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Message-ID: <CANgfPd_oi7GMeMNCJccuesxofQCPa1WPtzK=OnJXiR_-VR5mbA@mail.gmail.com>
Date: Mon, 15 Mar 2021 10:13:12 -0700
From: Ben Gardon <bgardon@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
Paolo Bonzini <pbonzini@...hat.com>,
Peter Shier <pshier@...gle.com>,
Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH 2/4] KVM: x86/mmu: Fix RCU usage for tdp_iter_root_pt
On Fri, Mar 12, 2021 at 8:22 AM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Thu, Mar 11, 2021, Ben Gardon wrote:
> > The root page table in the TDP MMU paging structure is not protected
> > with RCU, but rather by the root_count in the associated SP. As a result
> > it is safe for tdp_iter_root_pt to simply return a u64 *. This sidesteps
> > the complexities assoicated with propagating the __rcu annotation
> > around.
> >
> > Reported-by: kernel test robot <lkp@...xxxxxx>
> > Signed-off-by: Ben Gardon <bgardon@...gle.com>
> > ---
> > arch/x86/kvm/mmu/tdp_iter.c | 10 ++++++++--
> > arch/x86/kvm/mmu/tdp_iter.h | 2 +-
> > arch/x86/kvm/mmu/tdp_mmu.c | 4 ++--
> > 3 files changed, 11 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/x86/kvm/mmu/tdp_iter.c b/arch/x86/kvm/mmu/tdp_iter.c
> > index e5f148106e20..8e2c053533b6 100644
> > --- a/arch/x86/kvm/mmu/tdp_iter.c
> > +++ b/arch/x86/kvm/mmu/tdp_iter.c
> > @@ -159,8 +159,14 @@ void tdp_iter_next(struct tdp_iter *iter)
> > iter->valid = false;
> > }
> >
> > -tdp_ptep_t tdp_iter_root_pt(struct tdp_iter *iter)
> > +u64 *tdp_iter_root_pt(struct tdp_iter *iter)
> > {
> > - return iter->pt_path[iter->root_level - 1];
> > + /*
> > + * Though it is stored in an array of tdp_ptep_t for convenience,
> > + * the root PT is not actually protected by RCU, but by the root
> > + * count on the associated struct kvm_mmu_page. As a result it's
> > + * safe to rcu_dereference and return the value here.
>
> I'm not a big fan of this comment. It implies that calling tdp_iter_root_pt()
> without RCU protection is completely ok, but that's not true, as rcu_dereferecne()
> will complain when CONFIG_PROVE_RCU=1.
>
> There's also a good opportunity to streamline the the helper here, since both
> callers use the root only to get to the associated shadow page, and that's only
> done to get the as_id. If we provide tdp_iter_as_id() then the need for a
> comment goes away and we shave a few lines of code.
This is a good suggestion. I have a change to do this in another
series I was preparing to send out, but your suggestion below is even
better, so I'll add that to this series.
>
> That being said, an even better option would be to store as_id in the TDP iter.
> The cost on the stack is negligible, and while the early sptep->as_id lookup
> will be unnecessary in some cases, it will be a net win when setting multiple
> sptes, e.g. in mmu_notifier callbacks.
>
> Compile tested only...
>
> From 02fb9cd2aa52d0afd318e93661d0212ccdb54218 Mon Sep 17 00:00:00 2001
> From: Sean Christopherson <seanjc@...gle.com>
> Date: Fri, 12 Mar 2021 08:12:21 -0800
> Subject: [PATCH] KVM: x86/mmu: Store the address space ID in the TDP iterator
>
> Store the address space ID in the TDP iterator so that it can be
> retrieved without having to bounce through the root shadow page. This
> streamlines the code and fixes a Sparse warning about not properly using
> rcu_dereference() when grabbing the ID from the root on the fly.
>
> Reported-by: kernel test robot <lkp@...el.com>
> Cc: Ben Gardon <bgardon@...gle.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
> arch/x86/kvm/mmu/mmu_internal.h | 5 +++++
> arch/x86/kvm/mmu/tdp_iter.c | 7 +------
> arch/x86/kvm/mmu/tdp_iter.h | 3 ++-
> arch/x86/kvm/mmu/tdp_mmu.c | 23 +++++------------------
> 4 files changed, 13 insertions(+), 25 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
> index ec4fc28b325a..e844078d2374 100644
> --- a/arch/x86/kvm/mmu/mmu_internal.h
> +++ b/arch/x86/kvm/mmu/mmu_internal.h
> @@ -119,6 +119,11 @@ static inline bool kvm_mmu_put_root(struct kvm *kvm, struct kvm_mmu_page *sp)
> return !sp->root_count;
> }
>
> +static inline int kvm_mmu_page_as_id(struct kvm_mmu_page *sp)
> +{
> + return sp->role.smm ? 1 : 0;
> +}
> +
> /*
> * Return values of handle_mmio_page_fault, mmu.page_fault, and fast_page_fault().
