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

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Date:	Fri, 29 Apr 2016 13:40:07 -0700
From:	Dave Hansen <dave.hansen@...ux.intel.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Yu-cheng Yu <yu-cheng.yu@...el.com>
Cc:	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

On 04/29/2016 01:25 PM, Andy Lutomirski wrote:
> On Fri, Apr 29, 2016 at 1:07 PM, Yu-cheng Yu <yu-cheng.yu@...el.com> wrote:
>> On Fri, Apr 29, 2016 at 01:03:43PM -0700, Dave Hansen wrote:
>>> That's not feasible.  Think of dynamic libraries or just-in-time
>>> compilers.  What instruction set does /usr/bin/java use, for instance? :)
>>
>> The java argument is true. In that case or when the bitmask is
>> missing, we can allocate for all supported features.
> 
> I actually want to see us moving in the direction of unconditionally
> allocating everything on process startup.  If we can stop using CR0.TS
> entirely, I think everything will be better.

We can absolutely allocate the worst-case XSAVE buffer at task startup
for folks that never want to see a latency spike in the life of the app
no matter what.

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.  We also don't have to
do this with CR0.TS.  We'd actually use a combination of out-of-line
(not appended to task_struct) XSAVE buffers and XGETBV1 to check the
size of our XSAVE buffer before we call XSAVE* and resize it when needed.

Maybe nobody will ever care enough about 2kbytes/thread, though.