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

[Raj, Ashok]

On Fri, Jun 17, 2016 at 10:48:17PM +0200, Laszlo Ersek wrote:
> On 06/17/16 22:21, 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.
> 
> At the moment, my understanding of this feature is superficial, but the
> mechanisms involved in it don't seem complex. I don't expect
> difficulties implementing it, I just need the details that I asked for
> spelled out for me.
> 
> As to why we should be doing this in the guest firmware(s) -- "because
> that's what happens on physical machines too" :) Following the phys
> world to the letter in virt is not always a goal, but it's never wrong.

But the guest bios does nothing like the BIOS in the real platform.

for e.g. a real bios would have SMM handlers to work for implementing firmware
first mechanisms before notifying the OS. None of these exist in the 
virtalized world.