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Subject: Re: [PATCH v4 0/10] x86/xsaves: Fix XSAVES known issues
To: Ingo Molnar <mingo@kernel.org>
References: <cover.1457038929.git.yu-cheng.yu@intel.com>
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Cc: Andy Lutomirski <luto@amacapital.net>, Yu-cheng Yu <yu-cheng.yu@intel.com>,
        X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        "linux-kernel@vger.kernel.org" <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: <57278014.3050808@linux.intel.com>
Date: Mon, 2 May 2016 09:28:04 -0700
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On 04/30/2016 12:53 AM, Ingo Molnar wrote:
> 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?

Sounds sane to me.

BTW, I hacked up your "fpu performance" to compare XSAVE vs. XSAVES:

> [    0.048347] x86/fpu: Cost of: XSAVE                       insn          :   127 cycles
> [    0.049134] x86/fpu: Cost of: XSAVES                      insn          :   113 cycles
> [    0.048492] x86/fpu: Cost of: XRSTOR                      insn          :   120 cycles
> [    0.049267] x86/fpu: Cost of: XRSTORS                     insn          :   102 cycles

So I guess we can add that to the list of things that XSAVES is good
for.  Granted, the real-world benefit is probably hard to measure
because the cache residency of the XSAVE buffer isn't as good when
_actually_ context switching, but this at least shows a small
theoretical advantage for XSAVES.