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

[Binbin Wu]

On 11/21/2023 7:00 PM, Isaku Yamahata wrote:
> 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.
But I don't find it may block during freeing memory.
Did I miss anything?


> > > +
> > > +		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