From mboxrd@z Thu Jan 1 00:00:00 1970 From: mark.rutland@arm.com (Mark Rutland) Date: Mon, 12 Sep 2016 15:59:31 +0100 Subject: [PATCH] arm64: mm: move zero page from .bss to right before swapper_pg_dir In-Reply-To: <1473689784-29745-1-git-send-email-ard.biesheuvel@linaro.org> References: <1473689784-29745-1-git-send-email-ard.biesheuvel@linaro.org> Message-ID: <20160912145931.GB14165@leverpostej> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org Hi Ard, On Mon, Sep 12, 2016 at 03:16:24PM +0100, Ard Biesheuvel wrote: > static inline void cpu_set_reserved_ttbr0(void) > { > - unsigned long ttbr = virt_to_phys(empty_zero_page); > - > - asm( > - " msr ttbr0_el1, %0 // set TTBR0\n" > - " isb" > - : > - : "r" (ttbr)); > + /* > + * The zero page is located right before swapper_pg_dir, whose > + * physical address we can easily fetch from TTBR1_EL1. > + */ > + write_sysreg(read_sysreg(ttbr1_el1) - PAGE_SIZE, ttbr0_el1); > + isb(); > } As a heads-up, in arm64 for-next/core this is a write_sysreg() and an isb() thanks to [1,2]. This will need a rebase to avoid conflict. > /* > @@ -109,7 +108,8 @@ static inline void cpu_uninstall_idmap(void) > { > struct mm_struct *mm = current->active_mm; > > - cpu_set_reserved_ttbr0(); > + write_sysreg(virt_to_phys(empty_zero_page), ttbr0_el1); > + isb(); > local_flush_tlb_all(); > cpu_set_default_tcr_t0sz(); > > @@ -119,7 +119,8 @@ static inline void cpu_uninstall_idmap(void) > > static inline void cpu_install_idmap(void) > { > - cpu_set_reserved_ttbr0(); > + write_sysreg(virt_to_phys(empty_zero_page), ttbr0_el1); > + isb(); > local_flush_tlb_all(); > cpu_set_idmap_tcr_t0sz(); It would be worth a comment as to why we have to open-code these, so as to avoid "obvious" / "trivial cleanup" patches to this later. e.g. expand the comment in cpu_set_reserved_ttbr0 with: * In some cases (e.g. where cpu_replace_ttbr1 is used), TTBR1 may not * point at swapper_pg_dir, and this helper cannot be used. ... and add /* see cpu_set_reserved_ttbr0 */ to both of these above the write_sysreg(). > diff --git a/arch/arm64/include/asm/sections.h b/arch/arm64/include/asm/sections.h > index 4e7e7067afdb..44e94e234ba0 100644 > --- a/arch/arm64/include/asm/sections.h > +++ b/arch/arm64/include/asm/sections.h > @@ -26,5 +26,6 @@ extern char __hyp_text_start[], __hyp_text_end[]; > extern char __idmap_text_start[], __idmap_text_end[]; > extern char __irqentry_text_start[], __irqentry_text_end[]; > extern char __mmuoff_data_start[], __mmuoff_data_end[]; > +extern char __robss_start[], __robss_end[]; > > #endif /* __ASM_SECTIONS_H */ > diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S > index 5ce9b2929e0d..eae5036dc725 100644 > --- a/arch/arm64/kernel/vmlinux.lds.S > +++ b/arch/arm64/kernel/vmlinux.lds.S > @@ -209,9 +209,19 @@ SECTIONS > > BSS_SECTION(0, 0, 0) > > - . = ALIGN(PAGE_SIZE); > + . = ALIGN(SEGMENT_ALIGN); > + __robss_start = .; > idmap_pg_dir = .; > - . += IDMAP_DIR_SIZE; > + . = ALIGN(. + IDMAP_DIR_SIZE + PAGE_SIZE, SEGMENT_ALIGN); > + __robss_end = .; Is it really worth aligning this beyond PAGE_SIZE? We shouldn't be poking these very often, the padding is always larger than the number of used pages, and the swapper dir is relegated to page mappings regardless. > + /* > + * Put the zero page right before swapper_pg_dir so we can easily > + * obtain its physical address by subtracting PAGE_SIZE from the > + * contents of TTBR1_EL1. > + */ > + empty_zero_page = __robss_end - PAGE_SIZE; Further to the above, I think this would be clearer if defined in-line as with the idmap and swapper pgdirs (with page alignment). [...] > /* > - * Map the linear alias of the [_text, __init_begin) interval as > - * read-only/non-executable. This makes the contents of the > - * region accessible to subsystems such as hibernate, but > - * protects it from inadvertent modification or execution. > + * Map the linear alias of the intervals [_text, __init_begin) and > + * [robss_start, robss_end) as read-only/non-executable. This makes > + * the contents of these regions accessible to subsystems such > + * as hibernate, but protects them from inadvertent modification or > + * execution. For completeness, it may also be worth stating that we're mapping the gap between those as usual, since this will be freed. Then again, maybe not. ;) [...] > @@ -436,13 +442,19 @@ static void __init map_kernel_segment(pgd_t *pgd, void *va_start, void *va_end, > */ > static void __init map_kernel(pgd_t *pgd) > { > - static struct vm_struct vmlinux_text, vmlinux_rodata, vmlinux_init, vmlinux_data; > + static struct vm_struct vmlinux_text, vmlinux_rodata, vmlinux_init, > + vmlinux_data, vmlinux_robss, vmlinux_tail; > > map_kernel_segment(pgd, _text, _etext, PAGE_KERNEL_EXEC, &vmlinux_text); > map_kernel_segment(pgd, __start_rodata, __init_begin, PAGE_KERNEL, &vmlinux_rodata); > map_kernel_segment(pgd, __init_begin, __init_end, PAGE_KERNEL_EXEC, > &vmlinux_init); > - map_kernel_segment(pgd, _data, _end, PAGE_KERNEL, &vmlinux_data); > + map_kernel_segment(pgd, _data, __robss_start, PAGE_KERNEL, > + &vmlinux_data); > + map_kernel_segment(pgd, __robss_start, __robss_end, PAGE_KERNEL_RO, > + &vmlinux_robss); > + map_kernel_segment(pgd, __robss_end, _end, PAGE_KERNEL, > + &vmlinux_tail); Perhaps s/tail/swapper/ or s/tail/pgdir/ to make this clearer? Modulo the above, this looks good to me. Thanks, Mark. [1] http://lists.infradead.org/pipermail/linux-arm-kernel/2016-September/455284.html [2] http://lists.infradead.org/pipermail/linux-arm-kernel/2016-September/455290.html