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

[Sean Christopherson <seanjc@google.com>]

On Wed, Sep 25, 2024, Yan Zhao wrote:
> On Sat, Sep 14, 2024 at 05:27:32PM +0800, Yan Zhao wrote:
> > On Fri, Sep 13, 2024 at 10:23:00AM -0700, Sean Christopherson wrote:
> > > On Fri, Sep 13, 2024, Yan Zhao wrote:
> > > > This is a lock status report of TDX module for current SEAMCALL retry issue
> > > > based on code in TDX module public repo https://github.com/intel/tdx-module.git
> > > > branch TDX_1.5.05.
> > > > 
> > > > TL;DR:
> > > > - tdh_mem_track() can contend with tdh_vp_enter().
> > > > - tdh_vp_enter() contends with tdh_mem*() when 0-stepping is suspected.
> > > 
> > > The zero-step logic seems to be the most problematic.  E.g. if KVM is trying to
> > > install a page on behalf of two vCPUs, and KVM resumes the guest if it encounters
> > > a FROZEN_SPTE when building the non-leaf SPTEs, then one of the vCPUs could
> > > trigger the zero-step mitigation if the vCPU that "wins" and gets delayed for
> > > whatever reason.
> > > 
> > > Since FROZEN_SPTE is essentially bit-spinlock with a reaaaaaly slow slow-path,
> > > what if instead of resuming the guest if a page fault hits FROZEN_SPTE, KVM retries
> > > the fault "locally", i.e. _without_ redoing tdh_vp_enter() to see if the vCPU still
> > > hits the fault?
> > > 
> > > For non-TDX, resuming the guest and letting the vCPU retry the instruction is
> > > desirable because in many cases, the winning task will install a valid mapping
> > > before KVM can re-run the vCPU, i.e. the fault will be fixed before the
> > > instruction is re-executed.  In the happy case, that provides optimal performance
> > > as KVM doesn't introduce any extra delay/latency.
> > > 
> > > But for TDX, the math is different as the cost of a re-hitting a fault is much,
> > > much higher, especially in light of the zero-step issues.
> > > 
> > > E.g. if the TDP MMU returns a unique error code for the frozen case, and
> > > kvm_mmu_page_fault() is modified to return the raw return code instead of '1',
> > > then the TDX EPT violation path can safely retry locally, similar to the do-while
> > > loop in kvm_tdp_map_page().
> > > 
> > > The only part I don't like about this idea is having two "retry" return values,
> > > which creates the potential for bugs due to checking one but not the other.
> > > 
> > > Hmm, that could be avoided by passing a bool pointer as an out-param to communicate
> > > to the TDX S-EPT fault handler that the SPTE is frozen.  I think I like that
> > > option better even though the out-param is a bit gross, because it makes it more
> > > obvious that the "frozen_spte" is a special case that doesn't need attention for
> > > most paths.
> > Good idea.
> > But could we extend it a bit more to allow TDX's EPT violation handler to also
> > retry directly when tdh_mem_sept_add()/tdh_mem_page_aug() returns BUSY?
> I'm asking this because merely avoiding invoking tdh_vp_enter() in vCPUs seeing
> FROZEN_SPTE might not be enough to prevent zero step mitigation.

The goal isn't to make it completely impossible for zero-step to fire, it's to
make it so that _if_ zero-step fires, KVM can report the error to userspace without
having to retry, because KVM _knows_ that advancing past the zero-step isn't
something KVM can solve.

 : I'm not worried about any performance hit with zero-step, I'm worried about KVM
 : not being able to differentiate between a KVM bug and guest interference.  The
 : goal with a local retry is to make it so that KVM _never_ triggers zero-step,
 : unless there is a bug somewhere.  At that point, if zero-step fires, KVM can
   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 : report the error to userspace instead of trying to suppress guest activity, and
 : potentially from other KVM tasks too.

In other words, for the selftest you crafted, KVM reporting an error to userspace
due to zero-step would be working as intended.  

> E.g. in below selftest with a TD configured with pending_ve_disable=N,
> zero step mitigation can be triggered on a vCPU that is stuck in EPT violation
> vm exit for more than 6 times (due to that user space does not do memslot
> conversion correctly).
> 
> So, if vCPU A wins the chance to call tdh_mem_page_aug(), the SEAMCALL may
> contend with zero step mitigation code in tdh_vp_enter() in vCPU B stuck
> in EPT violation vm exits.