From: Andy Lutomirski <luto@kernel.org>
To: Len Brown <lenb@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>,
"Chang S. Bae" <chang.seok.bae@intel.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@kernel.org>, Borislav Petkov <bp@suse.de>,
X86 ML <x86@kernel.org>, Len Brown <len.brown@intel.com>,
Dave Hansen <dave.hansen@intel.com>,
"Liu, Jing2" <jing2.liu@intel.com>,
"Ravi V. Shankar" <ravi.v.shankar@intel.com>,
LKML <linux-kernel@vger.kernel.org>,
"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>
Subject: Re: [PATCH v2 22/22] x86/fpu/xstate: Introduce boot-parameters for control some state component support
Date: Tue, 24 Nov 2020 15:41:16 -0800 [thread overview]
Message-ID: <CALCETrX_1RkatiJxScjXhA38KTHvgrDxtj_e1YUs6fLTeHZufw@mail.gmail.com> (raw)
In-Reply-To: <CAJvTdKng_WMrmO_KtkuCxvvzGZ_HLcd84u2SwJiD_UJivb4Yaw@mail.gmail.com>
> On Nov 24, 2020, at 10:51 AM, Len Brown <lenb@kernel.org> wrote:
>
> On Fri, Nov 20, 2020 at 12:03 AM Andy Lutomirski <luto@kernel.org> wrote:
>>
>>> On Thu, Nov 19, 2020 at 3:37 PM Chang S. Bae <chang.seok.bae@intel.com> wrote:
>>> "xstate.enable=0x60000" will enable AMX on a system that does NOT have AMX
>>> compiled into XFEATURE_MASK_USER_ENABLED (assuming the kernel is new enough
>>> to support this feature).
>>>
>>
>> What's the purpose of xstate.enable? I can't really imagine it's
>> useful for AMX. I suppose it could be useful for hypothetical
>> post-AMX features, but that sounds extremely dangerous. Intel has
>> changed its strategy so many times on XSTATE extensibility that I find
>> it quite hard to believe that supporting unknown states is wise.
>
> Not hypothetical -- there are subsequent hardware features coming that
> will use the same
> exact XSTATE support that this series puts in place for AMX.
>
> We know that when those features ship in new hardware, there will be
> a set of customers who want to exercise those features immediately,
> but their kernel binary has not yet been re-compiled to see those
> features by-default.
>
> The purpose of "xstate.enable" is to empower those users to be able to
> explicitly enable support using their existing binary.
>
> You are right -- the feature isn't needed to enable AMX, unless somebody went to
> the trouble of building a kernel with the AMX source update, but chose
> to disable
> AMX-specific recognition, by-default.
>
>
We may want to taint the kernel if one of these flags is used because,
frankly, Intel’s track record is poor. Suppose we get a new feature
with PKRU-like semantics -- switching it blindly using
XSAVE(C,S,OPT,whatever) would simply incorrect. And XFD itself has
problems — supposedly it’s difficult or impossible to virtualize. It
wouldn’t utterly shock me if Intel were to drop IA32_XFD_ERR and
replace it with a new mechanism that’s less janky.
So this whole thing makes me quite nervous.
(I'm not a virtualization expert, but AIUI IA32_XFD_ERR has some
issues. If it's too late to fix those issues, Intel could probably
get away with completely dropping IA32_XFD_ERR from the spec -- OSes
can handle AMX just fine without it. Then the next XFD-able feature
could introduce a new improved way of reporting which feature
triggered #NM.)
