Re: [PATCH v10 052/108] KVM: x86/tdp_mmu: Ignore unsupported mmu operation on private GFNs

[Isaku Yamahata]

On Mon, Dec 05, 2022 at 02:23:35PM +0000,
"Wang, Wei W" <wei.w.wang@xxxxxxxxx> wrote:

> On Sunday, October 30, 2022 2:23 PM, Yamahata, Isaku wrote:
> > From: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> > 
> > Some KVM MMU operations (dirty page logging, page migration, aging page)
> > aren't supported for private GFNs (yet) with the first generation of TDX.
> > Silently return on unsupported TDX KVM MMU operations.
> > 
> > Signed-off-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> > ---
> >  arch/x86/kvm/mmu/mmu.c     |  3 ++
> >  arch/x86/kvm/mmu/tdp_mmu.c | 73
> > +++++++++++++++++++++++++++++++++++---
> >  arch/x86/kvm/x86.c         |  3 ++
> >  3 files changed, 74 insertions(+), 5 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index
> > 02e7b5cf3231..efc3b3f2dd12 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -6588,6 +6588,9 @@ static bool kvm_mmu_zap_collapsible_spte(struct
> > kvm *kvm,
> >  	for_each_rmap_spte(rmap_head, &iter, sptep) {
> >  		sp = sptep_to_sp(sptep);
> > 
> > +		/* Private page dirty logging is not supported yet. */
> > +		KVM_BUG_ON(is_private_sptep(sptep), kvm);
> > +
> >  		/*
> >  		 * We cannot do huge page mapping for indirect shadow pages,
> >  		 * which are found on the last rmap (level = 1) when not using diff --git
> > a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c index
> > 0e053b96444a..4b207ce83ffe 100644
> > --- a/arch/x86/kvm/mmu/tdp_mmu.c
> > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> > @@ -1469,7 +1469,8 @@ typedef bool (*tdp_handler_t)(struct kvm *kvm,
> > struct tdp_iter *iter,
> > 
> >  static __always_inline bool kvm_tdp_mmu_handle_gfn(struct kvm *kvm,
> >  						   struct kvm_gfn_range *range,
> > -						   tdp_handler_t handler)
> > +						   tdp_handler_t handler,
> > +						   bool only_shared)
> >  {
> >  	struct kvm_mmu_page *root;
> >  	struct tdp_iter iter;
> > @@ -1480,9 +1481,23 @@ static __always_inline bool
> > kvm_tdp_mmu_handle_gfn(struct kvm *kvm,
> >  	 * into this helper allow blocking; it'd be dead, wasteful code.
> >  	 */
> >  	for_each_tdp_mmu_root(kvm, root, range->slot->as_id) {
> > +		gfn_t start;
> > +		gfn_t end;
> > +
> > +		if (only_shared && is_private_sp(root))
> > +			continue;
> > +
> >  		rcu_read_lock();
> > 
> > -		tdp_root_for_each_leaf_pte(iter, root, range->start, range->end)
> > +		/*
> > +		 * For TDX shared mapping, set GFN shared bit to the range,
> > +		 * so the handler() doesn't need to set it, to avoid duplicated
> > +		 * code in multiple handler()s.
> > +		 */
> > +		start = kvm_gfn_for_root(kvm, root, range->start);
> > +		end = kvm_gfn_for_root(kvm, root, range->end);
> > +
> > +		tdp_root_for_each_leaf_pte(iter, root, start, end)
> >  			ret |= handler(kvm, &iter, range);
> > 
> >  		rcu_read_unlock();
> > @@ -1526,7 +1541,12 @@ static bool age_gfn_range(struct kvm *kvm, struct
> > tdp_iter *iter,
> > 
> >  bool kvm_tdp_mmu_age_gfn_range(struct kvm *kvm, struct kvm_gfn_range
> > *range)  {
> > -	return kvm_tdp_mmu_handle_gfn(kvm, range, age_gfn_range);
> > +	/*
> > +	 * First TDX generation doesn't support clearing A bit for private
