linux-kernel - Re: [PATCH v4 0/10] x86/xsaves: Fix XSAVES known issues

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Date:	Fri, 29 Apr 2016 13:03:43 -0700
From:	Dave Hansen <dave.hansen@...ux.intel.com>
To:	Yu-cheng Yu <yu-cheng.yu@...el.com>
Cc:	x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Andy Lutomirski <luto@...nel.org>,
	Borislav Petkov <bp@...e.de>,
	Sai Praneeth Prakhya <sai.praneeth.prakhya@...el.com>,
	"Ravi V. Shankar" <ravi.v.shankar@...el.com>,
	Fenghua Yu <fenghua.yu@...el.com>
Subject: Re: [PATCH v4 0/10] x86/xsaves: Fix XSAVES known issues

On 04/29/2016 12:57 PM, Yu-cheng Yu wrote:
> On Fri, Apr 29, 2016 at 11:09:23AM -0700, Dave Hansen wrote:
>> Once we *HAVE* XSAVES support, it also opens up the possibilities for
>> doing things like dynamic XSAVE buffer allocation.  For instance, let
>> threads that are not _using_ AVX-512 not waste the 2k of space for
>> it.
> 
> If we can somehow modify exec* system call to scan the executable
> binary (in user space) and pass along a bitmask containing xfeatures
> used in the binary, and XSAVES is enabled in the kernel, we can
> easily save a lot of memory.  The kernel only needs to allocate space
> for tasks that actually use xstates; most of them do not.

That's not feasible.  Think of dynamic libraries or just-in-time
compilers.  What instruction set does /usr/bin/java use, for instance? :)