From: Andrea Arcangeli <aarcange@redhat.com>
To: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
Thomas Gleixner <tglx@linutronix.de>,
Andy Lutomirski <luto@kernel.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Greg KH <gregkh@linuxfoundation.org>,
Dave Hansen <dave.hansen@intel.com>,
Andi Kleen <ak@linux.intel.com>,
Arjan Van De Ven <arjan.van.de.ven@intel.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH 6/7] x86/spec_ctrl: Add sysctl knobs to enable/disable SPEC_CTRL feature
Date: Fri, 5 Jan 2018 00:51:34 +0100 [thread overview]
Message-ID: <20180104235134.GC13348@redhat.com> (raw)
In-Reply-To: <ff9235b5-b215-9867-b154-fb760a699a52@linux.intel.com>
On Thu, Jan 04, 2018 at 03:26:52PM -0800, Tim Chen wrote:
> On 01/04/2018 02:54 PM, Peter Zijlstra wrote:
> > On Thu, Jan 04, 2018 at 09:56:47AM -0800, Tim Chen wrote:
> >> .macro ENABLE_IBRS
> >> - ALTERNATIVE "jmp 10f", "", X86_FEATURE_SPEC_CTRL
> >> + testl $SPEC_CTRL_IBRS_INUSE, spec_ctrl_ibrs
> >> + jz .Lskip_\@
> >> +
> >> PUSH_MSR_REGS
> >> WRMSR_ASM $MSR_IA32_SPEC_CTRL, $SPEC_CTRL_FEATURE_ENABLE_IBRS
> >> POP_MSR_REGS
> >> -10:
> >> +
> >> + jmp .Ldone_\@
> >> +.Lskip_\@:
> >> + /*
> >> + * prevent speculation beyond here as we could want to
> >> + * stop speculation by enabling IBRS
> >> + */
> >> + lfence
> >> +.Ldone_\@:
> >> .endm
> >
> >
> > Yeah no. We have jump labels for this stuff. There is no reason what so
> > ever to do dynamic tests for a variable that _never_ changes.
> >
>
> Admin can change spec_ctrl_ibrs value at run time, or when
> we scan new microcode. So it doesn't often change, but it could.
>
> There may be time when the admin wants to run the system
> in a more secure mode, and time when it is okay to leave out
> IBRS.
I think he meant to use STATIC_JUMP_IF_TRUE instead of testl
spec_ctrl_ibrs to avoid the conditional jump but still allow to
enable/disable the branch. I suggested using static key earlier too.
In older kernels there's not even the boilerplate to check the static
key in asm, so I used a pcp bitfield in a already guaranteed hot
cacheline so in practice it's as fast as static key but static key is
always theoretically preferable.
next prev parent reply other threads:[~2018-01-04 23:51 UTC|newest]
Thread overview: 114+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-01-04 17:56 [PATCH 0/7] IBRS patch series Tim Chen
2018-01-04 17:56 ` [PATCH 1/7] x86/feature: Detect the x86 feature to control Speculation Tim Chen
2018-01-04 19:58 ` Greg KH
2018-01-04 20:47 ` Tim Chen
2018-01-05 11:14 ` David Woodhouse
2018-01-05 15:14 ` Tom Lendacky
2018-01-05 17:07 ` Tim Chen
2018-01-05 13:09 ` Thomas Gleixner
2018-01-05 13:44 ` Andrea Arcangeli
2018-01-05 13:51 ` Thomas Gleixner
2018-01-04 17:56 ` [PATCH 2/7] x86/enter: MACROS to set/clear IBRS Tim Chen
2018-01-04 22:16 ` Peter Zijlstra
2018-01-04 22:21 ` Tim Chen
2018-01-04 22:23 ` Dave Hansen
2018-01-05 4:54 ` Andy Lutomirski
2018-01-05 5:05 ` Dave Hansen
2018-01-05 13:19 ` Thomas Gleixner
2018-01-04 17:56 ` [PATCH 3/7] x86/enter: Use IBRS on syscall and interrupts Tim Chen
2018-01-04 20:00 ` Greg KH
2018-01-04 20:26 ` Tim Chen
2018-01-04 20:45 ` Dave Hansen
2018-01-04 22:33 ` Peter Zijlstra
2018-01-04 23:12 ` Andrea Arcangeli
2018-01-05 0:08 ` Dave Hansen
2018-01-05 4:51 ` Andy Lutomirski
