M secure EPT entry
* vcpu 1: atomically update 2M level EPT entry to be present (unfreeze)
* vcpu 2: resume guest execution
          Depending on vcpu 1 state, vcpu 2 may result in EPT violation
          again or make progress on guest execution

Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
Co-developed-by: Kai Huang <kai.huang@intel.com>
Signed-off-by: Kai Huang <kai.huang@intel.com>
Co-developed-by: Yan Zhao <yan.y.zhao@intel.com>
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Co-developed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Message-ID: <20240718211230.1492011-15-rick.p.edgecombe@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

0 files changed, 0 insertions, 0 deletions