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From: "Andy Lutomirski" <luto@kernel.org>
To: "Len Brown" <lenb@kernel.org>
Cc: "Bae, Chang Seok" <chang.seok.bae@intel.com>,
	"Borislav Petkov" <bp@suse.de>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Ingo Molnar" <mingo@kernel.org>,
	"the arch/x86 maintainers" <x86@kernel.org>,
	"Brown, Len" <len.brown@intel.com>,
	"Dave Hansen" <dave.hansen@intel.com>,
	"Liu, Jing2" <jing2.liu@intel.com>,
	"Shankar, Ravi V" <ravi.v.shankar@intel.com>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v5 25/28] x86/fpu/xstate: Skip writing zeros to signal frame for dynamic user states if in INIT-state
Date: Mon, 24 May 2021 21:46:06 -0700	[thread overview]
Message-ID: <ef151c13-1126-43b1-ab55-4bd86fa4c2c0@www.fastmail.com> (raw)
In-Reply-To: <CAJvTdKmq2qKgkkbeQzQ6dG14+EWZ_eTbz6c6neGLFo_X=PCJeg@mail.gmail.com>



On Mon, May 24, 2021, at 11:15 AM, Len Brown wrote:
> On Sun, May 23, 2021 at 11:28 PM Andy Lutomirski <luto@kernel.org> wrote:
> 
> > But what happens if we don't have the XGETBV1 feature?  Are we making
> > AMX support depend on XGETBV1?
> 
> Yes, AMX systems always have XGETBV.
> 
> > How does this patch interact with "[PATCH v5 24/28] x86/fpu/xstate: Use
> > per-task xstate mask for saving xstate in signal frame"?  They seem to
> > be try to do something similar but not quite the same, and they seem to
> > be patching the same function.  The result seems odd.
> 
> The previous patch allowed non-AMX tasks to skip writing 8KB of zeros
> to their signal stack.  This patch builds on that to allow an AMX-task
> in INIT to also skip writing 8KB of zeros to its signal stack.

If we ever have a task that is “non-AMX” but has non-INIT AMX, then either we or the hardware has seriously screwed up.

So the logic boils down to: if (a) optimize; if (b) optimize; where a implies b.  I maintain my objection — I think this is redundant.

> 
> > Finally, isn't part of the point that we need to avoid even *allocating*
> > space for non-AMX-using tasks?  That would require writing out the
> > compacted format and/or fiddling with XCR0.
> 
> The current signal stack ABI is that the user receives all
> architectural state in
> uncompressed XTATE format on the signal stack.  They can XRESTOR it
> and the right thing happens.
> 
> We can optimize the data transfer (and we have), but we can't optimize the space
> without changing that ABI.

We are between a rock and a hard place here.  On the one hand, our ABI (supposedly — no one seems to have really confirmed this) has the signal frame in uncompacted form.  On the other hand, our ABI most definitely has signal frames under 2kB by a decent amount.

I think we should apply a simple patch to Linux to add a boot parameter x86_extra_signal_space=8192 that will pad the signal frame by 8192 bytes.  And then we ask people to seriously test their workloads with that parameter set. If things break, then AMX in its current proposed form may be off the table.

At least my dynamic XCR0 proposal has the property that legacy programs and AMX can reliably coexist on the same system.

  parent reply	other threads:[~2021-05-25  4:46 UTC|newest]

