On Mon, Nov 27, 2017 at 10:48:46AM +0100, Peter Zijlstra wrote: > On Mon, Nov 27, 2017 at 12:14:08AM +0100, Thomas Gleixner wrote: > > KAISER comes with overhead. The most expensive part is the CR3 switching in > > the entry code. > > > > Add a command line parameter which allows to disable KAISER at boot time. > > > > Most code pathes simply check a variable, but the entry code uses a static > > branch. The other code pathes cannot use a static branch because they are > > used before jump label patching is possible. Not an issue as the code > > pathes are not so performance sensitive as the entry/exit code. > > > > This makes KAISER depend on JUMP_LABEL and on a GCC which supports > > it, but that's a resonable requirement. > > > > The PGD allocation is still 8k when CONFIG_KAISER is enabled. This can be > > addressed on top of this. > > So in patch 15 Andy notes that we should probably also disable the > SYSCALL trampoline when we disable KAISER. > > https://lkml.kernel.org/r/20171124172411.19476-16-mingo@xxxxxxxxxx Could be a simple as this.. but I've not tested. diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c index f4f4ab8525bd..1be393a97421 100644 --- a/arch/x86/kernel/cpu/common.c +++ b/arch/x86/kernel/cpu/common.c @@ -1442,7 +1442,10 @@ void syscall_init(void) (entry_SYSCALL_64_trampoline - _entry_trampoline); wrmsr(MSR_STAR, 0, (__USER32_CS << 16) | __KERNEL_CS); - wrmsrl(MSR_LSTAR, SYSCALL64_entry_trampoline); + if (kaiser_enabled) + wrmsrl(MSR_LSTAR, SYSCALL64_entry_trampoline); + else + wrmsrl(MSR_LSTAR, (unsigned long)entry_SYSCALL_64); #ifdef CONFIG_IA32_EMULATION wrmsrl(MSR_CSTAR, (unsigned long)entry_SYSCALL_compat); -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
