linux-kernel - Re: [PATCH v2 1/3] KVM: x86: add a new page track hook track_remove

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Date:   Tue, 15 Nov 2022 07:22:14 +0800
From:   Yan Zhao <yan.y.zhao@...el.com>
To:     Sean Christopherson <seanjc@...gle.com>
CC:     <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <pbonzini@...hat.com>, <intel-gfx@...ts.freedesktop.org>,
        <intel-gvt-dev@...ts.freedesktop.org>, <zhenyuw@...ux.intel.com>
Subject: Re: [PATCH v2 1/3] KVM: x86: add a new page track hook
 track_remove_slot

On Mon, Nov 14, 2022 at 11:24:16PM +0000, Sean Christopherson wrote:
> On Tue, Nov 15, 2022, Yan Zhao wrote:
> > On Mon, Nov 14, 2022 at 04:32:34PM +0000, Sean Christopherson wrote:
> > > On Mon, Nov 14, 2022, Yan Zhao wrote:
> > > > On Sat, Nov 12, 2022 at 12:43:07AM +0000, Sean Christopherson wrote:
> > > > > On Sat, Nov 12, 2022, Yan Zhao wrote:
> > > > > > And I'm also not sure if a slots_arch_lock is required for
> > > > > > kvm_slot_page_track_add_page() and kvm_slot_page_track_remove_page().
> > > > > 
> > > > > It's not required.  slots_arch_lock protects interaction between memslot updates
> > > > In kvm_slot_page_track_add_page() and kvm_slot_page_track_remove_page(),
> > > > slot->arch.gfn_track[mode][index] is updated in update_gfn_track(),
> > > > do you know which lock is used to protect it?
> > > 
> > > mmu_lock protects the count, kvm->srcu protects the slot, and shadow_root_allocated
> > > protects that validity of gfn_track, i.e. shadow_root_allocated ensures that KVM
> > > allocates gfn_track for all memslots when shadow paging is activated.
> > Hmm, thanks for the reply.
> > but in direct_page_fault(),
> > if (page_fault_handle_page_track(vcpu, fault))
> > 	return RET_PF_EMULATE;
> > 
> > slot->arch.gfn_track is read without any mmu_lock is held.
> 
> That's a fast path that deliberately reads out of mmu_lock.  A false positive
> only results in unnecessary emulation, and any false positive is inherently prone
> to races anyways, e.g. fault racing with zap.
what about false negative?
If the fast path read 0 count, no page track write callback will be called and write
protection will be removed in the slow path.


Thanks
Yan
> 
> > > arch/x86/kvm/mmu/page_track.c-void __kvm_write_track_remove_gfn(struct kvm *kvm,
> > > arch/x86/kvm/mmu/page_track.c-                            struct kvm_memory_slot *slot, gfn_t gfn)
> > > arch/x86/kvm/mmu/page_track.c-{
> > > arch/x86/kvm/mmu/page_track.c-  lockdep_assert_held_write(&kvm->mmu_lock);
> > > arch/x86/kvm/mmu/page_track.c-
> > > arch/x86/kvm/mmu/page_track.c-  if (KVM_BUG_ON(!kvm_page_track_write_tracking_enabled(kvm), kvm))
> > > arch/x86/kvm/mmu/page_track.c-          return;
> > > arch/x86/kvm/mmu/page_track.c-
> > > arch/x86/kvm/mmu/page_track.c:  update_gfn_write_track(slot, gfn, -1);
> > yes, it will be helpful.
> > 
> > Besides, will WRITE_ONCE or atomic_add in update_gfn_write_track() to
> > update slot->arch.gfn_track be better?
> 
> WRITE_ONCE() won't suffice, it needs to be atomic.  Switching to atomic_inc/dec
> isn't worth it so long as KVM's shadow MMU takes mmu_lock for write, i.e. while
> the accounting is mutually exclusive for other reasons in both KVM and KVMGT.