Re: [Qemu-devel] [SeaBIOS] [PATCH v3] fw/msr_feature_control: add support to set MSR_IA32_FEATURE_CONTROL

[Haozhong Zhang]

On 07/06/16 09:44, Laszlo Ersek wrote:
> On 07/06/16 08:49, Haozhong Zhang wrote:
> > On 07/06/16 08:42, Laszlo Ersek wrote:
> >> On 07/06/16 08:28, Haozhong Zhang wrote:
> >>> Hi Ashok,
> >>>
> >>> On 07/06/16 02:18, Paolo Bonzini wrote:
> >>>>> I forgot to restore MSR_IA32_FEATURE_CONTROL in the resume path, and
> >>>>> MSR_IA32_FEATURE_CONTROL is zero after S3 resume.
> >>>>
> >>>> This is a bug.  Sorry Laszlo. :)
> >>>>
> >>>>> Not restore MSR_IA32_FEATURE_CONTROL during S3 resume does not affect
> >>>>> at least Linux guest (tested 4.5). Current QEMU may advise the guest
> >>>>> firmware to set bit 20 (for LMCE), bit 2 (for VMX) and bit 0 (lock
> >>>>> bit).
> >>>>> - For LMCE, Linux only checks bit 20 and bit 0 at boot time and then
> >>>>>   keeps using the result even after resume.
> >>>>
> >>>> On real hardware, LMCE would not be enabled after resume.  I'm not
> >>>> sure what would happen, but it wouldn't be good.
> >>>
> >>> Could you help to check if the LMCE bit in MSR_IA32_FEATURE_CONTROL is
> >>> set after S3 resume on the real hardware?
> >>
> >> The SDM says that IA32_FEATURE_CONTROL is zeroed on logical processor 
> >> reset.
> >>
> >>   23.7 ENABLING AND ENTERING VMX OPERATION
> >>
> >>   [...] VMXON is also controlled by the IA32_FEATURE_CONTROL MSR (MSR
> >>   address 3AH). This MSR is cleared to zero when a logical processor is
> >>   reset. [...]
> >>
> > 
> > Ah, I missed a bit in my question. I meant to check whether the
> > firmware on the real machine sets the LMCE bit and other necessary
> > bits in MSR_IA32_FEATURE_CONTROL after S3 resume.
> > 
> 
> I attached a minimal kernel module (reproducer / tester) to the github
> issue here:
> 
> https://github.com/tianocore/edk2/issues/97#issuecomment-230697897
> 
> We can use it for both testing the feature in guests, and for querying
> the MSR on physical machines.
>

Thanks for the kernel module! In fact, I'm using the rdmsr command
provided by msr-tools (which is included in some Linux distros).

> Specifically on my ThinkPad W541, the MSR has value 0x5:
> 
> [ 2885.877339] MSR 0x3a on CPU 0: 0x5
> [ 2908.151693] MSR 0x3a on CPU 1: 0x5
> 
> which, according to
> 
> #define FEATURE_CONTROL_LOCKED                                (1<<0)
> #define FEATURE_CONTROL_VMXON_ENABLED_INSIDE_SMX      (1<<1)
> #define FEATURE_CONTROL_VMXON_ENABLED_OUTSIDE_SMX     (1<<2)
> #define FEATURE_CONTROL_LMCE                          (1<<20)
> 
> corresponds to
> 
> (FEATURE_CONTROL_LOCKED | FEATURE_CONTROL_VMXON_ENABLED_OUTSIDE_SMX).
> 
> FEATURE_CONTROL_LMCE is not set at all. (I didn't do any S3 cycles in my
> current laptopt boot.)
> 
> My CPU is i7-4810MQ. It's a pretty modern laptop, so I think it is
> capable of LMCE, hardware-wise (if LMCE is hw-dependent, to begin with).
>

LMCE is supported only by server CPUs (Skylake-EX or later) and not
available on desktop CPUs.

For test in guest, you could follow what I added in "How to verify?"
https://github.com/tianocore/edk2/issues/97.

Thanks,
Haozhong

> In order to verify if my laptop was indeed capable of LCME, I read the
> IA32_MCG_CAP MSR as well:
> 
> # insmod ./rdmsr.ko msr=0x00000179
> MSR 0x179 on CPU 0: 0xc09
> 
> "MCG_LMCE_P" is bit 27 (value 0x8000000). So, apparently, I was wrong;
> my laptop does not support LMCE, and it's not surprising the BIOS
> doesn't set the LCME bit in the feature control MSR :)
> 
> Anyway I think you should be able to use the kernel module for
> experimenting with MSRs on other hosts.
> 
> Thanks
> Laszlo
>