linux-kernel - Re: [PATCH] KVM: x86/mmu: Grab nx_lpage_splits as an unsigned long before division

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Date:   Thu, 17 Jun 2021 19:10:38 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86/mmu: Grab nx_lpage_splits as an unsigned long
 before division

On 15/06/21 18:29, Sean Christopherson wrote:
> Snapshot kvm->stats.nx_lpage_splits into a local unsigned long to avoid
> 64-bit division on 32-bit kernels.  Casting to an unsigned long is safe
> because the maximum number of shadow pages, n_max_mmu_pages, is also an
> unsigned long, i.e. KVM will start recycling shadow pages before the
> number of splits can exceed a 32-bit value.
> 
>    ERROR: modpost: "__udivdi3" [arch/x86/kvm/kvm.ko] undefined!
> 
> Fixes: 7ee093d4f3f5 ("KVM: switch per-VM stats to u64")
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   arch/x86/kvm/mmu/mmu.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 720ceb0a1f5c..7d3e57678d34 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -6043,6 +6043,7 @@ static int set_nx_huge_pages_recovery_ratio(const char *val, const struct kernel
>   
>   static void kvm_recover_nx_lpages(struct kvm *kvm)
>   {
> +	unsigned long nx_lpage_splits = kvm->stat.nx_lpage_splits;
>   	int rcu_idx;
>   	struct kvm_mmu_page *sp;
>   	unsigned int ratio;
> @@ -6054,7 +6055,7 @@ static void kvm_recover_nx_lpages(struct kvm *kvm)
>   	write_lock(&kvm->mmu_lock);
>   
>   	ratio = READ_ONCE(nx_huge_pages_recovery_ratio);
> -	to_zap = ratio ? DIV_ROUND_UP(kvm->stat.nx_lpage_splits, ratio) : 0;
> +	to_zap = ratio ? DIV_ROUND_UP(nx_lpage_splits, ratio) : 0;
>   	for ( ; to_zap; --to_zap) {
>   		if (list_empty(&kvm->arch.lpage_disallowed_mmu_pages))
>   			break;
> 

Queued, thanks.

Paolo