Re: [PATCH v6 11/16] KVM: x86/tdp_mmu: Split the large page when zap leaf

[Isaku Yamahata]

On Tue, Nov 21, 2023 at 05:57:28PM +0800,
Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx> wrote:

> > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> > index 7873e9ee82ad..a209a67decae 100644
> > --- a/arch/x86/kvm/mmu/tdp_mmu.c
> > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> > @@ -964,6 +964,14 @@ bool kvm_tdp_mmu_zap_sp(struct kvm *kvm, struct kvm_mmu_page *sp)
> >   	return true;
> >   }
> > +
> > +static struct kvm_mmu_page *tdp_mmu_alloc_sp_for_split(struct kvm *kvm,
> > +						       struct tdp_iter *iter,
> > +						       bool shared);
> > +
> > +static int tdp_mmu_split_huge_page(struct kvm *kvm, struct tdp_iter *iter,
> > +				   struct kvm_mmu_page *sp, bool shared);
> > +
> >   /*
> >    * If can_yield is true, will release the MMU lock and reschedule if the
> >    * scheduler needs the CPU or there is contention on the MMU lock. If this
> > @@ -975,13 +983,15 @@ static bool tdp_mmu_zap_leafs(struct kvm *kvm, struct kvm_mmu_page *root,
> >   			      gfn_t start, gfn_t end, bool can_yield, bool flush,
> >   			      bool zap_private)
> >   {
> > +	bool is_private = is_private_sp(root);
> > +	struct kvm_mmu_page *split_sp = NULL;
> >   	struct tdp_iter iter;
> >   	end = min(end, tdp_mmu_max_gfn_exclusive());
> >   	lockdep_assert_held_write(&kvm->mmu_lock);
> > -	WARN_ON_ONCE(zap_private && !is_private_sp(root));
> > +	WARN_ON_ONCE(zap_private && !is_private);
> >   	if (!zap_private && is_private_sp(root))
> Can use is_private instead of is_private_sp(root) here as well.

I'll update it.

> 
> >   		return false;
> > @@ -1006,12 +1016,66 @@ static bool tdp_mmu_zap_leafs(struct kvm *kvm, struct kvm_mmu_page *root,
> >   		    !is_last_spte(iter.old_spte, iter.level))
> >   			continue;
> > +		if (is_private && kvm_gfn_shared_mask(kvm) &&
> > +		    is_large_pte(iter.old_spte)) {
> > +			gfn_t gfn = iter.gfn & ~kvm_gfn_shared_mask(kvm);
> > +			gfn_t mask = KVM_PAGES_PER_HPAGE(iter.level) - 1;
> > +			struct kvm_memory_slot *slot;
> > +			struct kvm_mmu_page *sp;
> > +
> > +			slot = gfn_to_memslot(kvm, gfn);
> > +			if (kvm_hugepage_test_mixed(slot, gfn, iter.level) ||
> > +			    (gfn & mask) < start ||
> > +			    end < (gfn & mask) + KVM_PAGES_PER_HPAGE(iter.level)) {
> > +				WARN_ON_ONCE(!can_yield);
> > +				if (split_sp) {
> > +					sp = split_sp;
> > +					split_sp = NULL;
> > +					sp->role = tdp_iter_child_role(&iter);
> > +				} else {
> > +					WARN_ON(iter.yielded);
> > +					if (flush && can_yield) {
> > +						kvm_flush_remote_tlbs(kvm);
> > +						flush = false;
> > +					}
> Is it necessary to do the flush here?

Because tdp_mmu_alloc_sp_for_split() may unlock mmu_lock and block.
While blocking, other thread operates on KVM MMU and gets confused due to
remaining TLB cache.


> > +					sp = tdp_mmu_alloc_sp_for_split(kvm, &iter, false);
> > +					if (iter.yielded) {
> > +						split_sp = sp;
> > +						continue;
> > +					}
> > +				}
> > +				KVM_BUG_ON(!sp, kvm);
> > +
> > +				tdp_mmu_init_sp(sp, iter.sptep, iter.gfn);
> > +				if (tdp_mmu_split_huge_page(kvm, &iter, sp, false)) {
> > +					kvm_flush_remote_tlbs(kvm);
> > +					flush = false;
> Why it needs to flush TLB immediately if tdp_mmu_split_huge_page() fails?

Hmm, we don't need it.  When breaking up page table, we need to tlb flush
before issuing TDH.MEM.PAGE.DEMOTE(), not after it.  Will remove those two lines.


> Also, when KVM MMU write lock is held, it seems tdp_mmu_split_huge_page()
> will not fail.

This can happen with TDX_OPERAND_BUSY with secure-ept tree lock with other
vcpus TDH.VP.ENTER(). TDH.VP.ENTER() can take exclusive lock of secure-EPT.


> But let's assume this condition can be triggered, since sp is
> local
> variable, it will lost its value after continue, and split_sp is also NULL,
> it will try to allocate a new sp, memory leakage here?

Nice catch. I'll add split_sp = sp;


> > +					/* force retry on this gfn. */
> > +					iter.yielded = true;
> > +				} else
> > +					flush = true;
> > +				continue;
> > +			}
> > +		}
> > +
> >   		tdp_mmu_iter_set_spte(kvm, &iter, SHADOW_NONPRESENT_VALUE);
> >   		flush = true;
> >   	}
> >   	rcu_read_unlock();
> > +	if (split_sp) {
> > +		WARN_ON(!can_yield);
> > +		if (flush) {
> > +			kvm_flush_remote_tlbs(kvm);
> > +			flush = false;
> > +		}
> Same here, why we need to do the flush here?
> Can we delay it till the caller do the flush?

No. Because we unlock mmu_lock and may block when freeing memory.


> > +
> > +		write_unlock(&kvm->mmu_lock);
> > +		tdp_mmu_free_sp(split_sp);
> > +		write_lock(&kvm->mmu_lock);
> > +	}
> > +
> >   	/*
> >   	 * Because this flow zaps _only_ leaf SPTEs, the caller doesn't need
> >   	 * to provide RCU protection as no 'struct kvm_mmu_page' will be freed.
> > @@ -1606,8 +1670,6 @@ static struct kvm_mmu_page *tdp_mmu_alloc_sp_for_split(struct kvm *kvm,
> >   	KVM_BUG_ON(kvm_mmu_page_role_is_private(role) !=
> >   		   is_private_sptep(iter->sptep), kvm);
> > -	/* TODO: Large page isn't supported for private SPTE yet. */
> > -	KVM_BUG_ON(kvm_mmu_page_role_is_private(role), kvm);
> >   	/*
> >   	 * Since we are allocating while under the MMU lock we have to be
> 
> 

-- 
Isaku Yamahata <isaku.yamahata@xxxxxxxxxxxxxxx>