Re: [PATCH] KVM: X86: Allow userspace to define the microcode version

[Konrad Rzeszutek Wilk]

On Tue, Apr 24, 2018 at 10:59:04AM +0800, Wanpeng Li wrote:
> 2018-04-18 18:36 GMT+08:00 Paolo Bonzini <pbonzini@xxxxxxxxxx>:
> > On 18/04/2018 11:03, Eduardo Habkost wrote:
> >>>> QEMU setting ucode_rev automatically using the host value when
> >>>> using "-cpu host" (with no need for explicit ucode_rev option)
> >>>> makes sense to me.
> >>> QEMU can't get the host value by rdmsr MSR_IA32_UCODE_REV directly
> >>> since rdmsr will #GP when ring !=0, any idea?
> >> By looking at kvm_get_msr_feature(), it looks like
> >> ioctl(system_fd, KVM_GET_MSRS) would return the host MSR value
> >> for us.
> >
> > Yes, that's exactly what it was introduced for (together with other MSRs
> > including VMX capabilities).
> 
> How about the live migration? What will happen if the source and
> destination machines have different microcode version?

You would need to include the microcode version in the migration stream.

But this brings another point - what if we want to manifest certain
new CPUID bits?

For example, see:
https://software.intel.com/sites/default/files/managed/1d/46/Retpoline-A-Branch-Target-Injection-Mitigation.pdf

5.3:
"To remedy this situation, an operating system running as a VM can query bit 2 of the
IA32_ARCH_CAPABILITIES MSR, known as âRSB Alternateâ (RSBA). When RSBA is set, it
indicates that the VM may run on a processor vulnerable to exploits of Empty RSB
conditions regardless of the processorâs DisplayFamily/DisplayModel signature, and
that the operating system should deploy appropriate mitigations. Virtual machine
managers (VMM) may set RSBA via MSR interception to indicate that a virtual machine
might run at some time in the future on a vulnerable processor."

Perhaps the guest should do a bit of sampling of various CPUIDs as the migration
has been done? Is there a nice KVM hook inside of the guest to do this?

> 
> Regards,
> Wanpeng Li