Re: [RFC,05/10] x86/speculation: Add basic IBRS support infrastructure

[Christophe de Dinechin]

> On 31 Jan 2018, at 11:15, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> 
> On Wed, 31 Jan 2018, Christophe de Dinechin wrote:
>>> On 30 Jan 2018, at 21:46, Alan Cox <gnomes@xxxxxxxxxxxxxxxxxxx> wrote:
>>> 
>>>> If you are ever going to migrate to Skylake, I think you should just
>>>> always tell the guests that you're running on Skylake. That way the
>>>> guests will always assume the worst case situation wrt Specte.
>>> 
>>> Unfortunately if you do that then guest may also decide to use other
>>> Skylake hardware features and pop its clogs when it finds out its actually
>>> running on Westmere or SandyBridge.
>>> 
>>> So you need to be able to both lie to the OS and user space via cpuid and
>>> also have a second 'but do skylake protections' that only mitigation
>>> aware software knows about.
>> 
>> Yes. The most desirable lie is different depending on whether you want to
>> allow virtualization features such as migration (where youâd gravitate
>> towards a CPU with less features) or whether you want to allow mitigation
>> (where youâd rather present the most fragile CPUID, probably Skylake).
>> 
>> Looking at some recent patches, Iâm concerned that the code being added
>> often assumes that the CPUID is the correct way to get that info.
>> I do not think this is correct. You really want specific information about
>> the host CPUID, not whatever KVM CPUID emulation makes up.
> 
> That wont cut it. If you have a heterogenous farm of systems, then you need:
> 
>  - All CPUs have to support IBRS/IBPB or at least hte hypervisor has to
>    pretend they do by providing fake MRS for that
> 
>  - Have a 'force IBRS/IBPB' mechanism so the guests don't discard it due
>    to missing CPU feature bits.
> 
> Though this gets worse. You have to make sure that the guest keeps _ALL_
> sorts of mitigation mechanisms enabled and does not decide to disable
> retpolines because IBRS/IBPB are "availableâ.

What you are saying is that itâs one thing to test at boot time, but
(at least) migration events should also cause a re-check. Agreed.
The alternative is to pessimistically enable mitigation in VMs.
I believe this is the current âstate of the artâ, i.e. enable
IBRS statically via a CPU type variant.

What is the best place to re-check anyway?

(Just out of curiosity: there are no non-symmetric systems
that mix CPUs of different generation, right?)


> 
> Good luck with making all that work.

:-)

> 
> Thanks,
> 
> 	tglx