Hi Mark, > -----Original Message----- > From: Mark Rutland [mailto:mark.rutl...@arm.com] > Sent: 2019年2月13日 19:31 > To: Peng Fan <peng....@nxp.com> > Cc: catalin.mari...@arm.com; will.dea...@arm.com; > ard.biesheu...@linaro.org; yaojun8558...@gmail.com; > cpan...@codeaurora.org; robin.mur...@arm.com; > linux-arm-ker...@lists.infradead.org; linux-kernel@vger.kernel.org; > van.free...@gmail.com > Subject: Re: [PATCH 3/3] arm64: mmu: pgd_pgtable_alloc: drop barrier > > On Wed, Feb 13, 2019 at 09:37:38AM +0000, Peng Fan wrote: > > The barriers are added for empty_zero_page, however the empty zero > > page has been moved to bss area by commit 5227cfa71f9e ("arm64: mm: > > place empty_zero_page in bss"), > > and it alreay added "dsb ishst" in head.S to make sure > > the empty zero page visible to PTW. > > The "zeroed page" the comment refers to is not empty_zero_page.
Thanks, I understand wrong. > > The page > the comment refers to is an arbitrary page returned by __get_free_page(), > which has been filled with zeroed at some point thanks to PGALLOC_GFP > containing __GFP_ZERO. > > > pgd_pgtable_alloc is only called by __create_pgd_mapping, and there > > are implicit barriers in __create_pgd_mapping, so we could remove the > > barrier pgd_pgtable_alloc > > I don't think this is true. > > Consider: > > create_pgd_mapping() > -> __create_pgd_mapping() > -> alloc_init_pud() > -> pgtable_alloc() // pgd_pgtable_alloc() > -> __pgd_populate() > > ... where AFAICT there is no barrier between pgtable_alloc() and > __pgd_populate(). > > Where is the barrier between the page zeroing and the update of the pgd > entry? I though the barrier in __pgd_populate could be enough, but this is not enough, we still need barrier before write pgd/pud/pmd/pte entries. So drop this patch. Thanks, Peng. > > Thanks, > Mark. > > > > > Signed-off-by: Peng Fan <peng....@nxp.com> > > --- > > > > Note: this patch tested on qemu-system-aarch64, not real hardware. > > > > arch/arm64/mm/mmu.c | 2 -- > > 1 file changed, 2 deletions(-) > > > > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c index > > f61fa7c8fd2e..04e3d4d070ce 100644 > > --- a/arch/arm64/mm/mmu.c > > +++ b/arch/arm64/mm/mmu.c > > @@ -371,8 +371,6 @@ static phys_addr_t pgd_pgtable_alloc(void) > > if (!ptr || !pgtable_page_ctor(virt_to_page(ptr))) > > BUG(); > > > > - /* Ensure the zeroed page is visible to the page table walker */ > > - dsb(ishst); > > return __pa(ptr); > > } > > > > -- > > 2.16.4 > >
