Subject: Re: [PATCH v4 0/10] x86/xsaves: Fix XSAVES known issues
To: Yu-cheng Yu <yu-cheng.yu@intel.com>
References: <cover.1457038929.git.yu-cheng.yu@intel.com>
 <5723A353.7060209@linux.intel.com> <20160429195741.GA15402@test-lenovo>
Cc: x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        linux-kernel@vger.kernel.org, Andy Lutomirski <luto@kernel.org>,
        Borislav Petkov <bp@suse.de>,
        Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Fenghua Yu <fenghua.yu@intel.com>
From: Dave Hansen <dave.hansen@linux.intel.com>
Message-ID: <5723BE1F.7040300@linux.intel.com>
Date: Fri, 29 Apr 2016 13:03:43 -0700
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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? :)