linux-kernel - Re: [PATCH v4 6/6] KVM: arm64: Use TLBI range-based intructions for unmap

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Date:   Mon, 29 May 2023 15:18:25 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Raghavendra Rao Ananta <rananta@...gle.com>
Cc:     Oliver Upton <oliver.upton@...ux.dev>,
        James Morse <james.morse@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Ricardo Koller <ricarkol@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jing Zhang <jingzhangos@...gle.com>,
        Colton Lewis <coltonlewis@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v4 6/6] KVM: arm64: Use TLBI range-based intructions for unmap

On Fri, 19 May 2023 01:52:31 +0100,
Raghavendra Rao Ananta <rananta@...gle.com> 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.
> 
> 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 this upon S2FWB in order to avoid walking the page-table
> again to perform the CMOs after issuing the TLBI.

But that's the real bottleneck. TLBIs are cheap compared to CMOs, even
on remarkably bad implementations. What is your plan to fix this?

> 
> 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@...gle.com>
> ---
>  arch/arm64/kvm/hyp/pgtable.c | 35 ++++++++++++++++++++++++++++++-----
>  1 file changed, 30 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> index b8f0dbd12f773..5832ee3418fb0 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -771,16 +771,34 @@ 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)
> +static bool stage2_unmap_defer_tlb_flush(struct kvm_pgtable *pgt)
>  {
> +	/*
> +	 * If FEAT_TLBIRANGE is implemented, defer the individial PTE
> +	 * TLB invalidations until the entire walk is finished, and
> +	 * then use the range-based TLBI instructions to do the
> +	 * invalidations. Condition this upon S2FWB in order to avoid
> +	 * a page-table walk again to perform the CMOs after TLBI.
> +	 */
> +	return system_supports_tlb_range() && stage2_has_fwb(pgt);
> +}
> +
> +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 kvm_pgtable *pgt = ctx->arg;
> +
>  	/*
>  	 * Clear the existing PTE, and perform break-before-make with
>  	 * TLB maintenance if it was valid.
>  	 */
>  	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(pgt))
> +			kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, mmu,
> +					ctx->addr, ctx->level);

This really doesn't match the comment anymore.

Overall, I'm very concerned that we lose the consistency property that
the current code has: once called, the TLBs and the page tables are
synchronised.

Yes, this patch looks correct. But it is also really fragile.

>  	}
>  
>  	mm_ops->put_page(ctx->ptep);
> @@ -1015,7 +1033,7 @@ 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),
> @@ -1029,13 +1047,20 @@ 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 kvm_pgtable_walker walker = {
>  		.cb	= stage2_unmap_walker,
>  		.arg	= pgt,
>  		.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(pgt))
> +		/* Perform the deferred TLB invalidations */
> +		kvm_call_hyp(__kvm_tlb_flush_vmid_range, pgt->mmu,
> +				addr, addr + size);

This "kvm_call_hyp(__kvm_tlb_flush_vmid_range,...)" could do with a
wrapper from the point where you introduce it.

> +
> +	return ret;
>  }
>  

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.