From: Andi Kleen <a...@linux.intel.com> Convert arch_prctl to use the new instructions to change fs/gs if available, instead of using MSRs.
This is merely a small performance optimization, no new functionality. With the new instructions the syscall is really obsolete, as everything can be set directly in ring 3. But the syscall is widely used by existing software, so we still support it. The syscall still enforces that the addresses are not in kernel space, even though that is not needed more. This is mainly so that the programs written for new CPUs do not suddenly fail on old CPUs. With the new instructions available it prefers to use them in the context switch, instead of using the old "use GDT segment rewrite" trick. Signed-off-by: Andi Kleen <a...@linux.intel.com> --- arch/x86/kernel/process_64.c | 45 ++++++++++++++++++++++++++++++++++++-------- 1 file changed, 37 insertions(+), 8 deletions(-) diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c index 9bad75a..7669b3b 100644 --- a/arch/x86/kernel/process_64.c +++ b/arch/x86/kernel/process_64.c @@ -486,15 +486,23 @@ long do_arch_prctl(struct task_struct *task, int code, unsigned long addr) int ret = 0; int doit = task == current; int cpu; + int fast_seg = boot_cpu_has(X86_FEATURE_FSGSBASE); switch (code) { case ARCH_SET_GS: + /* + * With fast_seg we don't need that check anymore, + * but keep it so that programs do not suddenly + * start failing when run on older CPUs. + * If you really want to set a address in kernel space + * use WRGSBASE directly. + */ if (addr >= TASK_SIZE_OF(task)) return -EPERM; cpu = get_cpu(); /* handle small bases via the GDT because that's faster to switch. */ - if (addr <= 0xffffffff) { + if (addr <= 0xffffffff && !fast_seg) { set_32bit_tls(task, GS_TLS, addr); if (doit) { load_TLS(&task->thread, cpu); @@ -506,8 +514,17 @@ long do_arch_prctl(struct task_struct *task, int code, unsigned long addr) task->thread.gsindex = 0; task->thread.gs = addr; if (doit) { - load_gs_index(0); - ret = wrmsrl_safe(MSR_KERNEL_GS_BASE, addr); + if (fast_seg) { + local_irq_disable(); + swapgs(); + loadsegment(gs, 0); + wrgsbase(addr); + swapgs(); + local_irq_enable(); + } else { + load_gs_index(0); + ret = wrmsrl_safe(MSR_KERNEL_GS_BASE, addr); + } } } put_cpu(); @@ -520,7 +537,7 @@ long do_arch_prctl(struct task_struct *task, int code, unsigned long addr) cpu = get_cpu(); /* handle small bases via the GDT because that's faster to switch. */ - if (addr <= 0xffffffff) { + if (addr <= 0xffffffff && !fast_seg) { set_32bit_tls(task, FS_TLS, addr); if (doit) { load_TLS(&task->thread, cpu); @@ -535,7 +552,10 @@ long do_arch_prctl(struct task_struct *task, int code, unsigned long addr) /* set the selector to 0 to not confuse __switch_to */ loadsegment(fs, 0); - ret = wrmsrl_safe(MSR_FS_BASE, addr); + if (fast_seg) + wrfsbase(addr); + else + ret = wrmsrl_safe(MSR_FS_BASE, addr); } } put_cpu(); @@ -544,6 +564,8 @@ long do_arch_prctl(struct task_struct *task, int code, unsigned long addr) unsigned long base; if (task->thread.fsindex == FS_TLS_SEL) base = read_32bit_tls(task, FS_TLS); + else if (doit && fast_seg) + base = rdfsbase(); else if (doit) rdmsrl(MSR_FS_BASE, base); else @@ -558,9 +580,16 @@ long do_arch_prctl(struct task_struct *task, int code, unsigned long addr) base = read_32bit_tls(task, GS_TLS); else if (doit) { savesegment(gs, gsindex); - if (gsindex) - rdmsrl(MSR_KERNEL_GS_BASE, base); - else + if (gsindex) { + if (fast_seg) { + local_irq_disable(); + swapgs(); + base = rdgsbase(); + swapgs(); + local_irq_enable(); + } else + rdmsrl(MSR_KERNEL_GS_BASE, base); + } else base = task->thread.gs; } else base = task->thread.gs; -- 1.9.3 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/