Re: [PATCH] arm64: kvm: fix gcc-10 shift warning

Will Deacon
2020-04-30T08:29:45Z

On Thu, Apr 30, 2020 at 09:02:51AM +0100, Marc Zyngier wrote:
> On Wed, 29 Apr 2020 20:56:20 +0200
> Arnd Bergmann <arnd@arndb.de> wrote:
>
> > gcc-10 warns that the 32-bit zero cannot be shifted more than
> > 32 bits to the right:
> >
> > arch/arm64/kvm/../../../virt/kvm/arm/mmu.c: In function 'clear_hyp_p4d_entry':
> > arch/arm64/include/asm/pgtable.h:630:35: error: right shift count >= width of type [-Werror=shift-count-overflow]
> > 630 | #define pud_index(addr) (((addr) >> PUD_SHIFT) & (PTRS_PER_PUD - 1))
> > | ^~
> > arch/arm64/include/asm/memory.h:271:45: note: in definition of macro '__phys_to_virt'
> > 271 | #define __phys_to_virt(x) ((unsigned long)((x) - physvirt_offset))
> > | ^
> > arch/arm64/include/asm/pgtable.h:633:42: note: in expansion of macro '__va'
> > 633 | #define pud_offset(dir, addr) ((pud_t *)__va(pud_offset_phys((dir), (addr))))
> > | ^~~~
> > arch/arm64/include/asm/pgtable.h:632:73: note: in expansion of macro 'pud_index'
> > 632 | #define pud_offset_phys(dir, addr) (p4d_page_paddr(READ_ONCE(*(dir))) + pud_index(addr) * sizeof(pud_t))
> > | ^~~~~~~~~
> > arch/arm64/include/asm/pgtable.h:633:47: note: in expansion of macro 'pud_offset_phys'
> > 633 | #define pud_offset(dir, addr) ((pud_t *)__va(pud_offset_phys((dir), (addr))))
> > | ^~~~~~~~~~~~~~~
> > arch/arm64/kvm/../../../virt/kvm/arm/mmu.c:510:36: note: in expansion of macro 'pud_offset'
> > 510 | pud_t *pud_table __maybe_unused = pud_offset(p4d, 0);
> > | ^~~~~~~~~~
> >
> > This is harmless, and the warning is a little bit silly for
> > a zero constant, but it's trivial to fix by making it an
> > unsigned long, so do that.
> >
> > Fixes: 22998131ab33 ("arm64: add support for folded p4d page tables")
> > Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> > ---
> > virt/kvm/arm/mmu.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/virt/kvm/arm/mmu.c b/virt/kvm/arm/mmu.c
> > index 48d4288c5f1b..534d9798c3cb 100644
> > --- a/virt/kvm/arm/mmu.c
> > +++ b/virt/kvm/arm/mmu.c
> > @@ -507,7 +507,7 @@ static void clear_hyp_pgd_entry(pgd_t *pgd)
> >
> > static void clear_hyp_p4d_entry(p4d_t *p4d)
> > {
> > - pud_t *pud_table __maybe_unused = pud_offset(p4d, 0);
> > + pud_t *pud_table __maybe_unused = pud_offset(p4d, 0UL);
> > VM_BUG_ON(p4d_huge(*p4d));
> > p4d_clear(p4d);
> > pud_free(NULL, pud_table);
>
> Acked-by: Marc Zyngier <maz@kernel.org>

Happy to queue via arm64 for 5.8. Does that work for you, Arnd, or were you
planning to get this in sooner than that?

Cheers,

Will