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

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Message-ID: <20160430075343.GA23063@gmail.com>
Date:	Sat, 30 Apr 2016 09:53:44 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Dave Hansen <dave.hansen@...ux.intel.com>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Yu-cheng Yu <yu-cheng.yu@...el.com>, X86 ML <x86@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <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

* Dave Hansen <dave.hansen@...ux.intel.com> wrote:

> But I also think it would be pretty nice if 'ls' didn't pay the 2k cost to have 
> AVX-512 state if it's not using AVX-512. [...]

A C library might decide to use AVX-512 memset(). RAM is cheap, while allocation 
complexity, especially in the kernel, has various other costs.

I mean, we should not worry about per thread allocation sizes that can be compared 
to the kernel stack size.

We can still use the compacted area handling instructions, because presumably 
those are the fastest and are also the most optimized ones? But I wouldn't use 
them to do dynamic allocation: just allocate the maximum possible FPU save area at 
task creation time and never again worry about that detail.

Ok?

Thanks,

	Ingo