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From: Len Brown <lenb@kernel.org>
To: Andy Lutomirski <luto@kernel.org>
Cc: "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, 1 Dec 2020 13:00:54 -0500	[thread overview]
Message-ID: <CAJvTdKnVqUw8qBWJOCssk0pBhUzXzj-qR+UJOoyzx=eSYCKKYQ@mail.gmail.com> (raw)
In-Reply-To: <CALCETrX_1RkatiJxScjXhA38KTHvgrDxtj_e1YUs6fLTeHZufw@mail.gmail.com>

> > On Fri, Nov 20, 2020 at 12:03 AM Andy Lutomirski <luto@kernel.org> wrote:

> We may want to taint the kernel if one of these flags is used

I agree with Andy.
I can imagine a distro would want to see the taint bit set if a user
overrides the default that they ship and support.

Chang,
Please taint when the enable flag is used -- Thanks!

> 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.

A valid concern.
To mitigate, there is a new CPUID bit on the way to tell us explicitly
which features are simple (require just XSAVE), versus which
features are not simple and require more complex support.n t
When that bit is locked down, we will add that sanity check here.

> 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.)

I don't follow the XFD concern.

There are a couple of cases.

If the hypervisor isn't aware of XFD, it neither uses it, or shows it
to the guests.

If the hypervisor is aware of XFD and supports TMUL, it allocates its
own context
switch buffers at boot time, and does not use the XFD optimization to
save space inside the hypervisor.  It can expose XFD to the guests,
(and context switch it when guests switch on/off physical CPUs)

The guest sees two cases, either XFD exists or it doesn't, just like
on real hardware.

thanks,
Len Brown, Intel Open Source Technology Center

      reply	other threads:[~2020-12-01 18:01 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
2020-12-01 18:00         ` Len Brown [this message]

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