linux-kernel - Re: [PATCH 1/2] KVM: x86/mmu: Don't rely on page-track mechanism to flush on memslot change

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Message-ID: <Y20wDoCz90jhxrU6@google.com>
Date:   Thu, 10 Nov 2022 17:08:30 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Yan Zhao <yan.y.zhao@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] KVM: x86/mmu: Don't rely on page-track mechanism to
 flush on memslot change

On Thu, Nov 10, 2022, Yan Zhao wrote:
> On Thu, Nov 10, 2022 at 01:48:20AM +0000, Sean Christopherson wrote:
> > Call kvm_mmu_zap_all_fast() directly when flushing a memslot instead of
> > bounding through the page-track mechanism.  KVM (unfortunately) needs to
> > zap and flush all page tables on memslot DELETE/MOVE irrespective of
> > whether KVM is shadowing guest page tables.
> > 
> > This will allow changing KVM to register a page-track notifier on the
> > first shadow root allocation, and will also allow deleting the misguided
> > kvm_page_track_flush_slot() hook itself once KVM-GT also moves to a
> > different method for reacting to memslot changes.
> >
> <...>
> > @@ -6021,7 +6014,6 @@ int kvm_mmu_init_vm(struct kvm *kvm)
> >  		return r;
> >  
> >  	node->track_write = kvm_mmu_pte_write;
> > -	node->track_flush_slot = kvm_mmu_invalidate_zap_pages_in_memslot;
> >  	kvm_page_track_register_notifier(kvm, node);
> >  
> >  	kvm->arch.split_page_header_cache.kmem_cache = mmu_page_header_cache;
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index e46e458c5b08..5da86fe3c113 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -12550,6 +12550,8 @@ void kvm_arch_flush_shadow_all(struct kvm *kvm)
> >  void kvm_arch_flush_shadow_memslot(struct kvm *kvm,
> >  				   struct kvm_memory_slot *slot)
> >  {
> > +	kvm_mmu_zap_all_fast(kvm);
> > +
> >  	kvm_page_track_flush_slot(kvm, slot);
> Could we move this kvm_page_track_flush_slot() to right before
> kvm_commit_memory_region()?

More or less.  The page-track stuff is x86-specific, just move it into x86's
kvm_arch_commit_memory_region().

> As KVM now does not need track_flush_slot any more and kvmgt is the only user
> to track_flush_slot, we can rename it to track_slot_changed to notify
> the new/deleted/moved slot.
> Do you think it's good?

Given that KVM/KVM-GT have never propery supported the MOVE case, and (IIUC) that
there's no danger to the kernel if KVM-GT fails to write-protect a moved memslot,
I would say just change the hook to ->remove_memslot().  I.e. even if the memslot
is being moved, simply notify KVM-GT that the old memslot is being removed.

E.g.

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 5a2821ca03b8..437e3832e377 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -12566,6 +12566,9 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
                                const struct kvm_memory_slot *new,
                                enum kvm_mr_change change)
 {
+       if (change == KVM_MR_DELETE || change == KVM_MR_MOVE)
+               kvm_page_track_remove_memslot(kvm, old);
+
        if (!kvm->arch.n_requested_mmu_pages &&
            (change == KVM_MR_CREATE || change == KVM_MR_DELETE)) {
                unsigned long nr_mmu_pages;