Re: [RFC PATCH 2/3] KVM: arm64: Fix handling of merging tables into a block entry

[Will Deacon]

On Tue, Dec 01, 2020 at 10:30:41AM +0800, wangyanan (Y) wrote:
> On 2020/12/1 0:01, Will Deacon wrote:
> > On Mon, Nov 30, 2020 at 11:24:19PM +0800, wangyanan (Y) wrote:
> > > On 2020/11/30 21:34, Will Deacon wrote:
> > > > On Mon, Nov 30, 2020 at 08:18:46PM +0800, Yanan Wang wrote:
> > > > > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> > > > > index 696b6aa83faf..fec8dc9f2baa 100644
> > > > > --- a/arch/arm64/kvm/hyp/pgtable.c
> > > > > +++ b/arch/arm64/kvm/hyp/pgtable.c
> > > > > @@ -500,6 +500,9 @@ static int stage2_map_walk_table_pre(u64 addr, u64 end, u32 level,
> > > > >    	return 0;
> > > > >    }
> > > > > +static void stage2_flush_dcache(void *addr, u64 size);
> > > > > +static bool stage2_pte_cacheable(kvm_pte_t pte);
> > > > > +
> > > > >    static int stage2_map_walk_leaf(u64 addr, u64 end, u32 level, kvm_pte_t *ptep,
> > > > >    				struct stage2_map_data *data)
> > > > >    {
> > > > > @@ -507,9 +510,17 @@ static int stage2_map_walk_leaf(u64 addr, u64 end, u32 level, kvm_pte_t *ptep,
> > > > >    	struct page *page = virt_to_page(ptep);
> > > > >    	if (data->anchor) {
> > > > > -		if (kvm_pte_valid(pte))
> > > > > +		if (kvm_pte_valid(pte)) {
> > > > > +			kvm_set_invalid_pte(ptep);
> > > > > +			kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, data->mmu,
> > > > > +				     addr, level);
> > > > >    			put_page(page);
> > > > This doesn't make sense to me: the page-table pages we're walking when the
> > > > anchor is set are not accessible to the hardware walker because we unhooked
> > > > the entire sub-table in stage2_map_walk_table_pre(), which has the necessary
> > > > TLB invalidation.
> > > > 
> > > > Are you seeing a problem in practice here?
> > > Yes, I indeed find a problem in practice.
> > > 
> > > When the migration was cancelled, a TLB conflic abort  was found in guest.
> > > 
> > > This problem is fixed before rework of the page table code, you can have a
> > > look in the following two links:
> > > 
> > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=3c3736cd32bf5197aed1410ae826d2d254a5b277
> > > 
> > > https://lists.cs.columbia.edu/pipermail/kvmarm/2019-March/035031.html
> > Ok, let's go through this, because I still don't see the bug. Please correct
> > me if you spot any mistakes:
> > 
> >    1. We have a block mapping for X => Y
> >    2. Dirty logging is enabled, so the block mapping is write-protected and
> >       ends up being split into page mappings
> >    3. Dirty logging is disabled due to a failed migration.
> > 
> > --- At this point, I think we agree that the state of the MMU is alright ---
> > 
> >    4. We take a stage-2 fault and want to reinstall the block mapping:
> > 
> >       a. kvm_pgtable_stage2_map() is invoked to install the block mapping
> >       b. stage2_map_walk_table_pre() finds a table where we would like to
> >          install the block:
> > 
> > 	i.   The anchor is set to point at this entry
> > 	ii.  The entry is made invalid
> > 	iii. We invalidate the TLB for the input address. This is
> > 	     TLBI IPAS2SE1IS without level hint and then TLBI VMALLE1IS.
> > 
> > 	*** At this point, the page-table pointed to by the old table entry
> > 	    is not reachable to the hardware walker ***
> > 
> >       c. stage2_map_walk_leaf() is called for each leaf entry in the
> >          now-unreachable subtree, dropping page-references for each valid
> > 	entry it finds.
> >       d. stage2_map_walk_table_post() is eventually called for the entry
> >          which we cleared back in b.ii, so we install the new block mapping.
> > 
> > You are proposing to add additional TLB invalidation to (c), but I don't
> > think that is necessary, thanks to the invalidation already performed in
> > b.iii. What am I missing here?
> 
> The point is at b.iii where the TLBI is not enough. There are many page
> mappings that we need to merge into a block mapping.
> 
> We invalidate the TLB for the input address without level hint at b.iii, but
> this operation just flush TLB for one page mapping, there
> 
> are still some TLB entries for the other page mappings in the cache, the MMU
> hardware walker can still hit these entries next time.

Ah, yes, I see. Thanks. I hadn't considered the case where there are table
entries beneath the anchor. So how about the diff below?

Will

--->8

diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
index 0271b4a3b9fe..12526d8c7ae4 100644
--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -493,7 +493,7 @@ static int stage2_map_walk_table_pre(u64 addr, u64 end, u32 level,
 		return 0;
 
 	kvm_set_invalid_pte(ptep);
-	kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, data->mmu, addr, 0);
+	/* TLB invalidation is deferred until the _post handler */
 	data->anchor = ptep;
 	return 0;
 }
@@ -547,11 +547,21 @@ static int stage2_map_walk_table_post(u64 addr, u64 end, u32 level,
 				      struct stage2_map_data *data)
 {
 	int ret = 0;
+	kvm_pte_t pte = *ptep;
 
 	if (!data->anchor)
 		return 0;
 
-	free_page((unsigned long)kvm_pte_follow(*ptep));
+	kvm_set_invalid_pte(ptep);
+
+	/*
+	 * Invalidate the whole stage-2, as we may have numerous leaf
+	 * entries below us which would otherwise need invalidating
+	 * individually.
+	 */
+	kvm_call_hyp(__kvm_tlb_flush_vmid, data->mmu);
+
+	free_page((unsigned long)kvm_pte_follow(pte));
 	put_page(virt_to_page(ptep));
 
 	if (data->anchor == ptep) {