linux-kernel - Re: [PATCH 01/10] KVM: Do not zero initialize 'pfn' in hva_to

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Message-ID: <3dccee6c-8682-66c8-6a22-e58630825443@redhat.com>
Date:   Thu, 16 Jun 2022 16:54:33 +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 01/10] KVM: Do not zero initialize 'pfn' in hva_to_pfn()

On 4/29/22 03:04, Sean Christopherson wrote:
> Drop the unnecessary initialization of the local 'pfn' variable in
> hva_to_pfn().  First and foremost, '0' is not an invalid pfn, it's a
> perfectly valid pfn on most architectures.  I.e. if hva_to_pfn() were to
> return an "uninitializd" pfn, it would actually be interpeted as a legal
> pfn by most callers.
> 
> Second, hva_to_pfn() can't return an uninitialized pfn as hva_to_pfn()
> explicitly sets pfn to an error value (or returns an error value directly)
> if a helper returns failure, and all helpers set the pfn on success.
> 
> Note, the zeroing of 'pfn' was introduced by commit 2fc843117d64 ("KVM:
> reorganize hva_to_pfn"), and was unnecessary and misguided paranoia even
> then.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   virt/kvm/kvm_main.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 0848430f36c6..04ed4334473c 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2567,7 +2567,7 @@ kvm_pfn_t hva_to_pfn(unsigned long addr, bool atomic, bool *async,
>   		     bool write_fault, bool *writable)
>   {
>   	struct vm_area_struct *vma;
> -	kvm_pfn_t pfn = 0;
> +	kvm_pfn_t pfn;
>   	int npages, r;
>   
>   	/* we can do it either atomically or asynchronously, not both */

I wonder if it was needed to avoid uninitialized variable warnings on 
"return pfn;"...

Paolo