Re: [PATCH v11 018/113] KVM: TDX: create/destroy VM structure

[Huang, Kai]

On Thu, 2023-01-26 at 21:59 +0000, Sean Christopherson wrote:
> On Thu, Jan 26, 2023, Huang, Kai wrote:
> > On Thu, 2023-01-26 at 17:28 +0000, Sean Christopherson wrote:
> > > In other words, once the PTE is zapped/blocked (branch is pruned), it's completely
> > > removed from the paging tree and no other tasks can access the branch (page table
> > > and its children).  I.e. the only remaining reference to the branch is the pointer
> > > handed to the RCU callback.  That means the RCU callback has exclusive access to the
> > > branch, i.e. can operate as if it were holding mmu_lock for write.  Furthermore, the
> > > RCU callback also doesn't need to flush TLBs because that was again done when
> > > pruning the branch.
> > > 
> > > It's the same idea that KVM already uses for root SPs, the only difference is how
> > > KVM determines that there is exactly one entity that holds a reference to the SP.
> > 
> > Right.  This works fine for normal non-TDX case.  However for TDX unfortunately
> > the access to the removed branch (or the removed sub-page-table) isn't that
> > "exclusive" as the SEAMCALL to truly zap that branch still needs to hold the
> > write lock of the entire Secure EPT tree, so it can still conflict with other
> > threads handling new faults.
> 
> I thought TDX was smart enough to read-lock only the part of the tree that it's
> actually consuming, and write-lock only the part of the tree that it's actually
> modifying?

The spec says there's only exclusive/shared access to the "whole Secure EPT
tree":

8.6. Secure EPT Concurrency

Secure EPT concurrency rules are designed to support the expected usage and yet
be as simple as possible.

Host-Side (SEAMCALL) Functions:
• Functions that manage Secure EPT acquire exclusive access to the whole Secure
EPT tree of the target TD.
• In specific cases where a Secure EPT entry update may collide with a
concurrent update done by the guest TD, such host-side functions update the
Secure EPT entry as a transaction, using atomic compare and exchange operations.
• TDH.MEM.SEPT.RD acquire shared access to the whole Secure EPT tree of the
target TD to help prevent changes to the tree while they execute.
• Other functions that only read Secure EPT for GPA-to-HPA translation (e.g.,
TDH.MR.EXTEND) acquire shared access to the whole Secure EPT tree of the target
TD to help prevent changes to the tree while they execute.

> 
> Hrm, but even if TDX takes a read-lock, there's still the problem of it needing
> to walk the upper levels, i.e. KVM needs to keep mid-level page tables reachable
> until they're fully removed.  Blech.  That should be a non-issue at this time
> though, as I don't think KVM will ever REMOVE a page table of a live guest.  I
> need to look at the PROMOTE/DEMOTE flows...

In this series, if I read correctly, when slot is removed/moved, private
mappings are zapped too.  It's kinda buried in:

[PATCH v11 043/113] KVM: x86/tdp_mmu: Don't zap private pages for unsupported
cases

(it's not easy to find -- I had to use 'git blame' in the actual repo to find
the commit, sigh.)