Re: [PATCH v7 12/12] KVM: arm64: Use TLBI range-based intructions for unmap

[Shaoqin Huang <shahuang@redhat.com>]

On 7/25/23 00:47, Raghavendra Rao Ananta wrote:
> On Mon, Jul 24, 2023 at 2:35 AM Shaoqin Huang <shahuang@redhat.com> wrote:
>>
>> Hi Raghavendra,
>>
>> On 7/22/23 10:22, Raghavendra Rao Ananta wrote:
>>> The current implementation of the stage-2 unmap walker traverses
>>> the given range and, as a part of break-before-make, performs
>>> TLB invalidations with a DSB for every PTE. A multitude of this
>>> combination could cause a performance bottleneck on some systems.
>>>
>>> Hence, if the system supports FEAT_TLBIRANGE, defer the TLB
>>> invalidations until the entire walk is finished, and then
>>> use range-based instructions to invalidate the TLBs in one go.
>>> Condition deferred TLB invalidation on the system supporting FWB,
>>> as the optimization is entirely pointless when the unmap walker
>>> needs to perform CMOs.
>>>
>>> Rename stage2_put_pte() to stage2_unmap_put_pte() as the function
>>> now serves the stage-2 unmap walker specifically, rather than
>>> acting generic.
>>>
>>> Signed-off-by: Raghavendra Rao Ananta <rananta@google.com>
>>> ---
>>>    arch/arm64/kvm/hyp/pgtable.c | 67 +++++++++++++++++++++++++++++++-----
>>>    1 file changed, 58 insertions(+), 9 deletions(-)
>>>
>>> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
>>> index 5ef098af1736..cf88933a2ea0 100644
>>> --- a/arch/arm64/kvm/hyp/pgtable.c
>>> +++ b/arch/arm64/kvm/hyp/pgtable.c
>>> @@ -831,16 +831,54 @@ static void stage2_make_pte(const struct kvm_pgtable_visit_ctx *ctx, kvm_pte_t n
>>>        smp_store_release(ctx->ptep, new);
>>>    }
>>>
>>> -static void stage2_put_pte(const struct kvm_pgtable_visit_ctx *ctx, struct kvm_s2_mmu *mmu,
>>> -                        struct kvm_pgtable_mm_ops *mm_ops)
>>> +struct stage2_unmap_data {
>>> +     struct kvm_pgtable *pgt;
>>> +     bool defer_tlb_flush_init;
>>> +};
>>> +
>>> +static bool __stage2_unmap_defer_tlb_flush(struct kvm_pgtable *pgt)
>>> +{
>>> +     /*
>>> +      * If FEAT_TLBIRANGE is implemented, defer the individual
>>> +      * TLB invalidations until the entire walk is finished, and
>>> +      * then use the range-based TLBI instructions to do the
>>> +      * invalidations. Condition deferred TLB invalidation on the
>>> +      * system supporting FWB, as the optimization is entirely
>>> +      * pointless when the unmap walker needs to perform CMOs.
>>> +      */
>>> +     return system_supports_tlb_range() && stage2_has_fwb(pgt);
>>> +}
>>> +
>>> +static bool stage2_unmap_defer_tlb_flush(struct stage2_unmap_data *unmap_data)
>>> +{
>>> +     bool defer_tlb_flush = __stage2_unmap_defer_tlb_flush(unmap_data->pgt);
>>> +
>>> +     /*
>>> +      * Since __stage2_unmap_defer_tlb_flush() is based on alternative
>>> +      * patching and the TLBIs' operations behavior depend on this,
>>> +      * track if there's any change in the state during the unmap sequence.
>>> +      */
>>> +     WARN_ON(unmap_data->defer_tlb_flush_init != defer_tlb_flush);
>>> +     return defer_tlb_flush;
>>> +}
>>> +
>>> +static void stage2_unmap_put_pte(const struct kvm_pgtable_visit_ctx *ctx,
>>> +                             struct kvm_s2_mmu *mmu,
>>> +                             struct kvm_pgtable_mm_ops *mm_ops)
>>>    {
>>> +     struct stage2_unmap_data *unmap_data = ctx->arg;
>>> +
>>>        /*
>>> -      * Clear the existing PTE, and perform break-before-make with
>>> -      * TLB maintenance if it was valid.
>>> +      * Clear the existing PTE, and perform break-before-make if it was
>>> +      * valid. Depending on the system support, the TLB maintenance for
>>> +      * the same can be deferred until the entire unmap is completed.
>>>         */
>>>        if (kvm_pte_valid(ctx->old)) {
>>>                kvm_clear_pte(ctx->ptep);
>>> -             kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, mmu, ctx->addr, ctx->level);
>>> +
>>> +             if (!stage2_unmap_defer_tlb_flush(unmap_data))
>> Why not directly check (unmap_data->defer_tlb_flush_init) here?
>>
> (Re-sending the reply as the previous one was formatted as HTML and
> was blocked by many lists)
> 
> No particular reason per say, but I was just going with the logic of
> determining if we need to defer the flush and the WARN_ON() parts
> separate.
> Any advantage if we directly check in stage2_unmap_put_pte() that I
> missed or is this purely for readability?

