Re: [PATCH v4 3/3] i386: publish advised value of MSR_IA32_FEATURE_CONTROL via fw_cfg

From: Haozhong Zhang <haozhong.zhang@intel.com>
To: "Raj, Ashok" <ashok.raj@intel.com>,
	Laszlo Ersek <lersek@redhat.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	rkrcmar@redhat.com
Cc: qemu-devel@nongnu.org, Richard Henderson <rth@twiddle.net>,
	Eduardo Habkost <ehabkost@redhat.com>,
	"Michael S . Tsirkin" <mst@redhat.com>,
	Marcelo Tosatti <mtosatti@redhat.com>,
	kvm@vger.kernel.org, Boris Petkov <bp@suse.de>,
	Tony Luck <tony.luck@intel.com>,
	Andi Kleen <andi.kleen@intel.com>
Subject: Re: [PATCH v4 3/3] i386: publish advised value of MSR_IA32_FEATURE_CONTROL via fw_cfg
Date: Mon, 20 Jun 2016 11:09:41 +0800	[thread overview]
Message-ID: <20160620030941.2kro4d44b5degiya@hz-desktop> (raw)
In-Reply-To: <20160617202118.GA8131@linux.intel.com>

On 06/17/16 13:21, Raj, Ashok wrote:
> On Fri, Jun 17, 2016 at 07:31:08PM +0200, Laszlo Ersek wrote:
> > >>
> > >> On 16/06/2016 08:06, Haozhong Zhang wrote:
> > >>> It's a prerequisite that certain bits of MSR_IA32_FEATURE_CONTROL should
> > >>> be set before some features (e.g. VMX and LMCE) can be used, which is
> > >>> usually done by the firmware. This patch adds a fw_cfg file
> > >>> "etc/msr_feature_control" which contains the advised value of
> > >>> MSR_IA32_FEATURE_CONTROL and can be used by guest firmware (e.g. SeaBIOS).
> > >>>
> 
> I'm sorry i'm joining this discussion a bit late returning from vacation. 
> In a real platform supporting LMCE, BIOS is responsible for setting the bits 
> for IA32_FEATURE_CONTROL correctly. There are good reasons why we want the 
> BIOS to play this role.
> 
> in a virtualized environment, do we really have to push the same requirement
> or would it suffice to just emulate it as we did in the early patches.
> 
> Not sure what exact problem is created by just simply supporting it within
> kvm/qemu and not needing the bios for the guest to also adapt these changes.
> 

In the current nested VMX implementation in QEMU, setup
MSR_IA32_FEATURE_CONTROL is left to guest. So I think, for LMCE which
is another feature involving MSR_IA32_FEATURE_CONTROL, we may follow
the existing code.

Paolo and Radim, is there any case that objects to setting
MSR_IA32_FEATURE_CONTROL in QEMU?

Thanks,
Haozhong