Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752653AbcLEVdF (ORCPT ); Mon, 5 Dec 2016 16:33:05 -0500 Received: from mail.kernel.org ([198.145.29.136]:54790 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752514AbcLEVc7 (ORCPT ); Mon, 5 Dec 2016 16:32:59 -0500 From: Andy Lutomirski To: x86@kernel.org Cc: One Thousand Gnomes , Borislav Petkov , "linux-kernel@vger.kernel.org" , Brian Gerst , Matthew Whitehead , Henrique de Moraes Holschuh , Peter Zijlstra , xen-devel , Juergen Gross , Boris Ostrovsky , Andrew Cooper , Andy Lutomirski , "H. Peter Anvin" Subject: [PATCH v3 4/4] x86/asm: Rewrite sync_core() to use IRET-to-self Date: Mon, 5 Dec 2016 13:32:43 -0800 Message-Id: X-Mailer: git-send-email 2.9.3 In-Reply-To: References: In-Reply-To: References: Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3639 Lines: 112 Aside from being excessively slow, CPUID is problematic: Linux runs on a handful of CPUs that don't have CPUID. Use IRET-to-self instead. IRET-to-self works everywhere, so it makes testing easy. For reference, On my laptop, IRET-to-self is ~110ns, CPUID(eax=1, ecx=0) is ~83ns on native and very very slow under KVM, and MOV-to-CR2 is ~42ns. While we're at it: sync_core() serves a very specific purpose. Document it. Cc: "H. Peter Anvin" Signed-off-by: Andy Lutomirski --- arch/x86/include/asm/processor.h | 77 ++++++++++++++++++++++++++++------------ 1 file changed, 55 insertions(+), 22 deletions(-) diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h index 64fbc937d586..201a956e345f 100644 --- a/arch/x86/include/asm/processor.h +++ b/arch/x86/include/asm/processor.h @@ -590,33 +590,66 @@ static __always_inline void cpu_relax(void) #define cpu_relax_lowlatency() cpu_relax() -/* Stop speculative execution and prefetching of modified code. */ +/* + * This function forces the icache and prefetched instruction stream to + * catch up with reality in two very specific cases: + * + * a) Text was modified using one virtual address and is about to be executed + * from the same physical page at a different virtual address. + * + * b) Text was modified on a different CPU, may subsequently be + * executed on this CPU, and you want to make sure the new version + * gets executed. This generally means you're calling this in a IPI. + * + * If you're calling this for a different reason, you're probably doing + * it wrong. + */ static inline void sync_core(void) { - int tmp; - -#ifdef CONFIG_X86_32 /* - * Do a CPUID if available, otherwise do a jump. The jump - * can conveniently enough be the jump around CPUID. + * There are quite a few ways to do this. IRET-to-self is nice + * because it works on every CPU, at any CPL (so it's compatible + * with paravirtualization), and it never exits to a hypervisor. + * The only down sides are that it's a bit slow (it seems to be + * a bit more than 2x slower than the fastest options) and that + * it unmasks NMIs. The "push %cs" is needed because, in + * paravirtual environments, __KERNEL_CS may not be a valid CS + * value when we do IRET directly. + * + * In case NMI unmasking or performance every becomes a problem, + * the next best option appears to be MOV-to-CR2 and an + * unconditional jump. That sequence also works on all CPUs, + * but it will fault at CPL3. + * + * CPUID is the conventional way, but it's nasty: it doesn't + * exist on some 486-like CPUs, and it usually exits to a + * hypervisor. */ - asm volatile("cmpl %2,%1\n\t" - "jl 1f\n\t" - "cpuid\n" - "1:" - : "=a" (tmp) - : "rm" (boot_cpu_data.cpuid_level), "ri" (0), "0" (1) - : "ebx", "ecx", "edx", "memory"); + register void *__sp asm(_ASM_SP); + +#ifdef CONFIG_X86_32 + asm volatile ( + "pushfl\n\t" + "pushl %%cs\n\t" + "pushl $1f\n\t" + "iret\n\t" + "1:" + : "+r" (__sp) : : "cc", "memory"); #else - /* - * CPUID is a barrier to speculative execution. - * Prefetched instructions are automatically - * invalidated when modified. - */ - asm volatile("cpuid" - : "=a" (tmp) - : "0" (1) - : "ebx", "ecx", "edx", "memory"); + unsigned long tmp; + + asm volatile ( + "movq %%ss, %0\n\t" + "pushq %0\n\t" + "pushq %%rsp\n\t" + "addq $8, (%%rsp)\n\t" + "pushfq\n\t" + "movq %%cs, %0\n\t" + "pushq %0\n\t" + "pushq $1f\n\t" + "iretq\n\t" + "1:" + : "=r" (tmp), "+r" (__sp) : : "cc", "memory"); #endif } -- 2.9.3