Re: [PATCH] KVM: x86: fix 32 bit build

From: Sean Christopherson <seanjc@google.com>
To: Maxim Levitsky <mlevitsk@redhat.com>
Cc: kvm@vger.kernel.org, Joerg Roedel <joro@8bytes.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
	Borislav Petkov <bp@alien8.de>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
	<x86@kernel.org>,
	"open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
	<linux-kernel@vger.kernel.org>, Jim Mattson <jmattson@google.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH] KVM: x86: fix 32 bit build
Date: Wed, 16 Jun 2021 15:59:23 +0000	[thread overview]
Message-ID: <YMof22ameZYUHNdi@google.com> (raw)
In-Reply-To: <20210616155032.1117176-1-mlevitsk@redhat.com>

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@redhat.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
> 