next prev parent reply other threads:[~2020-11-24 23:41 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-19 23:32 [PATCH v2 00/22] x86: Support Intel Advanced Matrix Extensions Chang S. Bae
2020-11-19 23:32 ` [PATCH v2 01/22] x86/fpu/xstate: Modify area init helper prototypes to access all the possible areas Chang S. Bae
2020-12-07 17:12 ` Borislav Petkov
2020-12-07 23:03 ` Bae, Chang Seok
2020-12-07 23:10 ` Borislav Petkov
2020-11-19 23:32 ` [PATCH v2 02/22] x86/fpu/xstate: Modify xstate copy " Chang S. Bae
2020-11-19 23:32 ` [PATCH v2 03/22] x86/fpu/xstate: Modify address finder " Chang S. Bae
2020-11-19 23:32 ` [PATCH v2 04/22] x86/fpu/xstate: Modify save and restore helper " Chang S. Bae
2020-11-19 23:32 ` [PATCH v2 05/22] x86/fpu/xstate: Introduce a new variable for dynamic user states Chang S. Bae
2020-11-19 23:32 ` [PATCH v2 06/22] x86/fpu/xstate: Outline dynamic xstate area size in the task context Chang S. Bae
2020-11-19 23:32 ` [PATCH v2 07/22] x86/fpu/xstate: Introduce helpers to manage an xstate area dynamically Chang S. Bae
2020-11-19 23:32 ` [PATCH v2 08/22] x86/fpu/xstate: Define the scope of the initial xstate data Chang S. Bae
2020-11-19 23:32 ` [PATCH v2 09/22] x86/fpu/xstate: Introduce wrapper functions for organizing xstate area access Chang S. Bae
2020-11-19 23:32 ` [PATCH v2 10/22] x86/fpu/xstate: Update xstate save function for supporting dynamic user xstate Chang S. Bae
2020-11-19 23:32 ` [PATCH v2 11/22] x86/fpu/xstate: Update xstate area address finder " Chang S. Bae
2020-11-19 23:32 ` [PATCH v2 12/22] x86/fpu/xstate: Update xstate context copy function for supporting dynamic area Chang S. Bae
2020-11-19 23:32 ` [PATCH v2 13/22] x86/fpu/xstate: Expand dynamic user state area on first use Chang S. Bae
2020-11-19 23:32 ` [PATCH v2 14/22] x86/fpu/xstate: Inherit dynamic user state when used in the parent Chang S. Bae
2020-11-20 5:08 ` Andy Lutomirski
2020-11-24 18:35 ` Len Brown
2020-11-19 23:32 ` [PATCH v2 15/22] x86/fpu/xstate: Support ptracer-induced xstate area expansion Chang S. Bae
2020-11-20 5:07 ` Andy Lutomirski
2020-11-24 18:22 ` Bae, Chang Seok
2020-11-24 18:33 ` Andy Lutomirski
2020-12-02 4:00 ` Bae, Chang Seok
2020-12-03 5:00 ` Andy Lutomirski
2020-12-08 19:12 ` Bae, Chang Seok
2020-11-19 23:32 ` [PATCH v2 16/22] x86/fpu/xstate: Extend the table for mapping xstate components with features Chang S. Bae
2020-11-19 23:32 ` [PATCH v2 17/22] x86/cpufeatures/amx: Enumerate Advanced Matrix Extension (AMX) feature bits Chang S. Bae
2020-11-19 23:32 ` [PATCH v2 18/22] x86/fpu/amx: Define AMX state components and have it used for boot-time checks Chang S. Bae
2020-11-19 23:32 ` [PATCH v2 19/22] x86/fpu/amx: Enable the AMX feature in 64-bit mode Chang S. Bae
2020-11-19 23:32 ` [PATCH v2 20/22] selftest/x86/amx: Include test cases for the AMX state management Chang S. Bae
2020-11-19 23:32 ` [PATCH v2 21/22] x86/fpu/xstate: Support dynamic user state in the signal handling path Chang S. Bae
2020-11-19 23:32 ` [PATCH v2 22/22] x86/fpu/xstate: Introduce boot-parameters for control some state component support Chang S. Bae
2020-11-20 5:02 ` Andy Lutomirski
2020-11-24 18:50 ` Len Brown
2020-11-24 23:41 ` Andy Lutomirski [this message]
2020-12-01 18:00 ` Len Brown
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