Re: [PATCH 09/21] KVM: TDX: Retry seamcall when TDX_OPERAND_BUSY with operand SEPT

[Yan Zhao <yan.y.zhao@intel.com>]

On Sat, Sep 14, 2024 at 05:27:32PM +0800, Yan Zhao wrote:
> > Similarly, can tdh_mem_page_aug() actually contend with tdg_mem_page_accept()?
> > The page isn't yet mapped, so why would the guest be allowed to take a lock on
> > the S-EPT entry?
> Before tdg_mem_page_accept() accepts a gpa and set rwx bits in a SPTE, if second
> tdh_mem_page_aug() is called on the same gpa, the second one may contend with
> tdg_mem_page_accept().
> 
> But given KVM does not allow the second tdh_mem_page_aug(), looks the contention
> between tdh_mem_page_aug() and tdg_mem_page_accept() will not happen.
I withdraw the reply above.

tdh_mem_page_aug() and tdg_mem_page_accept() both attempt to modify the same
SEPT entry, leading to contention.
- tdg_mem_page_accept() first walks the SEPT tree with no lock to get the SEPT
  entry. It then acquire the guest side lock of the found SEPT entry before
  checking entry state.
- tdh_mem_page_aug() first walks the SEPT tree with shared lock to locate the
  SEPT entry to modify, it then aquires host side lock of the SEPT entry before
  checking entry state.