2018-01-05 5:11 ` Dave Hansen
2018-01-05 12:01 ` Alan Cox
2018-01-05 13:35 ` Thomas Gleixner
2018-01-04 17:56 ` [PATCH 4/7] x86/idle: Disable IBRS entering idle and enable it on wakeup Tim Chen
2018-01-04 22:47 ` Peter Zijlstra
2018-01-04 23:00 ` Andrea Arcangeli
2018-01-04 23:22 ` Tim Chen
2018-01-04 23:42 ` Andrea Arcangeli
2018-01-04 23:45 ` Thomas Gleixner
2018-01-05 0:03 ` Andrea Arcangeli
2018-01-08 8:24 ` Peter Zijlstra
2018-01-04 17:56 ` [PATCH 5/7] x86: Use IBRS for firmware update path Tim Chen
2018-01-04 18:48 ` Alan Cox
2018-01-04 20:05 ` Greg KH
2018-01-04 20:08 ` Woodhouse, David
2018-01-05 16:08 ` gregkh
2018-01-05 16:37 ` Andrea Arcangeli
2018-01-04 20:21 ` Andrew Cooper
2018-01-04 20:48 ` Andrea Arcangeli
2018-01-04 20:51 ` Yves-Alexis Perez
2018-01-04 21:13 ` Tim Chen
2018-01-04 22:51 ` Peter Zijlstra
2018-01-05 13:40 ` Thomas Gleixner
2018-01-04 17:56 ` [PATCH 6/7] x86/spec_ctrl: Add sysctl knobs to enable/disable SPEC_CTRL feature Tim Chen
2018-01-04 18:33 ` Borislav Petkov
2018-01-04 18:36 ` Dave Hansen
2018-01-04 18:52 ` Borislav Petkov
2018-01-04 18:57 ` Andrea Arcangeli
2018-01-04 18:59 ` Dave Hansen
2018-01-04 19:06 ` Borislav Petkov
2018-01-05 13:48 ` Thomas Gleixner
2018-01-04 18:38 ` Andrea Arcangeli
2018-01-04 18:54 ` Dave Hansen
2018-01-04 18:56 ` Borislav Petkov
2018-01-04 18:55 ` Borislav Petkov
2018-01-04 18:34 ` Andrea Arcangeli
2018-01-04 19:02 ` Tim Chen
2018-01-04 18:50 ` Alan Cox
2018-01-04 20:16 ` Greg KH
2018-01-04 20:58 ` Tim Chen
2018-01-04 22:54 ` Peter Zijlstra
2018-01-04 23:26 ` Tim Chen
2018-01-04 23:51 ` Andrea Arcangeli [this message]
2018-01-04 23:59 ` Borislav Petkov
2018-01-05 0:07 ` Thomas Gleixner
2018-01-05 11:16 ` David Woodhouse
2018-01-06 1:29 ` Tim Chen
2018-01-04 17:56 ` [PATCH 7/7] x86/microcode: Recheck IBRS features on microcode reload Tim Chen
2018-01-04 18:28 ` Borislav Petkov
2018-01-04 18:34 ` Andrea Arcangeli
2018-01-04 18:50 ` Borislav Petkov
2018-01-04 19:10 ` Tim Chen
2018-01-05 13:32 ` Greg KH
2018-01-05 13:37 ` Borislav Petkov
2018-01-05 13:47 ` Greg KH
2018-01-05 15:28 ` Andrea Arcangeli
2018-01-04 19:00 ` [PATCH 0/7] IBRS patch series Linus Torvalds
2018-01-04 19:19 ` David Woodhouse
2018-01-04 19:33 ` Linus Torvalds
2018-01-04 19:39 ` David Woodhouse
2018-01-04 19:40 ` Andrew Cooper
2018-01-04 19:46 ` David Woodhouse
2018-01-04 21:22 ` Van De Ven, Arjan
2018-01-05 11:32 ` Paolo Bonzini
2018-01-05 12:09 ` Paul Turner
2018-01-05 14:45 ` Van De Ven, Arjan
2018-01-05 14:43 ` Andrea Arcangeli
2018-01-05 14:52 ` Van De Ven, Arjan
2018-01-05 15:03 ` Andrea Arcangeli
2018-01-05 14:54 ` Thomas Gleixner
2018-01-05 11:52 ` Paul Turner
2018-01-05 14:28 ` David Woodhouse
2018-01-05 14:42 ` Van De Ven, Arjan
2018-01-05 15:38 ` David Woodhouse
2018-01-05 16:05 ` Andrea Arcangeli
2018-01-05 16:37 ` David Woodhouse
2018-01-05 16:42 ` Andrea Arcangeli
2018-01-05 16:44 ` Van De Ven, Arjan
2018-01-05 16:46 ` David Woodhouse
2018-01-05 5:25 ` Florian Weimer
2018-01-05 11:05 ` David Woodhouse
2018-01-04 19:05 ` Justin Forbes
2018-01-04 19:10 ` Tim Chen
2018-01-04 21:01 ` Yves-Alexis Perez
2018-01-05 13:28 ` Greg KH
2018-01-05 13:47 ` Yves-Alexis Perez
2018-01-05 14:01 ` Greg KH
2018-01-05 14:26 ` Paolo Bonzini
2018-01-05 14:54 ` Yves-Alexis Perez
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