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Date:   Thu, 30 Apr 2020 09:02:51 +0100
From:   Marc Zyngier <maz@kernel.org>
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     Mike Rapoport <rppt@linux.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
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        Fenghua Yu <fenghua.yu@intel.com>,
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        Guan Xuetao <gxt@pku.edu.cn>,
        James Morse <james.morse@arm.com>,
        Jonas Bonn <jonas@southpole.se>,
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        Rich Felker <dalias@libc.org>,
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        Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Tony Luck <tony.luck@intel.com>, Will Deacon <will@kernel.org>,
        Yoshinori Sato <ysato@users.sourceforge.jp>,
        Stephen Rothwell <sfr@canb.auug.org.au>,
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        linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] arm64: kvm: fix gcc-10 shift warning
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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>

	M.
-- 
Jazz is not dead. It just smells funny...