netdev - RE: [RFC PATCH 0/3] kernel: add support for 256-bit IO access

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Mon, 19 Mar 2018 15:19:01 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Thomas Gleixner' <tglx@...utronix.de>
CC:     'Rahul Lakkireddy' <rahul.lakkireddy@...lsio.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "hpa@...or.com" <hpa@...or.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "ganeshgr@...lsio.com" <ganeshgr@...lsio.com>,
        "nirranjan@...lsio.com" <nirranjan@...lsio.com>,
        "indranil@...lsio.com" <indranil@...lsio.com>
Subject: RE: [RFC PATCH 0/3] kernel: add support for 256-bit IO access

From: Thomas Gleixner
> Sent: 19 March 2018 15:05
> 
> On Mon, 19 Mar 2018, David Laight wrote:
> > From: Rahul Lakkireddy
> > In principle it ought to be possible to get access to one or two
> > (eg) AVX registers by saving them to stack and telling the fpu
> > save code where you've put them.
> 
> No. We have functions for this and we are not adding new ad hoc magic.

I was thinking that a real API might do this...
Useful also for code that needs AVX-like registers to do things like CRCs.

> > OTOH, for x86, if the code always runs in process context (eg from a
> > system call) then, since the ABI defines them all as caller-saved
> > the AVX(2) registers, it is only necessary to ensure that the current
> > FPU registers belong to the current process once.
> > The registers can be set to zero by an 'invalidate' instruction on
> > system call entry (hope this is done) and after use.
> 
> Why would a system call touch the FPU registers? The kernel normally does
> not use FPU instructions and the code which explicitely does has to take
> care of save/restore. It would be performance madness to fiddle with the
> FPU stuff unconditionally if nothing uses it.

If system call entry reset the AVX registers then any FP save/restore
would be faster because the AVX registers wouldn't need to be saved
(and the cpu won't save them).
I believe the instruction to reset the AVX registers is fast.
The AVX registers only ever need saving if the process enters the
kernel through an interrupt.

	David