> *
> diff --git a/arch/x86/kvm/mmu/tdp_iter.c b/arch/x86/kvm/mmu/tdp_iter.c
> index e5f148106e20..55d0ce2185a5 100644
> --- a/arch/x86/kvm/mmu/tdp_iter.c
> +++ b/arch/x86/kvm/mmu/tdp_iter.c
> @@ -40,6 +40,7 @@ void tdp_iter_start(struct tdp_iter *iter, u64 *root_pt, int root_level,
> iter->gfn = round_gfn_for_level(iter->next_last_level_gfn, iter->level);
> tdp_iter_refresh_sptep(iter);
>
> + iter->as_id = kvm_mmu_page_as_id(sptep_to_sp(root_pt));
> iter->valid = true;
> }
>
> @@ -158,9 +159,3 @@ void tdp_iter_next(struct tdp_iter *iter)
> } while (try_step_up(iter));
> iter->valid = false;
> }
> -
> -tdp_ptep_t tdp_iter_root_pt(struct tdp_iter *iter)
> -{
> - return iter->pt_path[iter->root_level - 1];
> -}
> -
> diff --git a/arch/x86/kvm/mmu/tdp_iter.h b/arch/x86/kvm/mmu/tdp_iter.h
> index 4cc177d75c4a..df9c84713f5b 100644
> --- a/arch/x86/kvm/mmu/tdp_iter.h
> +++ b/arch/x86/kvm/mmu/tdp_iter.h
> @@ -36,6 +36,8 @@ struct tdp_iter {
> int min_level;
> /* The iterator's current level within the paging structure */
> int level;
> + /* The address space ID, i.e. SMM vs. regular. */
> + int as_id;
> /* A snapshot of the value at sptep */
> u64 old_spte;
> /*
> @@ -62,6 +64,5 @@ tdp_ptep_t spte_to_child_pt(u64 pte, int level);
> void tdp_iter_start(struct tdp_iter *iter, u64 *root_pt, int root_level,
> int min_level, gfn_t next_last_level_gfn);
> void tdp_iter_next(struct tdp_iter *iter);
> -tdp_ptep_t tdp_iter_root_pt(struct tdp_iter *iter);
>
> #endif /* __KVM_X86_MMU_TDP_ITER_H */
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 21cbbef0ee57..9f436aa14663 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -190,11 +190,6 @@ static void handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
> u64 old_spte, u64 new_spte, int level,
> bool shared);
>
> -static int kvm_mmu_page_as_id(struct kvm_mmu_page *sp)
> -{
> - return sp->role.smm ? 1 : 0;
> -}
> -
> static void handle_changed_spte_acc_track(u64 old_spte, u64 new_spte, int level)
> {
> if (!is_shadow_present_pte(old_spte) || !is_last_spte(old_spte, level))
> @@ -472,10 +467,6 @@ static inline bool tdp_mmu_set_spte_atomic(struct kvm *kvm,
> struct tdp_iter *iter,
> u64 new_spte)
> {
> - u64 *root_pt = tdp_iter_root_pt(iter);
> - struct kvm_mmu_page *root = sptep_to_sp(root_pt);
> - int as_id = kvm_mmu_page_as_id(root);
> -
> lockdep_assert_held_read(&kvm->mmu_lock);
>
> /*
> @@ -489,8 +480,8 @@ static inline bool tdp_mmu_set_spte_atomic(struct kvm *kvm,
> new_spte) != iter->old_spte)
> return false;
>
> - handle_changed_spte(kvm, as_id, iter->gfn, iter->old_spte, new_spte,
> - iter->level, true);
> + handle_changed_spte(kvm, iter->as_id, iter->gfn, iter->old_spte,
> + new_spte, iter->level, true);
>
> return true;
> }
> @@ -544,10 +535,6 @@ static inline void __tdp_mmu_set_spte(struct kvm *kvm, struct tdp_iter *iter,
> u64 new_spte, bool record_acc_track,
> bool record_dirty_log)
> {
> - tdp_ptep_t root_pt = tdp_iter_root_pt(iter);
> - struct kvm_mmu_page *root = sptep_to_sp(root_pt);
> - int as_id = kvm_mmu_page_as_id(root);
> -
> lockdep_assert_held_write(&kvm->mmu_lock);
>
> /*
> @@ -561,13 +548,13 @@ static inline void __tdp_mmu_set_spte(struct kvm *kvm, struct tdp_iter *iter,
>
> WRITE_ONCE(*rcu_dereference(iter->sptep), new_spte);
>
> - __handle_changed_spte(kvm, as_id, iter->gfn, iter->old_spte, new_spte,
> - iter->level, false);
> + __handle_changed_spte(kvm, iter->as_id, iter->gfn, iter->old_spte,
> + new_spte, iter->level, false);
> if (record_acc_track)
> handle_changed_spte_acc_track(iter->old_spte, new_spte,
> iter->level);
> if (record_dirty_log)
> - handle_changed_spte_dirty_log(kvm, as_id, iter->gfn,
> + handle_changed_spte_dirty_log(kvm, iter->as_id, iter->gfn,
> iter->old_spte, new_spte,
> iter->level);
> }
> --
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