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

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Message-ID: <YqtfpKof5IEBdKW4@google.com>
Date:   Thu, 16 Jun 2022 16:51:48 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.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 Thu, Jun 16, 2022, Paolo Bonzini wrote:
> 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;"...

That was my guess too, but IIRC I tried the old code with older compilers (gcc-7)
and couldn't trigger any warning.  So AFAICT, it was pure paranoia.