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From: Thomas Gleixner <tglx@linutronix.de>
To: "Chang S. Bae" <chang.seok.bae@intel.com>
Cc: Andy Lutomirski <luto@kernel.org>, Ingo Molnar <mingo@kernel.org>,
	"H . Peter Anvin" <hpa@zytor.com>,
	Andi Kleen <ak@linux.intel.com>,
	Ravi Shankar <ravi.v.shankar@intel.com>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v7 07/18] x86/fsgsbase/64: Preserve FS/GS state in __switch_to() if FSGSBASE is on
Date: Fri, 21 Jun 2019 17:22:09 +0200 (CEST)	[thread overview]
Message-ID: <alpine.DEB.2.21.1906211717500.5503@nanos.tec.linutronix.de> (raw)
In-Reply-To: <1557309753-24073-8-git-send-email-chang.seok.bae@intel.com>

On Wed, 8 May 2019, Chang S. Bae wrote:
> From: Andy Lutomirski <luto@kernel.org>
> 
> With the new FSGSBASE instructions, we can efficiently read and write
> the FSBASE and GSBASE in __switch_to().  Use that capability to preserve
> the full state.
> 
> This will enable user code to do whatever it wants with the new
> instructions without any kernel-induced gotchas.  (There can still be
> architectural gotchas: movl %gs,%eax; movl %eax,%gs may change GSBASE
> if WRGSBASE was used, but users are expected to read the CPU manual
> before doing things like that.)
> 
> This is a considerable speedup.  It seems to save about 100 cycles
> per context switch compared to the baseline 4.6-rc1 behavior on my
> Skylake laptop.
> 
> [ chang: 5~10% performance improvements were seen by a context switch
>   benchmark that ran threads with different FS/GSBASE values (to the
>   baseline 4.16). Minor edit on the changelog. ]
> 
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
> Reviewed-by: Andi Kleen <ak@linux.intel.com>
> Cc: H. Peter Anvin <hpa@zytor.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Ingo Molnar <mingo@kernel.org>
> ---
>  arch/x86/kernel/process_64.c | 34 ++++++++++++++++++++++++++++------
>  1 file changed, 28 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
> index 17421c3..e2089c9 100644
> --- a/arch/x86/kernel/process_64.c
> +++ b/arch/x86/kernel/process_64.c
> @@ -239,8 +239,18 @@ static __always_inline void save_fsgs(struct task_struct *task)
>  {
>  	savesegment(fs, task->thread.fsindex);
>  	savesegment(gs, task->thread.gsindex);
> -	save_base_legacy(task, task->thread.fsindex, FS);
> -	save_base_legacy(task, task->thread.gsindex, GS);
> +	if (static_cpu_has(X86_FEATURE_FSGSBASE)) {
> +		/*
> +		 * If FSGSBASE is enabled, we can't make any useful guesses
> +		 * about the base, and user code expects us to save the current
> +		 * value.  Fortunately, reading the base directly is efficient.
> +		 */
> +		task->thread.fsbase = rdfsbase();
> +		task->thread.gsbase = __rdgsbase_inactive();

This explodes when called from copy_thread_tls() when an interrupt hits
after switching GS in __rdgsbase_inactive(). Wrapped it with
local_irq_save/restore().

Oh well.

Thanks,

	tglx

  reply	other threads:[~2019-06-21 15:22 UTC|newest]