> > +	 * mapping, since there's no secure EPT API to support it.  However
> > +	 * it's a legitimate request for TDX guest.
> > +	 */
> > +	return kvm_tdp_mmu_handle_gfn(kvm, range, age_gfn_range, true);
> >  }
> > 
> >  static bool test_age_gfn(struct kvm *kvm, struct tdp_iter *iter, @@ -1537,7
> > +1557,8 @@ static bool test_age_gfn(struct kvm *kvm, struct tdp_iter *iter,
> > 
> >  bool kvm_tdp_mmu_test_age_gfn(struct kvm *kvm, struct kvm_gfn_range
> > *range)  {
> > -	return kvm_tdp_mmu_handle_gfn(kvm, range, test_age_gfn);
> > +	/* The first TDX generation doesn't support A bit. */
> > +	return kvm_tdp_mmu_handle_gfn(kvm, range, test_age_gfn, true);
> >  }
> > 
> >  static bool set_spte_gfn(struct kvm *kvm, struct tdp_iter *iter, @@ -1582,8
> > +1603,11 @@ bool kvm_tdp_mmu_set_spte_gfn(struct kvm *kvm, struct
> > kvm_gfn_range *range)
> >  	 * No need to handle the remote TLB flush under RCU protection, the
> >  	 * target SPTE _must_ be a leaf SPTE, i.e. cannot result in freeing a
> >  	 * shadow page.  See the WARN on pfn_changed in
> > __handle_changed_spte().
> > +	 *
> > +	 * .change_pte() callback should not happen for private page, because
> > +	 * for now TDX private pages are pinned during VM's life time.
> >  	 */
> > -	return kvm_tdp_mmu_handle_gfn(kvm, range, set_spte_gfn);
> > +	return kvm_tdp_mmu_handle_gfn(kvm, range, set_spte_gfn, true);
> >  }
> > 
> >  /*
> > @@ -1637,6 +1661,14 @@ bool kvm_tdp_mmu_wrprot_slot(struct kvm *kvm,
> > 
> >  	lockdep_assert_held_read(&kvm->mmu_lock);
> > 
> > +	/*
> > +	 * Because first TDX generation doesn't support write protecting private
> > +	 * mappings and kvm_arch_dirty_log_supported(kvm) = false, it's a bug
> > +	 * to reach here for guest TD.
> > +	 */
> > +	if (WARN_ON_ONCE(!kvm_arch_dirty_log_supported(kvm)))
> > +		return false;
> > +
> >  	for_each_valid_tdp_mmu_root_yield_safe(kvm, root, slot->as_id, true)
> >  		spte_set |= wrprot_gfn_range(kvm, root, slot->base_gfn,
> >  			     slot->base_gfn + slot->npages, min_level); @@ -1902,6
> > +1934,14 @@ bool kvm_tdp_mmu_clear_dirty_slot(struct kvm *kvm,
> > 
> >  	lockdep_assert_held_read(&kvm->mmu_lock);
> > 
> > +	/*
> > +	 * First TDX generation doesn't support clearing dirty bit,
> > +	 * since there's no secure EPT API to support it.  It is a
> > +	 * bug to reach here for TDX guest.
> > +	 */
> > +	if (WARN_ON_ONCE(!kvm_arch_dirty_log_supported(kvm)))
> > +		return false;
> > +
> 
> It might not be a good choice to intercept everywhere in kvm_mmu just as tdx
> doesn't support it. I'm thinking maybe we could do the check in tdx.c, which is
> much simpler. For example:
> 
> @@ -2592,6 +2605,12 @@ static void tdx_handle_changed_private_spte(struct kvm *kvm,
>         lockdep_assert_held(&kvm->mmu_lock);
> 
>         if (change->new.is_present) {
> +               /* Only flags change. This isn't supported currently. */
> +               KVM_BUG_ON(change->old.is_present, kvm);
> 
> Then we can have kvm_arch_dirty_log_supported completely removed.

Do you mean WARN_ON_ONCE()? If so, they can be removed from this patches because
the code should be blocked by "if (!kvm_arch_dirty_log_supported(kvm))" at the
caller.

-- 
Isaku Yamahata <isaku.yamahata@xxxxxxxxx>