Re: [PATCH] kvm: don't lose the higher 32 bits of tlbs_dirty

[Lai Jiangshan]

On Tue, Dec 15, 2020 at 1:20 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Sun, Dec 13, 2020, Lai Jiangshan wrote:
> > From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> >
> > In kvm_mmu_notifier_invalidate_range_start(), tlbs_dirty is used as:
> >       need_tlb_flush |= kvm->tlbs_dirty;
> > with need_tlb_flush's type being int and tlbs_dirty's type being long.
> >
> > It means that tlbs_dirty is always used as int and the higher 32 bits
> > is useless.
>
> It's probably worth noting in the changelog that it's _extremely_ unlikely this
> bug can cause problems in practice.  It would require encountering tlbs_dirty
> on a 4 billion count boundary, and KVM would need to be using shadow paging or
> be running a nested guest.

You are right, I don't consider it would cause problems in practice, and I
also tried to make tlbs_dirty as "int" and found that I have to change too
many places.

And you are right it is worth noting about it, I'm sorry for neglecting.

>
> > We can just change need_tlb_flush's type to long to
> > make full use of tlbs_dirty.
>
> Hrm, this does solve the problem, but I'm not a fan of continuing to use an
> integer variable as a boolean.  Rather than propagate tlbs_dirty to
> need_tlb_flush, what if this bug fix patch checks tlbs_dirty directly, and then
> a follow up patch converts need_tlb_flush to a bool and removes the unnecessary
> initialization (see below).
>
> E.g. the net result of both patches would be:
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 3abcb2ce5b7d..93b6986d3dfc 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -473,7 +473,8 @@ static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn,
>                                         const struct mmu_notifier_range *range)
>  {
>         struct kvm *kvm = mmu_notifier_to_kvm(mn);
> -       int need_tlb_flush = 0, idx;
> +       bool need_tlb_flush;
> +       int idx;
>
>         idx = srcu_read_lock(&kvm->srcu);
>         spin_lock(&kvm->mmu_lock);
> @@ -483,11 +484,10 @@ static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn,
>          * count is also read inside the mmu_lock critical section.
>          */
>         kvm->mmu_notifier_count++;
> -       need_tlb_flush = kvm_unmap_hva_range(kvm, range->start, range->end,
> -                                            range->flags);
> -       need_tlb_flush |= kvm->tlbs_dirty;
> +       need_tlb_flush = !!kvm_unmap_hva_range(kvm, range->start, range->end,
> +                                              range->flags);
>         /* we've to flush the tlb before the pages can be freed */
> -       if (need_tlb_flush)
> +       if (need_tlb_flush || kvm->tlbs_dirty)
>                 kvm_flush_remote_tlbs(kvm);
>
>         spin_unlock(&kvm->mmu_lock);
>
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: a4ee1ca4a36e ("KVM: MMU: delay flush all tlbs on sync_page path")

I searched back, found it and considered adding this fixes tag, but I was
afraid that this cleanup would be backported. Is it worth backporting such
patch which is extremely hardly a problem in practice?

>
> > Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> > ---
> >  virt/kvm/kvm_main.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 2541a17ff1c4..4e519a517e9f 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -470,7 +470,8 @@ static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn,
> >                                       const struct mmu_notifier_range *range)
> >  {
> >       struct kvm *kvm = mmu_notifier_to_kvm(mn);
> > -     int need_tlb_flush = 0, idx;
> > +     long need_tlb_flush = 0;
>
> need_tlb_flush doesn't need to be initialized here, it's explicitly set via the
> call to kvm_unmap_hva_range().
>
> > +     int idx;
> >
> >       idx = srcu_read_lock(&kvm->srcu);
> >       spin_lock(&kvm->mmu_lock);
> > --
> > 2.19.1.6.gb485710b
> >