Thread overview: 63+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-08 10:02 [PATCH v7 00/18] x86: Enable FSGSBASE instructions Chang S. Bae
2019-05-08 10:02 ` [PATCH v7 01/18] x86/fsgsbase/64: Fix ARCH_SET_FS/GS behaviors for a remote task Chang S. Bae
     [not found]   ` <74F4F506-2913-4013-9D81-A0C69FA8CDF1@intel.com>
     [not found]     ` <6420E1A5-B5AD-4028-AA91-AA4D5445AC83@intel.com>
2019-06-16 15:44       ` Bae, Chang Seok
2019-06-16 16:32         ` Thomas Gleixner
2019-06-22 10:04         ` [tip:x86/cpu] x86/ptrace: Prevent ptrace from clearing the FS/GS selector tip-bot for Chang S. Bae
2020-06-18 13:50         ` [tip: x86/fsgsbase] " tip-bot2 for Chang S. Bae
2019-05-08 10:02 ` [PATCH v7 02/18] selftests/x86/fsgsbase: Test ptracer-induced GSBASE write Chang S. Bae
2019-06-22 10:04   ` [tip:x86/cpu] " tip-bot for Chang S. Bae
2019-05-08 10:02 ` [PATCH v7 03/18] x86/fsgsbase/64: Add 'unsafe_fsgsbase' to enable CR4.FSGSBASE Chang S. Bae
2019-06-22 10:05   ` [tip:x86/cpu] x86/cpu: " tip-bot for Andy Lutomirski
2020-06-18 13:50   ` [tip: x86/fsgsbase] " tip-bot2 for Andy Lutomirski
2019-05-08 10:02 ` [PATCH v7 04/18] kbuild: Raise the minimum required binutils version to 2.21 Chang S. Bae
2019-06-22 10:06   ` [tip:x86/cpu] " tip-bot for Chang S. Bae
2019-05-08 10:02 ` [PATCH v7 05/18] x86/fsgsbase/64: Add intrinsics for FSGSBASE instructions Chang S. Bae
2019-06-22 10:06   ` [tip:x86/cpu] " tip-bot for Andi Kleen
2020-06-18 13:50   ` [tip: x86/fsgsbase] " tip-bot2 for Andi Kleen
2019-05-08 10:02 ` [PATCH v7 06/18] x86/fsgsbase/64: Enable FSGSBASE instructions in the helper functions Chang S. Bae
2019-06-22 10:07   ` [tip:x86/cpu] x86/fsgsbase/64: Enable FSGSBASE instructions in " tip-bot for Chang S. Bae
2020-06-18 13:50   ` [tip: x86/fsgsbase] " tip-bot2 for Chang S. Bae
2019-05-08 10:02 ` [PATCH v7 07/18] x86/fsgsbase/64: Preserve FS/GS state in __switch_to() if FSGSBASE is on Chang S. Bae
2019-06-21 15:22   ` Thomas Gleixner [this message]
2019-06-22 10:08   ` [tip:x86/cpu] x86/process/64: Use FSBSBASE in switch_to() if available tip-bot for Andy Lutomirski
2020-06-18 13:50   ` [tip: x86/fsgsbase] " tip-bot2 for Andy Lutomirski
2019-05-08 10:02 ` [PATCH v7 08/18] x86/fsgsbase/64: When copying a thread, use the FSGSBASE instructions Chang S. Bae
2019-06-22 10:09   ` [tip:x86/cpu] x86/process/64: Use FSGSBASE instructions on thread copy and ptrace tip-bot for Chang S. Bae
2020-06-18 13:50   ` [tip: x86/fsgsbase] " tip-bot2 for Chang S. Bae
2019-05-08 10:02 ` [PATCH v7 09/18] x86/entry/64: Add the READ_MSR_GSBASE macro Chang S. Bae
2019-05-08 10:02 ` [PATCH v7 10/18] x86/entry/64: Switch CR3 before SWAPGS on the paranoid entry Chang S. Bae
2019-06-22 10:09   ` [tip:x86/cpu] x86/entry/64: Switch CR3 before SWAPGS in " tip-bot for Chang S. Bae
