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From: Dave Hansen <dave.hansen@intel.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: "Brown, Len" <len.brown@intel.com>,
	Andy Lutomirski <luto@kernel.org>,
	"Bae, Chang Seok" <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>, "Liu, Jing2" <jing2.liu@intel.com>,
	"Shankar, Ravi V" <ravi.v.shankar@intel.com>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [RFC PATCH 13/22] x86/fpu/xstate: Expand dynamic user state area on first use
Date: Wed, 14 Oct 2020 09:29:21 -0700	[thread overview]
Message-ID: <69721125-4e1c-ca9c-ff59-8e1331933e6c@intel.com> (raw)
In-Reply-To: <CALCETrW8u5rUsZvoo5t4YtC+4boBVcK__-srtA1+-YX06QYD1w@mail.gmail.com>

On 10/14/20 9:10 AM, Andy Lutomirski wrote:
>> Actually, I think the modified optimization would survive such a scheme:
>>
>>  * copy page array into percpu area
>>  * XRSTORS from percpu area, modified optimization tuple is saved
>>  * run userspace
>>  * XSAVES back to percpu area.  tuple matches, modified optimization
>>    is still in play
>>  * copy percpu area back to page array
>>
>> Since the XRSTORS->XSAVES pair is both done to the percpu area, the
>> XSAVE tracking hardware never knows it isn't working on the "canonical"
>> buffer (the page array).
> I was suggesting something a little bit different.  We'd keep XMM,
> YMM, ZMM, etc state stored exactly the way we do now and, for
> AMX-using tasks, we would save the AMX state in an entirely separate
> buffer.  This way the pain of having a variable xstate layout is
> confined just to AMX tasks.

OK, got it.

So, we'd either need a second set of XSAVE/XRSTORs, or "manual" copying
of the registers out to memory.  We can preserve the modified
optimization if we're careful about ordering, but only for *ONE* of the
XSAVE buffers (if we use two).

> I'm okay with vmalloc() too, but I do think we need to deal with the
> various corner cases like allocation failing.

Yeah, agreed about handling the corner cases.  Also, if we preserve
plain old vmalloc() for now, we need good tracepoints or stats so we can
precisely figure out how many vmalloc()s (and IPIs) are due to AMX.

  reply	other threads:[~2020-10-14 16:29 UTC|newest]

Thread overview: 43+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-01 20:38 [RFC PATCH 00/22] x86: Support Intel Advanced Matrix Extensions Chang S. Bae
2020-10-01 20:38 ` [RFC PATCH 01/22] x86/fpu/xstate: Modify area init helper prototypes to access all the possible areas Chang S. Bae
2020-10-01 20:38 ` [RFC PATCH 02/22] x86/fpu/xstate: Modify xstate copy " Chang S. Bae
2020-10-01 20:38 ` [RFC PATCH 03/22] x86/fpu/xstate: Modify address finder " Chang S. Bae
2020-10-01 20:38 ` [RFC PATCH 04/22] x86/fpu/xstate: Modify save and restore helper " Chang S. Bae
2020-10-01 20:38 ` [RFC PATCH 05/22] x86/fpu/xstate: Introduce a new variable for dynamic user states Chang S. Bae
2020-10-01 20:38 ` [RFC PATCH 06/22] x86/fpu/xstate: Outline dynamic xstate area size in the task context Chang S. Bae
2020-10-01 20:38 ` [RFC PATCH 07/22] x86/fpu/xstate: Introduce helpers to manage an xstate area dynamically Chang S. Bae
2020-10-01 23:41   ` Andy Lutomirski
2020-10-13 22:00     ` Brown, Len
2020-10-01 20:38 ` [RFC PATCH 08/22] x86/fpu/xstate: Define the scope of the initial xstate data Chang S. Bae
2020-10-01 20:39 ` [RFC PATCH 09/22] x86/fpu/xstate: Introduce wrapper functions for organizing xstate area access Chang S. Bae
2020-10-01 20:39 ` [RFC PATCH 10/22] x86/fpu/xstate: Update xstate save function for supporting dynamic user xstate Chang S. Bae
2020-10-01 20:39 ` [RFC PATCH 11/22] x86/fpu/xstate: Update xstate area address finder " Chang S. Bae
2020-10-01 20:39 ` [RFC PATCH 12/22] x86/fpu/xstate: Update xstate context copy function for supporting dynamic area Chang S. Bae
2020-10-01 20:39 ` [RFC PATCH 13/22] x86/fpu/xstate: Expand dynamic user state area on first use Chang S. Bae
2020-10-01 23:39   ` Andy Lutomirski
2020-10-13 22:31     ` Brown, Len
2020-10-13 22:43       ` Dave Hansen
2020-10-14  1:11         ` Andy Lutomirski
2020-10-14  6:03           ` Dave Hansen
2020-10-14 16:10             ` Andy Lutomirski
2020-10-14 16:29               ` Dave Hansen [this message]
2020-11-03 21:32                 ` Bae, Chang Seok
2020-11-03 21:41                   ` Dave Hansen
2020-11-03 21:53                     ` Bae, Chang Seok
2020-10-14 10:41         ` Peter Zijlstra
2020-10-14  1:47       ` Andy Lutomirski
2020-10-01 20:39 ` [RFC PATCH 14/22] x86/fpu/xstate: Inherit dynamic user state when used in the parent Chang S. Bae
2020-10-01 20:39 ` [RFC PATCH 15/22] x86/fpu/xstate: Support ptracer-induced xstate area expansion Chang S. Bae
2020-10-01 20:39 ` [RFC PATCH 16/22] x86/fpu/xstate: Support dynamic user state in the signal handling path Chang S. Bae
2020-10-01 20:39 ` [RFC PATCH 17/22] x86/fpu/xstate: Extend the table for mapping xstate components with features Chang S. Bae
2020-10-01 20:39 ` [RFC PATCH 18/22] x86/cpufeatures/amx: Enumerate Advanced Matrix Extension (AMX) feature bits Chang S. Bae
2020-10-01 20:39 ` [RFC PATCH 19/22] x86/fpu/amx: Define AMX state components and have it used for boot-time checks Chang S. Bae
2020-10-01 20:39 ` [RFC PATCH 20/22] x86/fpu/amx: Enable the AMX feature in 64-bit mode Chang S. Bae
2020-10-01 20:39 ` [RFC PATCH 21/22] selftest/x86/amx: Include test cases for the AMX state management Chang S. Bae
2020-10-01 20:39 ` [RFC PATCH 22/22] x86/fpu/xstate: Introduce boot-parameters for control some state component support Chang S. Bae
2020-10-02  2:09   ` Randy Dunlap
2020-10-13 23:00     ` Brown, Len
2020-10-02 17:15   ` Andy Lutomirski
2020-10-02 17:26     ` Dave Hansen
2020-10-13 23:38     ` Brown, Len
2020-10-02 17:25   ` Dave Hansen

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