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

Marc Zyngier
2020-12-01T14:06:41Z

On 2020-12-01 13:46, Will Deacon wrote:
> 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);

That's a big hammer, and we so far have been pretty careful not to
over-invalidate. Is the block-replacing-table *without* an unmap
in between the only case where this triggers?

Thanks,

M.
--
Jazz is not dead. It just smells funny...