linux-kernel - Re: [PATCH V3 2/2] KVM: X86: Move PTE present check from loop body to __shadow_walk

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Message-ID: <4743aa1d-cae7-11a0-03f1-160617ae9a89@redhat.com>
Date:   Fri, 24 Sep 2021 11:56:23 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org
Cc:     Lai Jiangshan <laijs@...ux.alibaba.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        kvm@...r.kernel.org
Subject: Re: [PATCH V3 2/2] KVM: X86: Move PTE present check from loop body to
 __shadow_walk_next()

On 06/09/21 14:25, Lai Jiangshan wrote:
> But checking pte present in __shadow_walk_next() is a more prudent way of
> programing and loop bodies will not need to always check it. It allows us
> removing unneeded is_shadow_present_pte() in the loop bodies.
> 
> Terminating on !is_shadow_present_pte() is 100% the correct behavior, as
> walking past a !PRESENT SPTE would lead to attempting to read a the next
> level SPTE from a garbage iter->shadow_addr.  Even some paths that do_not_
> currently have a !is_shadow_present_pte() in the loop body is Ok since
> they will install present non-leaf SPTEs and the additional present check
> is just an NOP.
> 
> The checking result in __shadow_walk_next() will be propagated to
> shadow_walk_okay() for being used in any for(;;) loop.
> 
> Reviewed-by: Sean Christopherson<seanjc@...gle.com>
> Signed-off-by: Lai Jiangshan<laijs@...ux.alibaba.com>
> ---
> Changed from V2:
> 	Fix typo in the changelog reported by Sean
> 	Add Reviewed-by from Sean
> Changed from V1:
> 	Merge the two patches
> 	Update changelog
> 	Remove !is_shadow_present_pte() in FNAME(invlpg)
>   arch/x86/kvm/mmu/mmu.c         | 13 ++-----------
>   arch/x86/kvm/mmu/paging_tmpl.h |  2 +-
>   2 files changed, 3 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 538be037549d..26f6bd238a77 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -2223,7 +2223,7 @@ static bool shadow_walk_okay(struct kvm_shadow_walk_iterator *iterator)
>   static void __shadow_walk_next(struct kvm_shadow_walk_iterator *iterator,
>   			       u64 spte)
>   {
> -	if (is_last_spte(spte, iterator->level)) {
> +	if (!is_shadow_present_pte(spte) || is_last_spte(spte, iterator->level)) {
>   		iterator->level = 0;
>   		return;
>   	}
> @@ -3159,9 +3159,6 @@ static u64 *fast_pf_get_last_sptep(struct kvm_vcpu *vcpu, gpa_t gpa, u64 *spte)
>   	for_each_shadow_entry_lockless(vcpu, gpa, iterator, old_spte) {
>   		sptep = iterator.sptep;
>   		*spte = old_spte;
> -
> -		if (!is_shadow_present_pte(old_spte))
> -			break;
>   	}
>   
>   	return sptep;
> @@ -3721,9 +3718,6 @@ static int get_walk(struct kvm_vcpu *vcpu, u64 addr, u64 *sptes, int *root_level
>   		spte = mmu_spte_get_lockless(iterator.sptep);
>   
>   		sptes[leaf] = spte;
> -
> -		if (!is_shadow_present_pte(spte))
> -			break;
>   	}
>   
>   	return leaf;
> @@ -3838,11 +3832,8 @@ static void shadow_page_table_clear_flood(struct kvm_vcpu *vcpu, gva_t addr)
>   	u64 spte;
>   
>   	walk_shadow_page_lockless_begin(vcpu);
> -	for_each_shadow_entry_lockless(vcpu, addr, iterator, spte) {
> +	for_each_shadow_entry_lockless(vcpu, addr, iterator, spte)
>   		clear_sp_write_flooding_count(iterator.sptep);
> -		if (!is_shadow_present_pte(spte))
> -			break;
> -	}
>   	walk_shadow_page_lockless_end(vcpu);
>   }
>   
> diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
> index 4d559d2d4d66..72f358613786 100644
> --- a/arch/x86/kvm/mmu/paging_tmpl.h
> +++ b/arch/x86/kvm/mmu/paging_tmpl.h
> @@ -982,7 +982,7 @@ static void FNAME(invlpg)(struct kvm_vcpu *vcpu, gva_t gva, hpa_t root_hpa)
>   			FNAME(update_pte)(vcpu, sp, sptep, &gpte);
>   		}
>   
> -		if (!is_shadow_present_pte(*sptep) || !sp->unsync_children)
> +		if (!sp->unsync_children)

Queued both, thanks.

Paolo