linux-kernel - Re: [PATCH] KVM: Fix a warning in __kvm_gfn_to_hva_cache

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Message-ID: <20200511160537.GC24052@linux.intel.com>
Date:   Mon, 11 May 2020 09:05:37 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Peter Xu <peterx@...hat.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Oliver Upton <oupton@...gle.com>,
        Tony Cook <tony-cook@...pond.com>, zoran.davidovac@...il.com,
        euloanty@...e.com
Subject: Re: [PATCH] KVM: Fix a warning in __kvm_gfn_to_hva_cache_init()

+cc a few other people that have reported this at one time or another.

On Tue, May 05, 2020 at 10:12:45AM -0400, Peter Xu wrote:
> On Mon, May 04, 2020 at 06:39:29PM -0700, Sean Christopherson wrote:
> > On Mon, May 04, 2020 at 03:05:26PM -0400, Peter Xu wrote:
> > > GCC 10.0.1 gives me this warning when building KVM:
> > > 
> > >   warning: ‘nr_pages_avail’ may be used uninitialized in this function [-Wmaybe-uninitialized]
> > >   2442 |  for ( ; start_gfn <= end_gfn; start_gfn += nr_pages_avail) {
> > > 
> > > It should not happen, but silent it.
> > 
> > Heh, third times a charm?  This has been reported and proposed twice
> > before[1][2].  Are you using any custom compiler flags?  E.g. -O3 is known
> > to cause false positives with -Wmaybe-uninitialized.
> 
> No, what I did was only upgrading to Fedora 32 (which will auto-upgrade GCC),
> so it should be using the default params of whatever provided.
> 
> > 
> > If we do end up killing this warning, I'd still prefer to use
> > uninitialized_var() over zero-initializing the variable.
> > 
> > [1] https://lkml.kernel.org/r/20200218184756.242904-1-oupton@google.com
> > [2] https://bugzilla.kernel.org/show_bug.cgi?id=207173
> 
> OK, I didn't know this is a known problem and discussions going on.  But I
> guess it would be good to address this sooner because it could become a common
> warning very soon after people upgrades gcc.

Ya, others are hitting this as well.  It's especially painful with the
existence of KVM_WERROR.

Paolo, any preference on how to resolve this?  It would appear GCC 10 got
"smarter".