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

Stephen Rothwell
2020-04-29T22:43:13Z

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Hi all,

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);
> --
> 2.26.0
>

I have added that patch to linux-next today.

--
Cheers,
Stephen Rothwell

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