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.
