On Thu, Apr 7, 2016 at 5:31 PM, Andy Lutomirski <l...@kernel.org> wrote: > From: Borislav Petkov <b...@suse.de> > > Erratum 88 affects old AMD K8s, where a SWAPGS fails to cause an input > dependency on GS. Therefore, we need to MFENCE before it. > > But that MFENCE is expensive and unnecessary on the remaining x86 CPUs > out there so patch it out on the CPUs which don't require it.
This is basically identical to: https://lkml.kernel.org/g/1458576969-13309-4-git-send-email-a...@firstfloor.org Whoops! I thought I'd seen that somewhere but I couldn't spot it. Ingo, etc: we should probably apply one of those patches with a -stable tag (to mitigate the otherwise potentially unpleasant performance regression in here), but I don't really care which one. Andi's has a name for the bug that seems nicer by one character to me, but it would have to be (trivally) rebased. --Andy > > Signed-off-by: Borislav Petkov <b...@suse.de> > Cc: <sta...@vger.kernel.org> > Signed-off-by: Andy Lutomirski <l...@kernel.org > --- > arch/x86/entry/entry_64.S | 2 +- > arch/x86/include/asm/cpufeatures.h | 2 ++ > arch/x86/kernel/cpu/amd.c | 1 + > 3 files changed, 4 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S > index 858b555e274b..64d2033d1e49 100644 > --- a/arch/x86/entry/entry_64.S > +++ b/arch/x86/entry/entry_64.S > @@ -783,7 +783,7 @@ ENTRY(native_load_gs_index) > SWAPGS > gs_change: > movl %edi, %gs > -2: mfence /* workaround */ > +2: ALTERNATIVE "", "mfence", X86_BUG_SWAPGS_FENCE > SWAPGS > popfq > ret > diff --git a/arch/x86/include/asm/cpufeatures.h > b/arch/x86/include/asm/cpufeatures.h > index 2a052302bc43..7bfb6b70c745 100644 > --- a/arch/x86/include/asm/cpufeatures.h > +++ b/arch/x86/include/asm/cpufeatures.h > @@ -295,6 +295,8 @@ > #define X86_BUG_CLFLUSH_MONITOR X86_BUG(7) /* AAI65, CLFLUSH required > before MONITOR */ > #define X86_BUG_SYSRET_SS_ATTRS X86_BUG(8) /* SYSRET doesn't fix up > SS attrs */ > #define X86_BUG_NULL_SEG X86_BUG(9) /* Nulling a selector preserves > the base */ > +#define X86_BUG_SWAPGS_FENCE X86_BUG(10) /* SWAPGS without input dep on GS > */ > + > > #ifdef CONFIG_X86_32 > /* > diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c > index 6e47e3a916f1..b7cc9efe08b5 100644 > --- a/arch/x86/kernel/cpu/amd.c > +++ b/arch/x86/kernel/cpu/amd.c > @@ -632,6 +632,7 @@ static void init_amd_k8(struct cpuinfo_x86 *c) > */ > msr_set_bit(MSR_K7_HWCR, 6); > #endif > + set_cpu_bug(c, X86_BUG_SWAPGS_FENCE); > } > > static void init_amd_gh(struct cpuinfo_x86 *c) > -- > 2.5.5 > -- Andy Lutomirski AMA Capital Management, LLC
