Re: [PATCH v3 10/21] KVM: arm64: Add support for stage-2 page-aging in generic page-table

Gavin Shan
2020-09-03T04:34:54Z

Hi Will,

On 8/25/20 7:39 PM, Will Deacon wrote:
> Add stage-2 mkyoung(), mkold() and is_young() operations to the generic
> page-table code.
>
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: Quentin Perret <qperret@google.com>
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
> arch/arm64/include/asm/kvm_pgtable.h | 38 ++++++++++++
> arch/arm64/kvm/hyp/pgtable.c | 86 ++++++++++++++++++++++++++++
> 2 files changed, 124 insertions(+)
>

With the following one question resolved:

Reviewed-by: Gavin Shan <gshan@redhat.com>

> diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> index 8ab0d5f43817..ae56534f87a0 100644
> --- a/arch/arm64/include/asm/kvm_pgtable.h
> +++ b/arch/arm64/include/asm/kvm_pgtable.h
> @@ -173,6 +173,44 @@ int kvm_pgtable_stage2_map(struct kvm_pgtable *pgt, u64 addr, u64 size,
> */
> int kvm_pgtable_stage2_unmap(struct kvm_pgtable *pgt, u64 addr, u64 size);
>
> +/**
> + * kvm_pgtable_stage2_mkyoung() - Set the access flag in a page-table entry.
> + * @pgt: Page-table structure initialised by kvm_pgtable_stage2_init().
> + * @addr: Intermediate physical address to identify the page-table entry.
> + *
> + * If there is a valid, leaf page-table entry used to translate @addr, then
> + * set the access flag in that entry.
> + *
> + * Return: The old page-table entry prior to setting the flag, 0 on failure.
> + */
> +kvm_pte_t kvm_pgtable_stage2_mkyoung(struct kvm_pgtable *pgt, u64 addr);
> +
> +/**
> + * kvm_pgtable_stage2_mkold() - Clear the access flag in a page-table entry.
> + * @pgt: Page-table structure initialised by kvm_pgtable_stage2_init().
> + * @addr: Intermediate physical address to identify the page-table entry.
> + *
> + * If there is a valid, leaf page-table entry used to translate @addr, then
> + * clear the access flag in that entry.
> + *
> + * Note that it is the caller's responsibility to invalidate the TLB after
> + * calling this function to ensure that the updated permissions are visible
> + * to the CPUs.
> + *
> + * Return: The old page-table entry prior to clearing the flag, 0 on failure.
> + */
> +kvm_pte_t kvm_pgtable_stage2_mkold(struct kvm_pgtable *pgt, u64 addr);
> +
> +/**
> + * kvm_pgtable_stage2_is_young() - Test whether a page-table entry has the
> + * access flag set.
> + * @pgt: Page-table structure initialised by kvm_pgtable_stage2_init().
> + * @addr: Intermediate physical address to identify the page-table entry.
> + *
> + * Return: True if the page-table entry has the access flag set, false otherwise.
> + */
> +bool kvm_pgtable_stage2_is_young(struct kvm_pgtable *pgt, u64 addr);
> +
> /**
> * kvm_pgtable_walk() - Walk a page-table.
> * @pgt: Page-table structure initialised by kvm_pgtable_*_init().
> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> index 6f65d3841ec9..30713eb773e0 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -670,6 +670,92 @@ int kvm_pgtable_stage2_unmap(struct kvm_pgtable *pgt, u64 addr, u64 size)
> return kvm_pgtable_walk(pgt, addr, size, &walker);
> }
>
> +struct stage2_attr_data {
> + kvm_pte_t attr_set;
> + kvm_pte_t attr_clr;
> + kvm_pte_t pte;
> +};
> +
> +static int stage2_attr_walker(u64 addr, u64 end, u32 level, kvm_pte_t *ptep,
> + enum kvm_pgtable_walk_flags flag,
> + void * const arg)
> +{
> + kvm_pte_t pte = *ptep;
> + struct stage2_attr_data *data = arg;
> +
> + if (!kvm_pte_valid(pte))
> + return 0;
> +
> + data->pte = pte;
> + pte &= ~data->attr_clr;
> + pte |= data->attr_set;
> +
> + /*
> + * We may race with the CPU trying to set the access flag here,
> + * but worst-case the access flag update gets lost and will be
> + * set on the next access instead.
> + */
> + if (data->pte != pte)
> + WRITE_ONCE(*ptep, pte);
> +
> + return 0;
> +}
> +
> +static int stage2_update_leaf_attrs(struct kvm_pgtable *pgt, u64 addr,
> + u64 size, kvm_pte_t attr_set,
> + kvm_pte_t attr_clr, kvm_pte_t *orig_pte)
> +{
> + int ret;
> + kvm_pte_t attr_mask = KVM_PTE_LEAF_ATTR_LO | KVM_PTE_LEAF_ATTR_HI;
> + struct stage2_attr_data data = {
> + .attr_set = attr_set & attr_mask,
> + .attr_clr = attr_clr & attr_mask,
> + };
> + struct kvm_pgtable_walker walker = {
> + .cb = stage2_attr_walker,
> + .arg = &data,
> + .flags = KVM_PGTABLE_WALK_LEAF,
> + };
> +
> + ret = kvm_pgtable_walk(pgt, addr, size, &walker);
> + if (ret)
> + return ret;
> +
> + if (orig_pte)
> + *orig_pte = data.pte;
> + return 0;
> +}
> +

The @size is always 1 from the caller, which means the parameter
can be dropped from stage2_update_leaf_attrs(). In the meanwhile,
we don't know the page is mapped by PUD, PMD or PTE. So to have
fixed value ("1") looks meaningless.

> +kvm_pte_t kvm_pgtable_stage2_mkyoung(struct kvm_pgtable *pgt, u64 addr)
> +{
> + kvm_pte_t pte = 0;
> + stage2_update_leaf_attrs(pgt, addr, 1, KVM_PTE_LEAF_ATTR_LO_S2_AF, 0,
> + &pte);
> + dsb(ishst);
> + return pte;
> +}
> +
> +kvm_pte_t kvm_pgtable_stage2_mkold(struct kvm_pgtable *pgt, u64 addr)
> +{
> + kvm_pte_t pte = 0;
> + stage2_update_leaf_attrs(pgt, addr, 1, 0, KVM_PTE_LEAF_ATTR_LO_S2_AF,
> + &pte);
> + /*
> + * "But where's the TLBI?!", you scream.
> + * "Over in the core code", I sigh.
> + *
> + * See the '->clear_flush_young()' callback on the KVM mmu notifier.
> + */
> + return pte;
> +}
> +
> +bool kvm_pgtable_stage2_is_young(struct kvm_pgtable *pgt, u64 addr)
> +{
> + kvm_pte_t pte = 0;
> + stage2_update_leaf_attrs(pgt, addr, 1, 0, 0, &pte);
> + return pte & KVM_PTE_LEAF_ATTR_LO_S2_AF;
> +}
> +
> int kvm_pgtable_stage2_init(struct kvm_pgtable *pgt, struct kvm *kvm)
> {
> size_t pgd_sz;
>

Thanks,
Gavin