On August 4, 2020 9:48:40 PM PDT, Borislav Petkov <b...@suse.de> wrote: >On Tue, Aug 04, 2020 at 07:10:59PM -0700, Ricardo Neri wrote: >> The SERIALIZE instruction gives software a way to force the processor >to >> complete all modifications to flags, registers and memory from >previous >> instructions and drain all buffered writes to memory before the next >> instruction is fetched and executed. Thus, it serves the purpose of >> sync_core(). Use it when available. >> >> Commit 7117f16bf460 ("objtool: Fix ORC vs alternatives") enforced >stack >> invariance in alternatives. The iret-to-self does not comply with >such >> invariance. Thus, it cannot be used inside alternative code. Instead, >use >> an alternative that jumps to SERIALIZE when available. >> >> Cc: Andy Lutomirski <l...@kernel.org> >> Cc: Cathy Zhang <cathy.zh...@intel.com> >> Cc: Dave Hansen <dave.han...@linux.intel.com> >> Cc: Fenghua Yu <fenghua...@intel.com> >> Cc: "H. Peter Anvin" <h...@zytor.com> >> Cc: Kyung Min Park <kyung.min.p...@intel.com> >> Cc: Peter Zijlstra <pet...@infradead.org> >> Cc: "Ravi V. Shankar" <ravi.v.shan...@intel.com> >> Cc: Sean Christopherson <sean.j.christopher...@intel.com> >> Cc: linux-e...@vger.kernel.org >> Cc: linux-kernel@vger.kernel.org >> Suggested-by: Andy Lutomirski <l...@kernel.org> >> Signed-off-by: Ricardo Neri <ricardo.neri-calde...@linux.intel.com> >> --- >> This is a v2 from my initial submission [1]. The first three patches >of >> the series have been merged in Linus' tree. Hence, I am submitting >only >> this patch for review. >> >> [1]. https://lkml.org/lkml/2020/7/27/8 >> >> Changes since v1: >> * Support SERIALIZE using alternative runtime patching. >> (Peter Zijlstra, H. Peter Anvin) >> * Added a note to specify which version of binutils supports >SERIALIZE. >> (Peter Zijlstra) >> * Verified that (::: "memory") is used. (H. Peter Anvin) >> --- >> arch/x86/include/asm/special_insns.h | 2 ++ >> arch/x86/include/asm/sync_core.h | 10 +++++++++- >> 2 files changed, 11 insertions(+), 1 deletion(-) >> >> diff --git a/arch/x86/include/asm/special_insns.h >b/arch/x86/include/asm/special_insns.h >> index 59a3e13204c3..25cd67801dda 100644 >> --- a/arch/x86/include/asm/special_insns.h >> +++ b/arch/x86/include/asm/special_insns.h >> @@ -10,6 +10,8 @@ >> #include <linux/irqflags.h> >> #include <linux/jump_label.h> >> >> +/* Instruction opcode for SERIALIZE; supported in binutils >= 2.35. >*/ >> +#define __ASM_SERIALIZE ".byte 0xf, 0x1, 0xe8" >> /* >> * Volatile isn't enough to prevent the compiler from reordering the >> * read/write functions for the control registers and messing >everything up. >> diff --git a/arch/x86/include/asm/sync_core.h >b/arch/x86/include/asm/sync_core.h >> index fdb5b356e59b..201ea3d9a6bd 100644 >> --- a/arch/x86/include/asm/sync_core.h >> +++ b/arch/x86/include/asm/sync_core.h >> @@ -5,15 +5,19 @@ >> #include <linux/preempt.h> >> #include <asm/processor.h> >> #include <asm/cpufeature.h> >> +#include <asm/special_insns.h> >> >> #ifdef CONFIG_X86_32 >> static inline void iret_to_self(void) >> { >> asm volatile ( >> + ALTERNATIVE("", "jmp 2f", X86_FEATURE_SERIALIZE) >> "pushfl\n\t" >> "pushl %%cs\n\t" >> "pushl $1f\n\t" >> "iret\n\t" >> + "2:\n\t" >> + __ASM_SERIALIZE "\n" >> "1:" >> : ASM_CALL_CONSTRAINT : : "memory"); >> } >> @@ -23,6 +27,7 @@ static inline void iret_to_self(void) >> unsigned int tmp; >> >> asm volatile ( >> + ALTERNATIVE("", "jmp 2f", X86_FEATURE_SERIALIZE) > >Why is this and above stuck inside the asm statement? > >Why can't you simply do: > > if (static_cpu_has(X86_FEATURE_SERIALIZE)) { > asm volatile(__ASM_SERIALIZE ::: "memory"); > return; > } > >on function entry instead of making it more unreadable for no >particular >reason?
Because why use an alternative to jump over one instruction? I personally would prefer to have the IRET put out of line and have the call/jmp replaced by SERIALIZE inline. -- Sent from my Android device with K-9 Mail. Please excuse my brevity.