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

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Message-ID: <20171127102241.oj225ycxkc7rfvft@hirez.programming.kicks-ass.net>
Date:   Mon, 27 Nov 2017 11:22:41 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Rik van Riel <riel@...hat.com>, daniel.gruss@...k.tugraz.at,
        hughd@...gle.com, keescook@...gle.com, linux-mm@...ck.org,
        michael.schwarz@...k.tugraz.at, moritz.lipp@...k.tugraz.at,
        richard.fellner@...dent.tugraz.at
Subject: Re: [patch V2 5/5] x86/kaiser: Add boottime disable switch

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@kernel.org

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);