From: Borislav Petkov <bp@alien8.de>
To: Len Brown <lenb@kernel.org>
Cc: "Chang S. Bae" <chang.seok.bae@intel.com>,
Andy Lutomirski <luto@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@kernel.org>, X86 ML <x86@kernel.org>,
"Brown, Len" <len.brown@intel.com>,
Dave Hansen <dave.hansen@intel.com>,
thiago.macieira@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 v9 12/26] x86/fpu/xstate: Use feature disable (XFD) to protect dynamic user state
Date: Mon, 30 Aug 2021 19:53:04 +0200 [thread overview]
Message-ID: <YS0bAPaDGcDKftUp@zn.tnic> (raw)
In-Reply-To: <CAJvTdKn09GiAOgdsOR-+ooEO=bmj8VDL9e9sSAsu2UPx73a-Mw@mail.gmail.com>
On Tue, Aug 24, 2021 at 07:17:49PM -0400, Len Brown wrote:
> The problem with a system call to pre-allocate an AMX context switch
> buffer is that it doesn't actually deliver on the goal of guaranteeing
> no subsequent run-time failures due to OOM.
You mean you pre-allocate *everything*, i.e., you won't do any more
allocations but then you still might OOM?
Yeah, right.
Maybe from something else but not from your AMX-using process which has
prepared everything already.
> Even if your AMX thread pool threads were to invoke this system call
> as soon as possible... What is to say that the thread pool is created
> only at a time when memory is available? A thread could be created
> 24 hours into program execution under OOM conditions and this system
> call will return ENOMEM, and your program will in all likelihood
> throw up its arms and exit at the exact same place it would exit for
> transparently allocated buffers.
Well, if you preallocate everything you won't have to run for 24 hours,
encounter -ENOMEM and *lose* 24 hours worth of AMX computation. And then
the kernel won't have to do all kinds of crazy dance with dynamically
resizing buffers just because some small percentage of luserspace apps
decided to do AMX stuff.
> The program can equally cause the kernel to allocate an AMX context
> switch buffer by simply touching a TMM register -- and this can
> be done at exactly the same place in the program that calling a
> pre-allocate system call.
If the program touches a TMM register and it hasn't requested AMX
support upfront, it'll get killed.
> The advantage of the #NM over the syscall is that the programmer
> doesn't actually have to do anything. Also, transparently allocated
> buffers offer a theoretical benefit that a program may have many
> threads, but only a few may actually touch AMX, and so there is
> savings to be had by allocating buffers only for the threads that
> actually use the buffers.
The program already asked the kernel whether it can use AMX - it can
allocate the buffers for the threads too.
> XFD is used to *enforce* that AMX is not used without permission.
> Were we to not use the XFD feature, users would be able to stash
> data in TMM registers and even use TMUL without the kernel
> being able to prevent them from doing so. Then when they
> context switch or take a signal, the data in their TMM registers
> would mysteriously vanish...
>
> Much better to be able to tell them immediately that they are doing it
> wrong...
Ok, killing the program in the #NM handler if it hasn't requested AMX
prior makes sense.
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
next prev parent reply other threads:[~2021-08-30 17:52 UTC|newest]
Thread overview: 91+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-30 14:59 [PATCH v9 00/26] x86: Support Intel Advanced Matrix Extensions Chang S. Bae
2021-07-30 14:59 ` [PATCH v9 01/26] x86/fpu/xstate: Modify the initialization helper to handle both static and dynamic buffers Chang S. Bae
2021-07-30 14:59 ` [PATCH v9 02/26] x86/fpu/xstate: Modify state copy helpers " Chang S. Bae
2021-07-30 14:59 ` [PATCH v9 03/26] x86/fpu/xstate: Modify address finders " Chang S. Bae
2021-07-30 14:59 ` [PATCH v9 04/26] x86/fpu/xstate: Add a new variable to indicate dynamic user states Chang S. Bae
2021-07-30 14:59 ` [PATCH v9 05/26] x86/fpu/xstate: Add new variables to indicate dynamic XSTATE buffer size Chang S. Bae
2021-08-12 15:03 ` Borislav Petkov
2021-07-30 14:59 ` [PATCH v9 06/26] x86/fpu/xstate: Calculate and remember dynamic XSTATE buffer sizes Chang S. Bae
2021-08-12 16:36 ` Borislav Petkov
2021-07-30 14:59 ` [PATCH v9 07/26] x86/fpu/xstate: Convert the struct fpu 'state' field to a pointer Chang S. Bae
