Re: [patch V2 5/5] x86/kaiser: Add boottime disable switch

[Peter Zijlstra]

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);