2020-06-18 13:50   ` [tip: x86/fsgsbase] " tip-bot2 for Chang S. Bae
2019-05-08 10:02 ` [PATCH v7 11/18] x86/fsgsbase/64: Introduce the FIND_PERCPU_BASE macro Chang S. Bae
2019-06-22 10:10   ` [tip:x86/cpu] x86/entry/64: " tip-bot for Chang S. Bae
2020-06-18 13:50   ` [tip: x86/fsgsbase] " tip-bot2 for Chang S. Bae
2019-05-08 10:02 ` [PATCH v7 12/18] x86/fsgsbase/64: GSBASE handling with FSGSBASE in the paranoid path Chang S. Bae
2019-06-22 10:11   ` [tip:x86/cpu] x86/entry/64: Handle FSGSBASE enabled paranoid entry/exit tip-bot for Chang S. Bae
2019-06-29  7:21   ` [PATCH v7 12/18] x86/fsgsbase/64: GSBASE handling with FSGSBASE in the paranoid path Bae, Chang Seok
2019-06-29  7:37     ` Thomas Gleixner
2020-06-18 13:50   ` [tip: x86/fsgsbase] x86/entry/64: Handle FSGSBASE enabled paranoid entry/exit tip-bot2 for Chang S. Bae
2019-05-08 10:02 ` [PATCH v7 13/18] x86/fsgsbase/64: Document GSBASE handling in the paranoid path Chang S. Bae
2019-06-22 10:11   ` [tip:x86/cpu] x86/entry/64: " tip-bot for Chang S. Bae
2019-05-08 10:02 ` [PATCH v7 14/18] selftests/x86/fsgsbase: Test WRGSBASE Chang S. Bae
2019-06-22 10:12   ` [tip:x86/cpu] selftests/x86/fsgsbase: Test RD/WRGSBASE tip-bot for Andy Lutomirski
2019-05-08 10:02 ` [PATCH v7 15/18] selftests/x86/fsgsbase: Test ptracer-induced GSBASE write with FSGSBASE Chang S. Bae
2019-06-22 10:13   ` [tip:x86/cpu] " tip-bot for Chang S. Bae
2019-05-08 10:02 ` [PATCH v7 16/18] x86/fsgsbase/64: Enable FSGSBASE by default and add a chicken bit Chang S. Bae
2019-06-22 10:14   ` [tip:x86/cpu] x86/cpu: Enable FSGSBASE on 64bit " tip-bot for Andy Lutomirski
2020-06-18 13:50   ` [tip: x86/fsgsbase] " tip-bot2 for Andy Lutomirski
2019-05-08 10:02 ` [PATCH v7 17/18] x86/elf: Enumerate kernel FSGSBASE capability in AT_HWCAP2 Chang S. Bae
2019-06-22 10:14   ` [tip:x86/cpu] " tip-bot for Andi Kleen
2020-06-18 13:50   ` [tip: x86/fsgsbase] " tip-bot2 for Andi Kleen
2019-05-08 10:02 ` [PATCH v7 18/18] x86/fsgsbase/64: Add documentation for FSGSBASE Chang S. Bae
2019-06-14  6:54   ` Thomas Gleixner
2019-06-14 20:07     ` Bae, Chang Seok
     [not found]       ` <89BE934A-A392-4CED-83E5-CA4FADDAE6DF@intel.com>
2019-06-16  8:39         ` Thomas Gleixner
2019-06-16 12:34           ` Thomas Gleixner
2019-06-16 15:34             ` Bae, Chang Seok
2019-06-16 16:05               ` Thomas Gleixner
2019-06-16 20:48                 ` Bae, Chang Seok
2019-06-16 22:00                   ` Thomas Gleixner
     [not found]                     ` <8E2E84B6-BCCC-424D-A1A7-604828B389FB@intel.com>
2019-06-17  5:18                       ` Thomas Gleixner
2019-06-16 15:54     ` Randy Dunlap
2019-06-16 16:07       ` Thomas Gleixner
2019-06-22 10:15     ` [tip:x86/cpu] Documentation/x86/64: Add documentation for GS/FS addressing mode tip-bot for Thomas Gleixner

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