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

[Laszlo Ersek]

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.

Thanks
Laszlo