Re: [Qemu-devel] [PATCH] kvm: x86: Add support for -machine split-lock-ac

[Eduardo Habkost]

On Wed, Jul 04, 2018 at 03:38:32PM +0200, Paolo Bonzini wrote:
> On 04/07/2018 15:12, Jingqi Liu wrote:
> > Add the option split-lock-ac to control whether the #AC
> > exception is generated for split locked accesses, which
> > is introduced for the machine, there is an example to enable it:
> > -machine split-lock-ac=on
> > It is disabled on default.
> > 
> > When bit 29 of the MSR (33H) is set, the processor
> > causes an #AC exception to be issued instead of suppressing LOCK on
> > bus(during split lock access).
> 
> This should be a CPU feature, not a machine feature.  As mentioned in
> the review of the kernel patch, please work with Robert to use the same
> infrastructure for both MSR_TEST_CTL and MSR_IA32_ARCH_CAPABILITIES.
> 
> (Robert, does IceLake have this feature?  If so, we cannot create the
> CPU model until everything is in place).

I don't think we need to block the CPU model because QEMU+KVM
doesn't support some features yet, as long as kernel versions
capable of running the 3.0 version of IceLake-Server will be also
capable of running the 3.1 version of IceLake-Server.

Now, if that condition won't be true and we have some IceLake
features that will added only to more recent kernels, it might be
a good idea to hold the inclusion of the CPU model until
everything is in place.


> 
> BTW, why is the availability of the feature not exposed with a CPUID
> bit?  It doesn't make much sense.

I have the same question.  Without a CPUID bit, guests may block
the VM from being migrated to older hosts because the additional
section for the MSR will appear.

-- 
Eduardo