Messages in this thread | | | From | Andy Lutomirski <> | Date | Tue, 24 Mar 2015 12:07:50 -0700 | Subject | Re: [PATCH] x86/asm/entry/64: Improve the THREAD_INFO() macro explanation |
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Acked-by: Andy Lutomirski <luto@kernel.org>
On Tue, Mar 24, 2015 at 11:50 AM, Denys Vlasenko <dvlasenk@redhat.com> wrote: > On 03/24/2015 07:43 PM, Ingo Molnar wrote: >> From 0229a184997a7d4ad4398ee3ac2f5ae78c1c1a03 Mon Sep 17 00:00:00 2001 >> From: Ingo Molnar <mingo@kernel.org> >> Date: Tue, 24 Mar 2015 18:57:13 +0100 >> Subject: [PATCH] x86/asm/entry/64: Improve the THREAD_INFO() macro explanation >> >> Explain the background, and add a real example. >> >> Cc: Alexei Starovoitov <ast@plumgrid.com> >> Cc: Andy Lutomirski <luto@amacapital.net> >> Cc: Andy Lutomirski <luto@kernel.org> >> Cc: Borislav Petkov <bp@alien8.de> >> Cc: Borislav Petkov <bp@suse.de> >> Cc: Denys Vlasenko <dvlasenk@redhat.com> >> Cc: Frederic Weisbecker <fweisbec@gmail.com> >> Cc: H. Peter Anvin <hpa@zytor.com> >> Cc: Kees Cook <keescook@chromium.org> >> Cc: Linus Torvalds <torvalds@linux-foundation.org> >> Cc: Oleg Nesterov <oleg@redhat.com> >> Cc: Steven Rostedt <rostedt@goodmis.org> >> Cc: Will Drewry <wad@chromium.org> >> Signed-off-by: Ingo Molnar <mingo@kernel.org> >> --- >> arch/x86/include/asm/thread_info.h | 27 +++++++++++++++++++++++---- >> 1 file changed, 23 insertions(+), 4 deletions(-) >> >> diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h >> index ad0ee3423da5..813dfbb867a7 100644 >> --- a/arch/x86/include/asm/thread_info.h >> +++ b/arch/x86/include/asm/thread_info.h >> @@ -206,10 +206,29 @@ static inline unsigned long current_stack_pointer(void) >> _ASM_SUB $(THREAD_SIZE),reg ; >> >> /* >> - * ASM operand which evaluates to thread_info address >> - * if it is known that "reg" is exactly "off" bytes below stack top. >> - * Example (fetch thread_info->fieldname): >> - * mov TI_fieldname+THREAD_INFO(reg, off),%eax >> + * ASM operand which evaluates to a 'thread_info' address of >> + * the current task, if it is known that "reg" is exactly "off" >> + * bytes below the top of the stack currently. >> + * >> + * ( The kernel stack's size is known at build time, it is usually >> + * 2 or 4 pages, and the bottom of the kernel stack contains >> + * the thread_info structure. So to access the thread_info very >> + * quickly from assembly code we can calculate down from the >> + * top of the kernel stack to the bottom, using constant, >> + * build-time calculations only. ) >> + * >> + * For example, to fetch the current thread_info->flags value into %eax >> + * on x86-64 defconfig kernels, in syscall entry code where RSP is >> + * currently at exactly SIZEOF_PTREGS bytes away from the top of the >> + * stack: >> + * >> + * mov TI_flags+THREAD_INFO(%rsp, SIZEOF_PTREGS), %eax >> + * >> + * will translate to: >> + * >> + * 8b 84 24 b8 c0 ff ff mov -0x3f48(%rsp), %eax >> + * >> + * which is below the current RSP by almost 16K. >> */ >> #define THREAD_INFO(reg, off) ((off)-THREAD_SIZE)(reg) > > Acked-by: Denys Vlasenko <dvlasenk@redhat.com> >
-- Andy Lutomirski AMA Capital Management, LLC
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