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From: "Bae, Chang Seok" <chang.seok.bae@intel.com>
To: Andy Lutomirski <luto@kernel.org>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Juergen Gross <jgross@suse.com>,
	xen-devel <xen-devel@lists.xenproject.org>,
	Ingo Molnar <mingo@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	"H. Peter Anvin" <hpa@zytor.com>, Andi Kleen <ak@linux.intel.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"Metzger, Markus T" <markus.t.metzger@intel.com>,
	"Shankar, Ravi V" <ravi.v.shankar@intel.com>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [v3 04/12] x86/fsgsbase/64: Enable FSGSBASE instructions in the helper functions
Date: Thu, 25 Oct 2018 07:32:27 +0000	[thread overview]
Message-ID: <1EE75BE7-12C6-4BD9-9D31-E9C396D2015B@intel.com> (raw)
In-Reply-To: <CALCETrVcJ08c6yf6y3ruOA_Hx_7srkHw5iYfx1c2p8_ySiL2WA@mail.gmail.com>


> On Oct 24, 2018, at 12:16, Andy Lutomirski <luto@kernel.org> wrote:
> 
> On Tue, Oct 23, 2018 at 11:43 AM Chang S. Bae <chang.seok.bae@intel.com> wrote:
>> void x86_fsbase_write_cpu(unsigned long fsbase)
>> {
>> -       /*
>> -        * Set the selector to 0 as a notion, that the segment base is
>> -        * overwritten, which will be checked for skipping the segment load
>> -        * during context switch.
>> -        */
>> -       loadseg(FS, 0);
>> -       wrmsrl(MSR_FS_BASE, fsbase);
>> +       if (static_cpu_has(X86_FEATURE_FSGSBASE)) {
>> +               wrfsbase(fsbase);
>> +       } else {
>> +               /*
>> +                * Set the selector to 0 as a notion, that the segment base is
>> +                * overwritten, which will be checked for skipping the segment load
>> +                * during context switch.
>> +                */
>> +               loadseg(FS, 0);
>> +               wrmsrl(MSR_FS_BASE, fsbase);
>> +       }
>> }
>> 
>> void x86_gsbase_write_cpu_inactive(unsigned long gsbase)
>> {
>> -       /* Set the selector to 0 for the same reason as %fs above. */
>> -       loadseg(GS, 0);
>> -       wrmsrl(MSR_KERNEL_GS_BASE, gsbase);
>> +       if (static_cpu_has(X86_FEATURE_FSGSBASE)) {
>> +               wr_inactive_gsbase(gsbase);
>> +       } else {
>> +               /* Set the selector to 0 for the same reason as %fs above. */
>> +               loadseg(GS, 0);
>> +               wrmsrl(MSR_KERNEL_GS_BASE, gsbase);
> 
> I still don't get what this code is trying to do.  See other email.  I
> think it will straight up crash the kernel on some CPUs, since writing
> 0 to %%gs will zero out the *active* base on some CPUs.
> 

On those CPUs, how the old do_arch_prctl_64() worked?
loadseg(GS, 0) eventually hits the native_load_gs_index entry, where actual
mov …, %gs is wrapped by two SWAPGSes. So, it won’t cause the side effect
of overwriting the *active* base, I think.

> I think that, if you really want some fancy optimization for the
> non-FSGSBASE case, you need to pull that out into the callers of these
> helpers.


  parent reply	other threads:[~2018-10-25  7:32 UTC|newest]

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-10-23 18:42 [v3 00/12] x86: Enable FSGSBASE instructions Chang S. Bae
2018-10-23 18:42 ` [v3 01/12] taint: Introduce a new taint flag (insecure) Chang S. Bae
2018-10-24 18:50   ` Andy Lutomirski
2018-10-23 18:42 ` [v3 02/12] x86/fsgsbase/64: Add 'unsafe_fsgsbase' to enable CR4.FSGSBASE Chang S. Bae
2018-10-24 18:51   ` Andy Lutomirski
2018-10-23 18:42 ` [v3 03/12] x86/fsgsbase/64: Add intrinsics/macros for FSGSBASE instructions Chang S. Bae
2018-10-24 18:53   ` Andy Lutomirski
2018-10-24 19:21     ` Andi Kleen
2018-10-25 23:14       ` Andy Lutomirski
2018-10-25 23:31         ` Linus Torvalds
2018-10-26  0:09           ` Andy Lutomirski
2018-10-23 18:42 ` [v3 04/12] x86/fsgsbase/64: Enable FSGSBASE instructions in the helper functions Chang S. Bae
2018-10-24 19:16   ` Andy Lutomirski
2018-10-24 19:41     ` [Xen-devel] " Andrew Cooper
2018-10-25  6:09       ` Juergen Gross
2018-10-25 23:08         ` Andrew Cooper
2018-10-25 23:11           ` Andy Lutomirski
2018-10-25 23:14             ` Andrew Cooper
2018-10-25  7:32     ` Bae, Chang Seok [this message]
2018-10-25 23:00       ` Andy Lutomirski
2018-10-25 23:03         ` Bae, Chang Seok
2018-10-25 23:16     ` Andy Lutomirski
2018-10-23 18:42 ` [v3 05/12] x86/fsgsbase/64: Preserve FS/GS state in __switch_to() if FSGSBASE is on Chang S. Bae
2018-10-24 19:21   ` Andy Lutomirski
2018-10-24 19:36     ` Bae, Chang Seok
2018-10-23 18:42 ` [v3 06/12] x86/fsgsbase/64: When copying a thread, use the FSGSBASE instructions if available Chang S. Bae
2018-10-23 18:42 ` [v3 07/12] x86/fsgsbase/64: Introduce the new FIND_PERCPU_BASE macro Chang S. Bae
2018-10-26  0:25   ` Andy Lutomirski
2018-10-26  0:59     ` Nadav Amit
2018-10-23 18:42 ` [v3 08/12] x86/fsgsbase/64: Use the per-CPU base as GSBASE at the paranoid_entry Chang S. Bae
2018-10-23 18:42 ` [v3 09/12] selftests/x86/fsgsbase: Test WRGSBASE Chang S. Bae
2018-10-23 18:42 ` [v3 10/12] x86/fsgsbase/64: Enable FSGSBASE by default and add a chicken bit Chang S. Bae
2018-10-23 18:42 ` [v3 11/12] x86/elf: Enumerate kernel FSGSBASE capability in AT_HWCAP2 Chang S. Bae
2018-10-23 18:42 ` [v3 12/12] x86/fsgsbase/64: Add documentation for FSGSBASE Chang S. Bae

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