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From: Ben Gardon <bgardon@google.com>
To: Paolo Bonzini <pbonzini@redhat.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 23/28] KVM: x86/mmu: Allow parallel page faults for the TDP MMU
Date: Wed, 3 Feb 2021 09:46:30 -0800	[thread overview]
Message-ID: <CANgfPd-ELyPrn5z0N+o8R6Ci=O25XF+EDU-HDGgvVXGV7uF-dQ@mail.gmail.com> (raw)
In-Reply-To: <d2c4ae90-1e60-23ed-4bda-24cf88db04c9@redhat.com>

On Wed, Feb 3, 2021 at 4:40 AM Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> On 02/02/21 19:57, Ben Gardon wrote:
> >
> > -     write_lock(&vcpu->kvm->mmu_lock);
> > +
> > +     if (is_tdp_mmu_root(vcpu->kvm, vcpu->arch.mmu->root_hpa))
> > +             read_lock(&vcpu->kvm->mmu_lock);
> > +     else
> > +             write_lock(&vcpu->kvm->mmu_lock);
> > +
>
> I'd like to make this into two helper functions, but I'm not sure about
> the naming:
>
> - kvm_mmu_read_lock_for_root/kvm_mmu_read_unlock_for_root: not precise
> because it's really write-locked for shadow MMU roots
>
> - kvm_mmu_lock_for_root/kvm_mmu_unlock_for_root: not clear that TDP MMU
> operations will need to operate in shared-lock mode
>
> I prefer the first because at least it's the conservative option, but
> I'm open to other opinions and suggestions.
>
> Paolo
>

Of the above two options, I like the second one, though I'd be happy
with either. I agree the first is more conservative, in that it's
clear the MMU lock could be shared. It feels a little misleading,
though to have read in the name of the function but then acquire the
write lock, especially since there's code below that which expects the
write lock. I don't know of a good way to abstract this into a helper
without some comments to make it clear what's going on, but maybe
there's a slightly more open-coded compromise:
if (!kvm_mmu_read_lock_for_root(vcpu->kvm, vcpu->arch.mmu->root_hpa))
         write_lock(&vcpu->kvm->mmu_lock);
or
enum kvm_mmu_lock_mode lock_mode =
get_mmu_lock_mode_for_root(vcpu->kvm, vcpu->arch.mmu->root_hpa);
....
kvm_mmu_lock_for_mode(lock_mode);

Not sure if either of those are actually clearer, but the latter
trends in the direction the RCF took, having an enum to capture
read/write and whether or not yo yield in a lock mode parameter.

  reply	other threads:[~2021-02-03 17:47 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
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 [this message]
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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