2021-08-12 17:09 ` Borislav Petkov
2021-07-30 14:59 ` [PATCH v9 08/26] x86/fpu/xstate: Introduce helpers to manage the XSTATE buffer dynamically Chang S. Bae
2021-08-12 19:44 ` Borislav Petkov
2021-08-13 8:04 ` Bae, Chang Seok
2021-08-13 10:04 ` Borislav Petkov
2021-08-13 19:43 ` Bae, Chang Seok
2021-08-18 9:28 ` Borislav Petkov
2021-08-18 19:46 ` Bae, Chang Seok
2021-08-25 16:01 ` Bae, Chang Seok
2021-08-30 17:07 ` Borislav Petkov
2021-08-30 23:39 ` Bae, Chang Seok
2021-08-16 18:33 ` Bae, Chang Seok
2021-08-16 18:53 ` Borislav Petkov
2021-08-30 17:45 ` Dave Hansen
2021-08-30 23:39 ` Bae, Chang Seok
2021-07-30 14:59 ` [PATCH v9 09/26] x86/fpu/xstate: Update the XSTATE save function to support dynamic states Chang S. Bae
2021-07-30 14:59 ` [PATCH v9 10/26] x86/fpu/xstate: Update the XSTATE buffer address finder " Chang S. Bae
2021-08-18 11:33 ` Borislav Petkov
2021-08-18 19:47 ` Bae, Chang Seok
2021-08-30 17:18 ` Borislav Petkov
2021-08-30 23:38 ` Bae, Chang Seok
2021-07-30 14:59 ` [PATCH v9 11/26] x86/fpu/xstate: Update the XSTATE context copy function " Chang S. Bae
2021-08-18 12:03 ` Borislav Petkov
2021-08-18 19:47 ` Bae, Chang Seok
2021-07-30 14:59 ` [PATCH v9 12/26] x86/fpu/xstate: Use feature disable (XFD) to protect dynamic user state Chang S. Bae
2021-08-18 16:24 ` Borislav Petkov
2021-08-18 17:20 ` Thiago Macieira
2021-08-18 17:46 ` Borislav Petkov
2021-08-18 17:58 ` Thiago Macieira
2021-08-18 18:10 ` Borislav Petkov
2021-08-24 22:51 ` Len Brown
2021-08-18 20:43 ` Bae, Chang Seok
2021-08-18 21:04 ` Thiago Macieira
2021-08-18 21:12 ` Bae, Chang Seok
2021-08-18 22:27 ` Thiago Macieira
2021-08-19 1:21 ` Andy Lutomirski
2021-08-19 16:06 ` Thiago Macieira
2021-08-18 21:17 ` Borislav Petkov
2021-08-18 21:37 ` Bae, Chang Seok
2021-08-19 8:00 ` Borislav Petkov
2021-08-19 15:24 ` Bae, Chang Seok
2021-08-24 23:22 ` Len Brown
2021-08-30 17:31 ` Borislav Petkov
2021-09-17 3:48 ` Len Brown
2021-08-18 19:47 ` Bae, Chang Seok
2021-08-24 22:21 ` Len Brown
2021-08-30 17:41 ` Borislav Petkov
2021-08-31 21:44 ` Len Brown
2021-08-24 23:17 ` Len Brown
2021-08-30 17:53 ` Borislav Petkov [this message]
2021-08-31 22:07 ` Len Brown
2021-08-31 22:11 ` Dave Hansen
2021-08-30 18:04 ` Dave Hansen
2021-08-31 22:15 ` Len Brown
2021-08-31 22:16 ` Len Brown
2021-08-31 22:39 ` Thiago Macieira
2021-08-31 22:44 ` Len Brown
2021-07-30 14:59 ` [PATCH v9 13/26] x86/fpu/xstate: Support ptracer-induced XSTATE buffer expansion Chang S. Bae
2021-07-30 14:59 ` [PATCH v9 14/26] x86/arch_prctl: Create ARCH_SET_STATE_ENABLE/ARCH_GET_STATE_ENABLE Chang S. Bae
2021-08-06 16:46 ` Thiago Macieira
2021-08-09 22:08 ` Bae, Chang Seok
2021-08-09 23:42 ` Thiago Macieira
2021-08-10 0:57 ` Bae, Chang Seok
2021-08-13 19:44 ` Bae, Chang Seok
2021-07-30 14:59 ` [PATCH v9 15/26] x86/fpu/xstate: Support both legacy and expanded signal XSTATE size Chang S. Bae
2021-07-30 14:59 ` [PATCH v9 16/26] x86/fpu/xstate: Adjust the XSAVE feature table to address gaps in state component numbers Chang S. Bae
2021-07-30 14:59 ` [PATCH v9 17/26] x86/fpu/xstate: Disable XSTATE support if an inconsistent state is detected Chang S. Bae
2021-07-30 14:59 ` [PATCH v9 18/26] x86/cpufeatures/amx: Enumerate Advanced Matrix Extension (AMX) feature bits Chang S. Bae
2021-07-30 14:59 ` [PATCH v9 19/26] x86/fpu/amx: Define AMX state components and have it used for boot-time checks Chang S. Bae
2021-07-30 14:59 ` [PATCH v9 20/26] x86/fpu/amx: Initialize child's AMX state Chang S. Bae
2021-07-30 14:59 ` [PATCH v9 21/26] x86/fpu/amx: Enable the AMX feature in 64-bit mode Chang S. Bae
2021-07-30 14:59 ` [PATCH v9 22/26] x86/fpu/xstate: Skip writing zeros to signal frame for dynamic user states if in INIT-state Chang S. Bae
2021-07-30 14:59 ` [PATCH v9 23/26] selftest/x86/amx: Test cases for the AMX state management Chang S. Bae
2021-07-30 14:59 ` [PATCH v9 24/26] x86/insn/amx: Add TILERELEASE instruction to the opcode map Chang S. Bae
2021-07-30 14:59 ` [PATCH v9 25/26] intel_idle/amx: Add SPR support with XTILEDATA capability Chang S. Bae
2021-07-30 18:41 ` Dave Hansen
2021-08-03 21:32 ` Bae, Chang Seok
2021-08-03 21:38 ` Dave Hansen
2021-08-03 21:43 ` Brown, Len
2021-07-30 20:15 ` Dave Hansen
2021-07-30 14:59 ` [PATCH v9 26/26] x86/fpu/xstate: Add a sanity check for XFD state when saving XSTATE Chang S. Bae
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