Re: [PATCH 06/14] KVM: arm64: Return next table from map callbacks

David Matlack
2022-09-07T21:32:42Z

On Tue, Aug 30, 2022 at 07:41:24PM +0000, Oliver Upton wrote:
> The map walkers install new page tables during their traversal. Return
> the newly-installed table PTE from the map callbacks to point the walker
> at the new table w/o rereading the ptep.
>
> Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
> ---
> arch/arm64/kvm/hyp/pgtable.c | 9 +++++----
> 1 file changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> index 331f6e3b2c20..f911509e6512 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -202,13 +202,12 @@ static inline int __kvm_pgtable_visit(struct kvm_pgtable_walk_data *data,
> if (!table && (flags & KVM_PGTABLE_WALK_LEAF)) {
> ret = kvm_pgtable_visitor_cb(data, addr, level, ptep, &pte,
> KVM_PGTABLE_WALK_LEAF);
> - pte = *ptep;
> - table = kvm_pte_table(pte, level);
> }
>
> if (ret)
> goto out;

Rather than passing a pointer to the local variable pte and requiring
all downstream code to update it (and deal with dereferencing to read
the old pte), wouldn't it be simpler to just re-read the PTE here? e.g.

/*
* Explicitly re-read the PTE since it may have been modified
* during the TABLE_PRE or LEAF callback.
*/
pte = kvm_pte_read(ptep);

This should also result in better behavior once parallelization is
introduced, because it will prevent the walker from traversing down and
doing a bunch of work on page tables that are in the process of being
freed by some other thread.

>
> + table = kvm_pte_table(pte, level);
> if (!table) {

nit: Technically there's no reason to set @table again. e.g. This could
just be:

if (!kvm_pte_table(pte, level)) {

> data->addr = ALIGN_DOWN(data->addr, kvm_granule_size(level));
> data->addr += kvm_granule_size(level);
> @@ -427,6 +426,7 @@ static int hyp_map_walker(u64 addr, u64 end, u32 level, kvm_pte_t *ptep, kvm_pte
> new = kvm_init_table_pte(childp, mm_ops);
> mm_ops->get_page(ptep);
> smp_store_release(ptep, new);
> + *old = new;
>
> return 0;
> }
> @@ -768,7 +768,7 @@ static int stage2_map_walker_try_leaf(u64 addr, u64 end, u32 level,
> }
>
> static int stage2_map_walk_leaf(u64 addr, u64 end, u32 level, kvm_pte_t *ptep,
> - struct stage2_map_data *data);
> + kvm_pte_t *old, struct stage2_map_data *data);
>
> static int stage2_map_walk_table_pre(u64 addr, u64 end, u32 level,
> kvm_pte_t *ptep, kvm_pte_t *old,
> @@ -791,7 +791,7 @@ static int stage2_map_walk_table_pre(u64 addr, u64 end, u32 level,
> */
> kvm_call_hyp(__kvm_tlb_flush_vmid, data->mmu);
>
> - ret = stage2_map_walk_leaf(addr, end, level, ptep, data);
> + ret = stage2_map_walk_leaf(addr, end, level, ptep, old, data);
>
> mm_ops->put_page(ptep);
> mm_ops->free_removed_table(childp, level + 1, pgt);
> @@ -832,6 +832,7 @@ static int stage2_map_walk_leaf(u64 addr, u64 end, u32 level, kvm_pte_t *ptep,
> new = kvm_init_table_pte(childp, mm_ops);
> mm_ops->get_page(ptep);
> smp_store_release(ptep, new);
> + *old = new;
>
> return 0;
> }
> --
> 2.37.2.672.g94769d06f0-goog
>