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Date:   Thu, 19 Jan 2023 21:36:32 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     "Huang, Kai" <kai.huang@intel.com>
Cc:     "pbonzini@redhat.com" <pbonzini@redhat.com>,
        "isaku.yamahata@gmail.com" <isaku.yamahata@gmail.com>,
        "Shahar, Sagi" <sagis@google.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Aktas, Erdem" <erdemaktas@google.com>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "dmatlack@google.com" <dmatlack@google.com>,
        "zhi.wang.linux@gmail.com" <zhi.wang.linux@gmail.com>,
        "sean.j.christopherson@intel.com" <sean.j.christopherson@intel.com>,
        "Yamahata, Isaku" <isaku.yamahata@intel.com>
Subject: Re: [PATCH v11 018/113] KVM: TDX: create/destroy VM structure
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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

In other words, when mmu_lock is held for read, KVM relies on atomic SPTE updates.
With the mirror=>s-ept scheme, updates are no longer atomic and everything falls
apart.

Gracefully retrying only papers over the visible failures, the really problematic
scenarios are where multiple updates race and _don't_ trigger conflicts in the TDX
module.

> We previously used spinlock around the SEAMCALLs to avoid, but looks that is
> not preferred.

That doesn't address the race above either.  And even if it did, serializing all
S-EPT SEAMCALLs for a VM is not an option, at least not in the long term.

The least invasive idea I have is expand the TDP MMU's concept of "frozen" SPTEs
and freeze (a.k.a. lock) the SPTE (KVM's mirror) until the corresponding S-EPT
update completes.

The other idea is to scrap the mirror concept entirely, though I gotta imagine
that would provide pretty awful performance.

diff --git a/arch/x86/kvm/mmu/spte.h b/arch/x86/kvm/mmu/spte.h
index 0d8deefee66c..bcb398e71475 100644
--- a/arch/x86/kvm/mmu/spte.h
+++ b/arch/x86/kvm/mmu/spte.h
@@ -198,9 +198,9 @@ extern u64 __read_mostly shadow_nonpresent_or_rsvd_mask;
 /* Removed SPTEs must not be misconstrued as shadow present PTEs. */
 static_assert(!(REMOVED_SPTE & SPTE_MMU_PRESENT_MASK));
 
-static inline bool is_removed_spte(u64 spte)
+static inline bool is_frozen_spte(u64 spte)
 {
-       return spte == REMOVED_SPTE;
+       return spte == REMOVED_SPTE || spte & FROZEN_SPTE;
 }
 
 /* Get an SPTE's index into its parent's page table (and the spt array). */
diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index bba33aea0fb0..7f34eccadf98 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -651,6 +651,9 @@ static inline int tdp_mmu_set_spte_atomic(struct kvm *kvm,
 
        lockdep_assert_held_read(&kvm->mmu_lock);
 
+       if (<is TDX> && new_spte != REMOVED_SPTE)
+               new_spte |= FROZEN_SPTE;
+
        /*
         * Note, fast_pf_fix_direct_spte() can also modify TDP MMU SPTEs and
         * does not hold the mmu_lock.
@@ -662,6 +665,9 @@ static inline int tdp_mmu_set_spte_atomic(struct kvm *kvm,
                              new_spte, iter->level, true);
        handle_changed_spte_acc_track(iter->old_spte, new_spte, iter->level);
 
+       if (<is TDX> && new_spte != REMOVED_SPTE)
+               __kvm_tdp_mmu_write_spte(iter->sptep, new_spte);
+
        return 0;
 }