Thread overview: 75+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-23 19:32 [PATCH v5 00/28] x86: Support Intel Advanced Matrix Extensions Chang S. Bae
2021-05-23 19:32 ` [PATCH v5 01/28] x86/fpu/xstate: Modify the initialization helper to handle both static and dynamic buffers Chang S. Bae
2021-05-23 19:32 ` [PATCH v5 02/28] x86/fpu/xstate: Modify state copy helpers " Chang S. Bae
2021-05-23 19:32 ` [PATCH v5 03/28] x86/fpu/xstate: Modify address finders " Chang S. Bae
2021-05-23 19:32 ` [PATCH v5 04/28] x86/fpu/xstate: Modify the context restore helper " Chang S. Bae
2021-05-23 19:32 ` [PATCH v5 05/28] x86/fpu/xstate: Add a new variable to indicate dynamic user states Chang S. Bae
2021-05-23 19:32 ` [PATCH v5 06/28] x86/fpu/xstate: Add new variables to indicate dynamic xstate buffer size Chang S. Bae
2021-05-23 19:32 ` [PATCH v5 07/28] x86/fpu/xstate: Calculate and remember dynamic xstate buffer sizes Chang S. Bae
2021-05-23 19:32 ` [PATCH v5 08/28] x86/fpu/xstate: Convert the struct fpu 'state' field to a pointer Chang S. Bae
2021-05-23 19:32 ` [PATCH v5 09/28] x86/fpu/xstate: Introduce helpers to manage the xstate buffer dynamically Chang S. Bae
2021-05-23 19:32 ` [PATCH v5 10/28] x86/fpu/xstate: Define the scope of the initial xstate data Chang S. Bae
2021-05-23 19:32 ` [PATCH v5 11/28] x86/fpu/xstate: Update the xstate save function to support dynamic states Chang S. Bae
2021-05-23 19:32 ` [PATCH v5 12/28] x86/fpu/xstate: Update the xstate buffer address finder " Chang S. Bae
2021-05-23 19:32 ` [PATCH v5 13/28] x86/fpu/xstate: Update the xstate context copy function " Chang S. Bae
2021-05-23 19:32 ` [PATCH v5 14/28] x86/fpu/xstate: Prevent unauthorised use of dynamic user state Chang S. Bae
2021-06-16 16:17   ` Dave Hansen
2021-06-16 16:27   ` Dave Hansen
2021-06-16 18:12     ` Andy Lutomirski
2021-06-16 18:47       ` Bae, Chang Seok
2021-06-16 19:01         ` Dave Hansen
2021-06-16 19:23           ` Bae, Chang Seok
2021-06-16 19:28             ` Dave Hansen
2021-06-16 19:37               ` Bae, Chang Seok
2021-06-28 10:11               ` Liu, Jing2
2021-06-29 17:43           ` Bae, Chang Seok
2021-06-29 17:54             ` Dave Hansen
2021-06-29 18:35               ` Bae, Chang Seok
2021-06-29 18:50                 ` Dave Hansen
2021-06-29 19:13                   ` Bae, Chang Seok
2021-06-29 19:26                     ` Dave Hansen
2021-05-23 19:32 ` [PATCH v5 15/28] x86/arch_prctl: Create ARCH_GET_XSTATE/ARCH_PUT_XSTATE Chang S. Bae
2021-05-24 23:10   ` Len Brown
2021-05-25 17:27     ` Borislav Petkov
2021-05-25 17:33       ` Dave Hansen
2021-05-26  0:38     ` Len Brown
2021-05-27 11:14       ` second, sync-alloc syscall Borislav Petkov
2021-05-27 13:59         ` Len Brown
2021-05-27 19:35           ` Andy Lutomirski
2021-05-25 15:46   ` [PATCH v5 15/28] x86/arch_prctl: Create ARCH_GET_XSTATE/ARCH_PUT_XSTATE Dave Hansen
2021-05-23 19:32 ` [PATCH v5 16/28] x86/fpu/xstate: Support ptracer-induced xstate buffer expansion Chang S. Bae
2021-05-23 19:32 ` [PATCH v5 17/28] x86/fpu/xstate: Adjust the XSAVE feature table to address gaps in state component numbers Chang S. Bae
2021-05-23 19:32 ` [PATCH v5 18/28] x86/fpu/xstate: Disable xstate support if an inconsistent state is detected Chang S. Bae
2021-05-23 19:32 ` [PATCH v5 19/28] x86/cpufeatures/amx: Enumerate Advanced Matrix Extension (AMX) feature bits Chang S. Bae
2021-05-23 19:32 ` [PATCH v5 20/28] x86/fpu/amx: Define AMX state components and have it used for boot-time checks Chang S. Bae
2021-05-23 19:32 ` [PATCH v5 21/28] x86/fpu/amx: Initialize child's AMX state Chang S. Bae
2021-05-24  3:09   ` Andy Lutomirski
2021-05-24 17:37     ` Len Brown
2021-05-24 18:13       ` Andy Lutomirski
2021-05-24 18:21         ` Len Brown
2021-05-25  3:44           ` Andy Lutomirski
2021-05-23 19:32 ` [PATCH v5 22/28] x86/fpu/amx: Enable the AMX feature in 64-bit mode Chang S. Bae
2021-05-23 19:32 ` [PATCH v5 23/28] selftest/x86/amx: Test cases for the AMX state management Chang S. Bae
2021-05-23 19:32 ` [PATCH v5 24/28] x86/fpu/xstate: Use per-task xstate mask for saving xstate in signal frame Chang S. Bae
2021-05-24  3:15   ` Andy Lutomirski
2021-05-24 18:06     ` Len Brown
2021-05-25  4:47       ` Andy Lutomirski
2021-05-25 14:04         ` Len Brown
2021-05-23 19:32 ` [PATCH v5 25/28] x86/fpu/xstate: Skip writing zeros to signal frame for dynamic user states if in INIT-state Chang S. Bae
2021-05-24  3:25   ` Andy Lutomirski
2021-05-24 18:15     ` Len Brown
2021-05-24 18:29       ` Dave Hansen
2021-05-25  4:46       ` Andy Lutomirski [this message]
2021-05-23 19:32 ` [PATCH v5 26/28] selftest/x86/amx: Test case for AMX state copy optimization in signal delivery Chang S. Bae
2021-05-23 19:32 ` [PATCH v5 27/28] x86/insn/amx: Add TILERELEASE instruction to the opcode map Chang S. Bae
2021-05-23 19:32 ` [PATCH v5 28/28] x86/fpu/amx: Clear the AMX state when appropriate Chang S. Bae
2021-05-24  3:13   ` Andy Lutomirski
2021-05-24 14:10     ` Dave Hansen
2021-05-24 17:32       ` Len Brown
2021-05-24 17:39         ` Dave Hansen
2021-05-24 18:24           ` Len Brown
2021-05-27 11:56             ` Peter Zijlstra
2021-05-27 14:02               ` Len Brown
2021-05-24 14:06   ` Dave Hansen
2021-05-24 17:34     ` Len Brown
2021-05-24 21:11       ` [PATCH v5-fix " Chang S. Bae

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