linux-kernel - Re: [PATCH] x86/FPU: Fix FPU handling on legacy FPU machines

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Date:	Sat, 12 Mar 2016 18:47:23 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Fenghua Yu <fenghua.yu@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"x86@...nel.org" <x86@...nel.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Bryan O'Donoghue <pure.logic@...us-software.ie>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andy Shevchenko <andy.shevchenko@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Yu, Yu-cheng" <yu-cheng.yu@...el.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] x86/FPU: Fix FPU handling on legacy FPU machines

On Sat, Mar 12, 2016 at 09:21:16AM -0800, Andy Lutomirski wrote:
> For reference, what are the QEMU options and boot options you used to
> trigger this?  I'm asking because I tested eagerfpu=on without fxsr a
> few weeks ago in QEMU, and I didn't trigger it.  Maybe I needed to
> force KVM off or something.

$ qemu-system-i386
-enable-kvm
-gdb tcp::1234
-cpu 486
-m 2048
-hda /home/boris/kvm/debian/sid-i386.img
-boot menu=off,order=c
-localtime
-net nic,model=rtl8139,macaddr=12:35:12:34:56:78
-net user,hostfwd=tcp::1235-:22
-usbdevice tablet
-kernel /home/boris/kernel/linux-2.6/arch/x86/boot/bzImage
-append "root=/dev/sda1 resume=/dev/sdb1 debug ignore_loglevel log_buf_len=16M earlyprintk=ttyS0,115200 console=ttyS0,115200 console=tty0 "
-monitor pty
-soundhw hda
-serial file:/home/boris/kvm/test-i386-1235.log
-snapshot
-name "Debian i386:1235"
-smp 1
-virtfs local,path=/tmp,mount_tag=tmp,security_model=none


I basically tried to get it as close to 486er as possible. I had also
CONFIG_M486=y for the guest kernel.

> Off the top of my head, I'm guessing that when I wrote "x86/fpu: Fix
> FNSAVE usage in eagerfpu mode", I was inadvertently using a bastardize
> combination of FNSAVE and FXRSTOR-of-init-state, KVM let the FXRSTOR
> through despite not advertising it in CPUID, and it papered over the
> init issue because the wrong init state format was hidden by using the
> wrong instruction to load it.

Yeah, I think qemu looks at CPUID bits for some stuff but probably not
all. And I also think we should go and fix such issues there so that we
can have as accurate an emulation as possible.

> Sigh. Yet more reason for Intel to add chicken bits to *turn off* new
> features.

Amen to that.

-- 
Regards/Gruss,
    Boris.

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