linux-kernel - Re: [PATCH v2 20/28] KVM: x86/mmu: Use atomic ops to set SPTEs in TDP MMU map

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Date:   Thu, 1 Apr 2021 19:32:07 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Ben Gardon <bgardon@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Peter Xu <peterx@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Peter Shier <pshier@...gle.com>,
        Peter Feiner <pfeiner@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>
Subject: Re: [PATCH v2 20/28] KVM: x86/mmu: Use atomic ops to set SPTEs in TDP
 MMU map

On 01/04/21 18:50, Ben Gardon wrote:
>> retry:
>>                   if (is_shadow_present_pte(iter.old_spte)) {
>>                          if (is_large_pte(iter.old_spte)) {
>>                                  if (!tdp_mmu_zap_spte_atomic(vcpu->kvm, &iter))
>>                                          break;
>>
>>                                  /*
>>                                   * The iter must explicitly re-read the SPTE because
>>                                   * the atomic cmpxchg failed.
>>                                   */
>>                                  iter.old_spte = READ_ONCE(*rcu_dereference(iter.sptep));
>>                                  goto retry;
>>                          }
>>                   } else {
>>                          ...
>>                  }
>>
>> ?
> To be honest, that feels less readable to me. For me retry implies
> that we failed to make progress and need to repeat an operation, but
> the reality is that we did make progress and there are just multiple
> steps to replace the large SPTE with a child PT.

You're right, it's makes no sense---I misremembered the direction of
tdp_mmu_zap_spte_atomic's return value.  I was actually thinking of this:

> Another option which could improve readability and performance would
> be to use the retry to repeat failed cmpxchgs instead of breaking out
> of the loop. Then we could avoid retrying the page fault each time a
> cmpxchg failed, which may happen a lot as vCPUs allocate intermediate
> page tables on boot. (Probably less common for leaf entries, but
> possibly useful there too.)

which would be

retry:
                  if (is_shadow_present_pte(iter.old_spte)) {
                        if (is_large_pte(iter.old_spte) &&
                            !tdp_mmu_zap_spte_atomic(vcpu->kvm, &iter)) {
                                 /*
                                  * The iter must explicitly re-read the SPTE because
                                  * the atomic cmpxchg failed.
                                  */
                                 iter.old_spte = READ_ONCE(*rcu_dereference(iter.sptep));
                                 goto retry;
                             }
                             /* XXX move this to tdp_mmu_zap_spte_atomic? */
                             iter.old_spte = 0;
                        } else {
                             continue;
                        }
                  }
                  sp = alloc_tdp_mmu_page(vcpu, iter.gfn, iter.level);
                  child_pt = sp->spt;

                  new_spte = make_nonleaf_spte(child_pt,
                                               !shadow_accessed_mask);

                  if (!tdp_mmu_set_spte_atomic(vcpu->kvm, &iter,
                                              new_spte)) {
                       tdp_mmu_free_sp(sp);
                       /*
                        * The iter must explicitly re-read the SPTE because
                        * the atomic cmpxchg failed.
                        */
                       iter.old_spte = READ_ONCE(*rcu_dereference(iter.sptep));
                       goto retry;
                  }
                  tdp_mmu_link_page(vcpu->kvm, sp, true,
                                    huge_page_disallowed &&
                                    req_level >= iter.level);

                  trace_kvm_mmu_get_page(sp, true);

which survives at least a quick smoke test of booting a 20-vCPU Windows
guest.  If you agree I'll turn this into an actual patch.