From: Paolo Bonzini <pbonzini@redhat.com>
To: Ben Gardon <bgardon@google.com>
Cc: LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
Peter Xu <peterx@redhat.com>,
Sean Christopherson <seanjc@google.com>,
Peter Shier <pshier@google.com>,
Peter Feiner <pfeiner@google.com>,
Junaid Shahid <junaids@google.com>,
Jim Mattson <jmattson@google.com>,
Yulei Zhang <yulei.kernel@gmail.com>,
Wanpeng Li <kernellwp@gmail.com>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
Xiao Guangrong <xiaoguangrong.eric@gmail.com>
Subject: Re: [PATCH v2 20/28] KVM: x86/mmu: Use atomic ops to set SPTEs in TDP MMU map
Date: Thu, 1 Apr 2021 19:32:07 +0200 [thread overview]
Message-ID: <e94bc2f3-b948-0176-0253-b487bf2aa787@redhat.com> (raw)
In-Reply-To: <CANgfPd85U_YwDdXc1Dkn-UzKpae5FRzYshLFABAU_xHTs0i3Hg@mail.gmail.com>
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.
next prev parent reply other threads:[~2021-04-01 17:45 UTC|newest]
Thread overview: 65+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-02 18:57 [PATCH v2 00/28] Allow parallel MMU operations with TDP MMU Ben Gardon
2021-02-02 18:57 ` [PATCH v2 01/28] KVM: x86/mmu: change TDP MMU yield function returns to match cond_resched Ben Gardon
2021-02-02 18:57 ` [PATCH v2 02/28] KVM: x86/mmu: Add comment on __tdp_mmu_set_spte Ben Gardon
2021-02-02 18:57 ` [PATCH v2 03/28] KVM: x86/mmu: Add lockdep when setting a TDP MMU SPTE Ben Gardon
2021-02-02 18:57 ` [PATCH v2 04/28] KVM: x86/mmu: Don't redundantly clear TDP MMU pt memory Ben Gardon
2021-02-02 18:57 ` [PATCH v2 05/28] KVM: x86/mmu: Factor out handling of removed page tables Ben Gardon
2021-02-02 18:57 ` [PATCH v2 06/28] locking/rwlocks: Add contention detection for rwlocks Ben Gardon
2021-02-09 20:39 ` Guenter Roeck
2021-02-09 21:46 ` Waiman Long
2021-02-09 22:25 ` Guenter Roeck
2021-02-10 0:27 ` Waiman Long
2021-02-10 0:41 ` Waiman Long
2021-02-10 6:04 ` Guenter Roeck
2021-02-10 14:57 ` Waiman Long
2021-02-10 3:32 ` Waiman Long
2021-02-10 15:15 ` Waiman Long
2021-02-02 18:57 ` [PATCH v2 07/28] sched: Add needbreak " Ben Gardon
2021-02-02 18:57 ` [PATCH v2 08/28] sched: Add cond_resched_rwlock Ben Gardon
2021-02-02 18:57 ` [PATCH v2 09/28] KVM: x86/mmu: Fix braces in kvm_recover_nx_lpages Ben Gardon
2021-02-02 18:57 ` [PATCH v2 10/28] KVM: x86/mmu: Fix TDP MMU zap collapsible SPTEs Ben Gardon
2021-02-03 9:43 ` Paolo Bonzini
2021-02-02 18:57 ` [PATCH v2 11/28] KVM: x86/mmu: Merge flush and non-flush tdp_mmu_iter_cond_resched Ben Gardon
2021-02-02 18:57 ` [PATCH v2 12/28] KVM: x86/mmu: Rename goal_gfn to next_last_level_gfn Ben Gardon
2021-02-02 18:57 ` [PATCH v2 13/28] KVM: x86/mmu: Ensure forward progress when yielding in TDP MMU iter Ben Gardon
2021-02-05 23:42 ` Sean Christopherson
2021-02-02 18:57 ` [PATCH v2 14/28] KVM: x86/mmu: Yield in TDU MMU iter even if no SPTES changed Ben Gardon
2021-02-02 18:57 ` [PATCH v2 15/28] KVM: x86/mmu: Skip no-op changes in TDP MMU functions Ben Gardon
2021-02-02 18:57 ` [PATCH v2 16/28] KVM: x86/mmu: Clear dirtied pages mask bit before early break Ben Gardon
2021-02-02 18:57 ` [PATCH v2 17/28] KVM: x86/mmu: Protect TDP MMU page table memory with RCU Ben Gardon
2021-02-02 18:57 ` [PATCH v2 18/28] KVM: x86/mmu: Use an rwlock for the x86 MMU Ben Gardon
2021-02-02 18:57 ` [PATCH v2 19/28] KVM: x86/mmu: Factor out functions to add/remove TDP MMU pages Ben Gardon
2021-02-02 18:57 ` [PATCH v2 20/28] KVM: x86/mmu: Use atomic ops to set SPTEs in TDP MMU map Ben Gardon
2021-02-03 2:48 ` kernel test robot
2021-02-03 11:14 ` Paolo Bonzini
2021-02-06 0:26 ` Sean Christopherson
2021-02-08 10:32 ` Paolo Bonzini
2021-04-01 10:32 ` Paolo Bonzini
2021-04-01 16:50 ` Ben Gardon
2021-04-01 17:32 ` Paolo Bonzini [this message]
2021-04-01 18:09 ` Sean Christopherson
2021-02-02 18:57 ` [PATCH v2 21/28] KVM: x86/mmu: Flush TLBs after zap in TDP MMU PF handler Ben Gardon
2021-02-06 0:29 ` Sean Christopherson
2021-02-02 18:57 ` [PATCH v2 22/28] KVM: x86/mmu: Mark SPTEs in disconnected pages as removed Ben Gardon
2021-02-03 11:17 ` Paolo Bonzini
2021-02-02 18:57 ` [PATCH v2 23/28] KVM: x86/mmu: Allow parallel page faults for the TDP MMU Ben Gardon
2021-02-03 12:39 ` Paolo Bonzini
2021-02-03 17:46 ` Ben Gardon
2021-02-03 18:30 ` Paolo Bonzini
2021-02-06 0:12 ` Sean Christopherson
2021-02-02 18:57 ` [PATCH v2 24/28] KVM: x86/mmu: Allow zap gfn range to operate under the mmu read lock Ben Gardon
2021-02-03 11:25 ` Paolo Bonzini
2021-02-03 11:26 ` Paolo Bonzini
2021-02-03 18:31 ` Ben Gardon
2021-02-03 18:32 ` Paolo Bonzini
2021-02-02 18:57 ` [PATCH v2 25/28] KVM: x86/mmu: Allow zapping collapsible SPTEs to use MMU " Ben Gardon
2021-02-03 11:34 ` Paolo Bonzini
2021-02-03 18:51 ` Ben Gardon
2021-02-02 18:57 ` [PATCH v2 26/28] KVM: x86/mmu: Allow enabling / disabling dirty logging under " Ben Gardon
2021-02-03 11:38 ` Paolo Bonzini
2021-02-02 18:57 ` [PATCH v2 27/28] KVM: selftests: Add backing src parameter to dirty_log_perf_test Ben Gardon
2021-02-02 18:57 ` [PATCH v2 28/28] KVM: selftests: Disable dirty logging with vCPUs running Ben Gardon
2021-02-03 10:07 ` Paolo Bonzini
2021-02-03 11:00 ` [PATCH v2 00/28] Allow parallel MMU operations with TDP MMU Paolo Bonzini
2021-02-03 17:54 ` Sean Christopherson
2021-02-03 18:13 ` Paolo Bonzini
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