On Thu, 2023-01-19 at 21:36 +0000, Sean Christopherson wrote: > On Thu, Jan 19, 2023, Huang, Kai wrote: > > On Thu, 2023-01-19 at 15:37 +0000, Sean Christopherson wrote: > > > On Thu, Jan 19, 2023, Huang, Kai wrote: > > > > On Tue, 2023-01-17 at 21:01 +0000, Sean Christopherson wrote: > > > > > On Tue, Jan 17, 2023, Sean Christopherson wrote: > > > > > > On Tue, Jan 17, 2023, Zhi Wang wrote: > > > > > Oh, the other important piece I forgot to mention is that dropping mmu_lock deep > > > > > in KVM's MMU in order to wait isn't always an option. Most flows would play nice > > > > > with dropping mmu_lock and sleeping, but some paths, e.g. from the mmu_notifier, > > > > > (conditionally) disallow sleeping. > > > > > > > > Could we do something similar to tdp_mmu_iter_cond_resched() but not simple busy > > > > retrying "X times", at least at those paths that can release mmu_lock()? > > > > > > That's effectively what happens by unwinding up the stak with an error code. > > > Eventually the page fault handler will get the error and retry the guest. > > > > > > > Basically we treat TDX_OPERAND_BUSY as seamcall_needbreak(), similar to > > > > rwlock_needbreak(). I haven't thought about details though. > > > > > > I am strongly opposed to that approach. I do not want to pollute KVM's MMU code > > > with a bunch of retry logic and error handling just because the TDX module is > > > ultra paranoid and hostile to hypervisors. > > > > Right. But IIUC there's legal cases that SEPT SEAMCALL can return BUSY due to > > multiple threads trying to read/modify SEPT simultaneously in case of TDP MMU. > > For instance, parallel page faults on different vcpus on private pages. I > > believe this is the main reason to retry. > > Um, crud. I think there's a bigger issue. KVM always operates on its copy of the > S-EPT tables and assumes the the real S-EPT tables will always be synchronized with > KVM's mirror. That assumption doesn't hold true without serializing SEAMCALLs in > some way. E.g. if a SPTE is zapped and mapped at the same time, we can end up with: > > vCPU0 vCPU1 > ===== ===== > mirror[x] = xyz > old_spte = mirror[x] > mirror[x] = REMOVED_SPTE > sept[x] = REMOVED_SPTE > sept[x] = xyz IIUC this case cannot happen, as the two steps in the vcpu0 are within read lock, which prevents from vcpu1, which holds the write lock during zapping SPTE.
