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From: Len Brown <lenb@kernel.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: "Chang S. Bae" <chang.seok.bae@intel.com>,
	Borislav Petkov <bp@suse.de>, Andy Lutomirski <luto@kernel.org>,
	Ingo Molnar <mingo@kernel.org>, X86 ML <x86@kernel.org>,
	"Brown, Len" <len.brown@intel.com>,
	Dave Hansen <dave.hansen@intel.com>,
	"Liu, Jing2" <jing2.liu@intel.com>,
	"Ravi V. Shankar" <ravi.v.shankar@intel.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v4 14/22] x86/fpu/xstate: Expand the xstate buffer on the first use of dynamic user state
Date: Mon, 29 Mar 2021 12:06:38 -0400	[thread overview]
Message-ID: <CAJvTdKm4qKw8D8b+NokBsdiE5yBj=LTiH50VuxJY2aAL8qQq6g@mail.gmail.com> (raw)
In-Reply-To: <CAJvTdKnBRmogm6zF0KyDtx1VC_bpRa8_H1P9mxtMP06fy8a57g@mail.gmail.com>

On Mon, Mar 29, 2021 at 11:43 AM Len Brown <lenb@kernel.org> wrote:
>
> On Mon, Mar 29, 2021 at 9:33 AM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> > > I found the author of this passage, and he agreed to revise it to say this
> > > was targeted primarily at VMMs.
> >
> > Why would this only a problem for VMMs?
>
> VMMs may have to emulate different hardware for different guest OS's,
> and they would likely "context switch" XCR0 to achieve that.
>
> As switching XCR0 at run-time would confuse the heck out of user-space,
> it was not imagined that a bare-metal OS would do that.

to clarify...
*switching* XCR0 on context switch is slow, but perfectly legal.

*changing* XCR0 during the lifetime of a process, in any of its tasks,
on any of its CPUs, will confuse any software that uses xgetbv/XCR0
to calculate the size of XSAVE buffers for userspace threading.


-- 
Len Brown, Intel Open Source Technology Center

  reply	other threads:[~2021-03-29 16:07 UTC|newest]

