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.
Signed-off-by: Peter Xu <[email protected]>
---
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 74bdb7bf3295..2da293885a67 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -2425,7 +2425,7 @@ static int __kvm_gfn_to_hva_cache_init(struct kvm_memslots *slots,
gfn_t start_gfn = gpa >> PAGE_SHIFT;
gfn_t end_gfn = (gpa + len - 1) >> PAGE_SHIFT;
gfn_t nr_pages_needed = end_gfn - start_gfn + 1;
- gfn_t nr_pages_avail;
+ gfn_t nr_pages_avail = 0;
/* Update ghc->generation before performing any error checks. */
ghc->generation = slots->generation;
--
2.26.2
On 5/5/20 5:05 AM, 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.
>
> Signed-off-by: Peter Xu <[email protected]>
> ---
> virt/kvm/kvm_main.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
Reviewed-by: Gavin Shan <[email protected]>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 74bdb7bf3295..2da293885a67 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2425,7 +2425,7 @@ static int __kvm_gfn_to_hva_cache_init(struct kvm_memslots *slots,
> gfn_t start_gfn = gpa >> PAGE_SHIFT;
> gfn_t end_gfn = (gpa + len - 1) >> PAGE_SHIFT;
> gfn_t nr_pages_needed = end_gfn - start_gfn + 1;
> - gfn_t nr_pages_avail;
> + gfn_t nr_pages_avail = 0;
>
> /* Update ghc->generation before performing any error checks. */
> ghc->generation = slots->generation;
>
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.
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/[email protected]
[2] https://bugzilla.kernel.org/show_bug.cgi?id=207173
>
> Signed-off-by: Peter Xu <[email protected]>
> ---
> 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 74bdb7bf3295..2da293885a67 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2425,7 +2425,7 @@ static int __kvm_gfn_to_hva_cache_init(struct kvm_memslots *slots,
> gfn_t start_gfn = gpa >> PAGE_SHIFT;
> gfn_t end_gfn = (gpa + len - 1) >> PAGE_SHIFT;
> gfn_t nr_pages_needed = end_gfn - start_gfn + 1;
> - gfn_t nr_pages_avail;
> + gfn_t nr_pages_avail = 0;
>
> /* Update ghc->generation before performing any error checks. */
> ghc->generation = slots->generation;
> --
> 2.26.2
>
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/[email protected]
> [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.
Thanks,
--
Peter Xu
+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/[email protected]
> > [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".
On Mon, May 11, 2020 at 9:05 AM Sean Christopherson
<[email protected]> wrote:
>
> +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/[email protected]
> > > [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".
Seems that doing absolutely nothing was the fix here :) See:
78a5255ffb6a ("Stop the ad-hoc games with -Wno-maybe-initialized")
--
Thanks,
Oliver
On Mon, May 11, 2020 at 10:04:29AM -0700, Oliver Upton wrote:
> On Mon, May 11, 2020 at 9:05 AM Sean Christopherson
> <[email protected]> wrote:
> >
> > +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/[email protected]
> > > > [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".
>
> Seems that doing absolutely nothing was the fix here :) See:
>
> 78a5255ffb6a ("Stop the ad-hoc games with -Wno-maybe-initialized")
Ah, perfect! Thanks Oliver.