This is a note to let you know that I've just added the patch titled x86/asm: Rewrite sync_core() to use IRET-to-self
to the 4.9-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: x86-asm-rewrite-sync_core-to-use-iret-to-self.patch and it can be found in the queue-4.9 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <sta...@vger.kernel.org> know about it. From c198b121b1a1d7a7171770c634cd49191bac4477 Mon Sep 17 00:00:00 2001 From: Andy Lutomirski <l...@kernel.org> Date: Fri, 9 Dec 2016 10:24:08 -0800 Subject: x86/asm: Rewrite sync_core() to use IRET-to-self From: Andy Lutomirski <l...@kernel.org> commit c198b121b1a1d7a7171770c634cd49191bac4477 upstream. 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. Signed-off-by: Andy Lutomirski <l...@kernel.org> Cc: Juergen Gross <jgr...@suse.com> Cc: One Thousand Gnomes <gno...@lxorguk.ukuu.org.uk> Cc: Peter Zijlstra <pet...@infradead.org> Cc: Brian Gerst <brge...@gmail.com> Cc: Matthew Whitehead <tedheads...@gmail.com> Cc: Borislav Petkov <b...@alien8.de> Cc: Henrique de Moraes Holschuh <h...@hmh.eng.br> Cc: Andrew Cooper <andrew.coop...@citrix.com> Cc: Boris Ostrovsky <boris.ostrov...@oracle.com> Cc: xen-devel <xen-de...@lists.xen.org> Link: http://lkml.kernel.org/r/5c79f0225f68bc8c40335612bf624511abb78941.1481307769.git.l...@kernel.org Signed-off-by: Thomas Gleixner <t...@linutronix.de> Cc: Evalds Iodzevics <evalds.iodzev...@gmail.com> Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org> --- arch/x86/include/asm/processor.h | 80 ++++++++++++++++++++++++++++----------- 1 file changed, 58 insertions(+), 22 deletions(-) --- a/arch/x86/include/asm/processor.h +++ b/arch/x86/include/asm/processor.h @@ -596,33 +596,69 @@ static __always_inline void cpu_relax(vo #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 ever 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 (i.e. Xen PV and lguest). + * + * 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. + * + * Like all of Linux's memory ordering operations, this is a + * compiler barrier as well. */ - 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) : : "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 int tmp; + + asm volatile ( + "mov %%ss, %0\n\t" + "pushq %q0\n\t" + "pushq %%rsp\n\t" + "addq $8, (%%rsp)\n\t" + "pushfq\n\t" + "mov %%cs, %0\n\t" + "pushq %q0\n\t" + "pushq $1f\n\t" + "iretq\n\t" + "1:" + : "=&r" (tmp), "+r" (__sp) : : "cc", "memory"); #endif } Patches currently in stable-queue which might be from l...@kernel.org are queue-4.9/cpu-hotplug-prevent-crash-when-cpu-bringup-fails-on-config_hotplug_cpu-n.patch queue-4.9/x86-smp-enforce-config_hotplug_cpu-when-smp-y.patch queue-4.9/x86-asm-rewrite-sync_core-to-use-iret-to-self.patch _______________________________________________ Xen-devel mailing list Xen-devel@lists.xenproject.org https://lists.xenproject.org/mailman/listinfo/xen-devel