Thread overview: 78+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-21 18:56 [PATCH v4 00/22] x86: Support Intel Advanced Matrix Extensions Chang S. Bae
2021-02-21 18:56 ` [PATCH v4 01/22] x86/fpu/xstate: Modify the initialization helper to handle both static and dynamic buffers Chang S. Bae
2021-03-10 13:40   ` Borislav Petkov
2021-02-21 18:56 ` [PATCH v4 02/22] x86/fpu/xstate: Modify state copy helpers " Chang S. Bae
2021-02-21 18:56 ` [PATCH v4 03/22] x86/fpu/xstate: Modify address finders " Chang S. Bae
2021-02-21 18:56 ` [PATCH v4 04/22] x86/fpu/xstate: Modify the context restore helper " Chang S. Bae
2021-02-21 18:56 ` [PATCH v4 05/22] x86/fpu/xstate: Add a new variable to indicate dynamic user states Chang S. Bae
2021-02-21 18:56 ` [PATCH v4 06/22] x86/fpu/xstate: Add new variables to indicate dynamic xstate buffer size Chang S. Bae
2021-02-21 18:56 ` [PATCH v4 07/22] x86/fpu/xstate: Calculate and remember dynamic xstate buffer sizes Chang S. Bae
2021-02-21 18:56 ` [PATCH v4 08/22] x86/fpu/xstate: Convert the struct fpu 'state' field to a pointer Chang S. Bae
2021-02-21 18:56 ` [PATCH v4 09/22] x86/fpu/xstate: Introduce helpers to manage the xstate buffer dynamically Chang S. Bae
2021-02-21 18:56 ` [PATCH v4 10/22] x86/fpu/xstate: Define the scope of the initial xstate data Chang S. Bae
2021-02-21 18:56 ` [PATCH v4 11/22] x86/fpu/xstate: Update the xstate save function to support dynamic states Chang S. Bae
2021-02-21 18:56 ` [PATCH v4 12/22] x86/fpu/xstate: Update the xstate buffer address finder " Chang S. Bae
2021-02-21 18:56 ` [PATCH v4 13/22] x86/fpu/xstate: Update the xstate context copy function " Chang S. Bae
2021-02-21 18:56 ` [PATCH v4 14/22] x86/fpu/xstate: Expand the xstate buffer on the first use of dynamic user state Chang S. Bae
2021-03-20 22:13   ` Thomas Gleixner
2021-03-20 22:21     ` Andy Lutomirski
2021-03-23 21:01       ` Len Brown
2021-03-24  3:14         ` Liu, Jing2
2021-03-24 21:09           ` Len Brown
2021-03-24 21:26             ` Andy Lutomirski
2021-03-24 21:30               ` Dave Hansen
2021-03-24 21:42                 ` Andy Lutomirski
2021-03-24 21:58                   ` Dave Hansen
2021-03-24 22:12                     ` Andy Lutomirski
2021-03-25  5:12             ` Liu, Jing2
2021-03-25  6:59               ` Bae, Chang Seok
2021-03-25  7:26                 ` Liu, Jing2
2021-03-23 21:52     ` Bae, Chang Seok
2021-03-24 14:24       ` Dave Hansen
2021-03-29 13:14     ` Len Brown
2021-03-29 13:33       ` Thomas Gleixner
2021-03-29 15:43         ` Len Brown
2021-03-29 16:06           ` Len Brown [this message]
2021-03-29 17:43             ` Andy Lutomirski
2021-03-29 18:57               ` Len Brown
2021-03-29 18:49           ` Thomas Gleixner
2021-03-29 22:16             ` Len Brown
2021-03-30  8:28               ` Thomas Gleixner
2021-03-30 16:38                 ` Len Brown
2021-03-26 16:34   ` Jann Horn
2021-03-29 18:14     ` Bae, Chang Seok
2021-02-21 18:56 ` [PATCH v4 15/22] x86/fpu/xstate: Support ptracer-induced xstate buffer expansion Chang S. Bae
2021-02-21 18:56 ` [PATCH v4 16/22] x86/fpu/xstate: Extend the table to map state components with features Chang S. Bae
2021-03-20 21:25   ` Thomas Gleixner
2021-03-23 21:52     ` Bae, Chang Seok
2021-02-21 18:56 ` [PATCH v4 17/22] x86/cpufeatures/amx: Enumerate Advanced Matrix Extension (AMX) feature bits Chang S. Bae
2021-02-21 18:56 ` [PATCH v4 18/22] x86/fpu/amx: Define AMX state components and have it used for boot-time checks Chang S. Bae
2021-03-20 21:31   ` Thomas Gleixner
2021-03-23 21:52     ` Bae, Chang Seok
2021-02-21 18:56 ` [PATCH v4 19/22] x86/fpu/amx: Enable the AMX feature in 64-bit mode Chang S. Bae
2021-03-20 21:26   ` Thomas Gleixner
2021-03-23 21:51     ` Bae, Chang Seok
2021-02-21 18:56 ` [PATCH v4 20/22] selftest/x86/amx: Include test cases for the AMX state management Chang S. Bae
2021-02-21 18:56 ` [PATCH v4 21/22] x86/fpu/xstate: Support dynamic user state in the signal handling path Chang S. Bae
2021-02-21 18:56 ` [PATCH v4 22/22] x86/fpu/xstate: Introduce boot-parameters to control state component support Chang S. Bae
2021-02-21 19:30   ` Randy Dunlap
2021-02-21 20:10     ` Bae, Chang Seok
2021-02-21 20:37       ` Randy Dunlap
2021-03-20 20:56   ` Thomas Gleixner
2021-03-25 22:59     ` Len Brown
2021-03-25 23:10       ` Dave Hansen
2021-03-26 15:27         ` Len Brown
2021-03-26 19:22           ` Thomas Gleixner
2021-03-26  1:41       ` Andy Lutomirski
2021-03-26 15:33         ` Len Brown
2021-03-26 15:48           ` Andy Lutomirski
2021-03-26 17:53             ` Len Brown
2021-03-26 18:12               ` Andy Lutomirski
2021-03-27  4:53                 ` Len Brown
2021-03-27 22:20                   ` Thomas Gleixner
2021-03-29 13:31                     ` Len Brown
2021-03-29 14:10                       ` Thomas Gleixner
2021-03-26 18:17               ` Borislav Petkov
2021-03-27  4:41                 ` Len Brown
2021-03-26  1:50       ` Thomas Gleixner
2021-03-26 15:36         ` Len Brown

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