Hi Raghavendra,

I just wondering if before the stage2 walk, we want to defer the tlb 
flush, but if when walk the stage2, the stage2_unmap_defer_tlb_flush() 
trigger the WARN_ON() and return don't defer the tlb flush, it will 
flush the ctx->addr's tlb. But before the WARN_ON() triggered, these tlb 
will not be flushed, since when walk stage2 done in the 
kvm_pgtable_stage2_unmap(), the stage2_unmap_defer_tlb_flush() still 
trigger the WARN_ON() and don't use the tlb range-based flush. Thus some 
of the tlb are not flushed.

If we directly check the (unmap_data->defer_tlb_flush_init), this isn't 
change during walking the stage2, so the WARN_ON() should only trigger 
in kvm_pgtable_stage2_unmap()->stage2_unmap_defer_tlb_flush().

I'm not sure if it's right since I just think once we set up use the 
TLBI range-based flush, the result of the 
__stage2_unmap_defer_tlb_flush() shouldn't change. Otherwise there must 
have some stale TLB entry.

Thanks,
Shaoqin

> 
>>> +                     kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, mmu,
>>> +                                     ctx->addr, ctx->level);
>> Small indent hint. The ctx->addr can align with __kvm_tlb_flush_vmid_ipa.
>>
> Ah, yes. I'll adjust this if I send out a v8.
> 
> Thank you.
> Raghavendra
>> Thanks,
>> Shaoqin
>>>        }
>>>
>>>        mm_ops->put_page(ctx->ptep);
>>> @@ -1070,7 +1108,8 @@ int kvm_pgtable_stage2_set_owner(struct kvm_pgtable *pgt, u64 addr, u64 size,
>>>    static int stage2_unmap_walker(const struct kvm_pgtable_visit_ctx *ctx,
>>>                               enum kvm_pgtable_walk_flags visit)
>>>    {
>>> -     struct kvm_pgtable *pgt = ctx->arg;
>>> +     struct stage2_unmap_data *unmap_data = ctx->arg;
>>> +     struct kvm_pgtable *pgt = unmap_data->pgt;
>>>        struct kvm_s2_mmu *mmu = pgt->mmu;
>>>        struct kvm_pgtable_mm_ops *mm_ops = ctx->mm_ops;
>>>        kvm_pte_t *childp = NULL;
>>> @@ -1098,7 +1137,7 @@ static int stage2_unmap_walker(const struct kvm_pgtable_visit_ctx *ctx,
>>>         * block entry and rely on the remaining portions being faulted
>>>         * back lazily.
>>>         */
>>> -     stage2_put_pte(ctx, mmu, mm_ops);
>>> +     stage2_unmap_put_pte(ctx, mmu, mm_ops);
>>>
>>>        if (need_flush && mm_ops->dcache_clean_inval_poc)
>>>                mm_ops->dcache_clean_inval_poc(kvm_pte_follow(ctx->old, mm_ops),
>>> @@ -1112,13 +1151,23 @@ static int stage2_unmap_walker(const struct kvm_pgtable_visit_ctx *ctx,
>>>
>>>    int kvm_pgtable_stage2_unmap(struct kvm_pgtable *pgt, u64 addr, u64 size)
>>>    {
>>> +     int ret;
>>> +     struct stage2_unmap_data unmap_data = {
>>> +             .pgt = pgt,
>>> +             .defer_tlb_flush_init = __stage2_unmap_defer_tlb_flush(pgt),
>>> +     };
>>>        struct kvm_pgtable_walker walker = {
>>>                .cb     = stage2_unmap_walker,
>>> -             .arg    = pgt,
>>> +             .arg    = &unmap_data,
>>>                .flags  = KVM_PGTABLE_WALK_LEAF | KVM_PGTABLE_WALK_TABLE_POST,
>>>        };
>>>
>>> -     return kvm_pgtable_walk(pgt, addr, size, &walker);
>>> +     ret = kvm_pgtable_walk(pgt, addr, size, &walker);
>>> +     if (stage2_unmap_defer_tlb_flush(&unmap_data))
>>> +             /* Perform the deferred TLB invalidations */
>>> +             kvm_tlb_flush_vmid_range(pgt->mmu, addr, size);
>>> +
>>> +     return ret;
>>>    }
>>>
>>>    struct stage2_attr_data {
>>
>> --
>> Shaoqin
>>
> 

-- 
Shaoqin


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