Date: Sat, 12 Mar 2016 18:47:23 +0100
From: Borislav Petkov <bp@alien8.de>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Fenghua Yu <fenghua.yu@intel.com>, Thomas Gleixner <tglx@linutronix.de>,
        "x86@kernel.org" <x86@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "Bryan O'Donoghue" <pure.logic@nexus-software.ie>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>, Ingo Molnar <mingo@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Andy Shevchenko <andy.shevchenko@gmail.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "Yu, Yu-cheng" <yu-cheng.yu@intel.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH] x86/FPU: Fix FPU handling on legacy FPU machines
Message-ID: <20160312174722.GE23410@pd.tnic>
References: <20160311090840.GA8486@gmail.com>
 <20160311094802.GA4312@pd.tnic>
 <1457694124.2007.12.camel@nexus-software.ie>
 <20160311112610.GC4312@pd.tnic>
 <20160311113206.GD4312@pd.tnic>
 <CA+55aFyHQ0+zM5pBNWakA_ti4gutLX5VdounyuduGeFGTmvksg@mail.gmail.com>
 <20160311220313.GG4312@pd.tnic>
 <56E34197.8080209@linux.intel.com>
 <20160311222023.GH4312@pd.tnic>
 <CALCETrXFDtZ9qkyaPTNqos+=Q8iDSxbjaC36ikg5dJBc8+sToA@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <CALCETrXFDtZ9qkyaPTNqos+=Q8iDSxbjaC36ikg5dJBc8+sToA@mail.gmail.com>
User-Agent: Mutt/1.5.24 (2015-08-30)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1788
Lines: 52

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.

ECO tip #101: Trim your mails when you reply.