linux-kernel - Re: [PATCH] KVM: x86: fix 32 bit build

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Date:   Wed, 16 Jun 2021 15:59:23 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     kvm@...r.kernel.org, Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@...r.kernel.org>, Jim Mattson <jmattson@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH] KVM: x86: fix 32 bit build

On Wed, Jun 16, 2021, Maxim Levitsky wrote:
> Now that kvm->stat.nx_lpage_splits is 64 bit, use DIV_ROUND_UP_ULL
> when doing division.

I went the "cast to an unsigned long" route.  I prefer the cast approach because
to_zap is also an unsigned long, i.e. using DIV_ROUND_UP_ULL() could look like a
truncation bug.  In practice, nx_lpage_splits can't be more than an unsigned long
so it's largely a moot point, I just like the more explicit "this is doing
something odd".

https://lkml.kernel.org/r/20210615162905.2132937-1-seanjc@google.com

> Fixes: 7ee093d4f3f5 ("KVM: switch per-VM stats to u64")
> Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>
> ---
>  arch/x86/kvm/mmu/mmu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 720ceb0a1f5c..97372225f183 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -6054,7 +6054,7 @@ 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_ULL(kvm->stat.nx_lpage_splits, ratio) : 0;
>  	for ( ; to_zap; --to_zap) {
>  		if (list_empty(&kvm->arch.lpage_disallowed_mmu_pages))
>  			break;
> -- 
> 2.26.3
>