Re: kern/47248: Using -march=native on Intel I5 causes illegal instructions

[David Laight <david%l8s.co.uk@localhost>]

The following reply was made to PR kern/47248; it has been noted by GNATS.

From: David Laight <david%l8s.co.uk@localhost>
To: gnats-bugs%NetBSD.org@localhost
Cc: kern-bug-people%netbsd.org@localhost, gnats-admin%netbsd.org@localhost,
        netbsd-bugs%netbsd.org@localhost
Subject: Re: kern/47248: Using -march=native on Intel I5 causes illegal 
instructions
Date: Sun, 30 Dec 2012 16:42:40 +0000

 On Mon, Nov 26, 2012 at 10:10:00AM +0000, jaapb%NetBSD.org@localhost wrote:
 > >Synopsis:       Using -march=native on Intel I5 causes illegal instructions
 
 The gcc code that converts -march=native into a list of -mxxx options
 (-mcx16 -msahf -maes -mpclmul -mpopcnt -mavx on my i7) is only checking
 that the cpu supports the extra instructions, not that the OS also has
 support.
 
 The Intel docs vol 1 secton 13.5 tell you to check the OXSAVE bit
 as well as the AVX one (otherwise it is likely to be an old OS).
 The doc also says to check that the XCR0 bits indicate that AVX
 can actually be used - but I suspect the kernel may want to 'lazy-set'
 that bit.
 
 Checking OXSAVE is easy, fixed in rev 1.2 of gcc's driver_i386.c
 
        David
 
 -- 
 David Laight: david%l8s.co.uk@localhost