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

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Message-ID: <20200430082927.GA18615@willie-the-truck>
Date:   Thu, 30 Apr 2020 09:29:28 +0100
From:   Will Deacon <will@...nel.org>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Arnd Bergmann <arnd@...db.de>, Mike Rapoport <rppt@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Brian Cain <bcain@...eaurora.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Christophe Leroy <christophe.leroy@....fr>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Guan Xuetao <gxt@....edu.cn>,
        James Morse <james.morse@....com>,
        Jonas Bonn <jonas@...thpole.se>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Ley Foon Tan <ley.foon.tan@...el.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Mackerras <paulus@...ba.org>,
        Rich Felker <dalias@...c.org>,
        Russell King <linux@...linux.org.uk>,
        Stafford Horne <shorne@...il.com>,
        Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Tony Luck <tony.luck@...el.com>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Paolo Bonzini <pbonzini@...hat.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: kvm: fix gcc-10 shift warning

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@...db.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@...db.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@...nel.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