Re: [PATCH 2/2] KVM: x86: Allow userspace to define what's the microcode version

["Sironi, Filippo" <sironi@amazon.de>]

> On 27. Nov 2017, at 14:09, Steve Rutherford <srutherford@google.com> wrote:
> 
> On Mon, Nov 27, 2017 at 3:58 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
>> On 26/11/2017 17:41, Filippo Sironi wrote:
>>> ... that the guest should see.
>>> Guest operating systems may check the microcode version to decide whether
>>> to disable certain features that are known to be buggy up to certain
>>> microcode versions.  Address the issue by making the microcode version
>>> that the guest should see settable.
>>> The rationale for having userspace specifying the microcode version, rather
>>> than having the kernel picking it, is to ensure consistency for live-migrated
>>> instances; we don't want them to see a microcode version increase without a
>>> reset.
>>> 
>>> Signed-off-by: Filippo Sironi <sironi@amazon.de>
>>> ---
>>> arch/x86/kvm/x86.c       | 23 +++++++++++++++++++++++
>>> include/uapi/linux/kvm.h |  3 +++
>>> 2 files changed, 26 insertions(+)
>>> 
>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>> index 925c3e29cad3..741588f27ebc 100644
>>> --- a/arch/x86/kvm/x86.c
>>> +++ b/arch/x86/kvm/x86.c
>>> @@ -4033,6 +4033,29 @@ long kvm_arch_vm_ioctl(struct file *filp,
>>>      } u;
>>> 
>>>      switch (ioctl) {
>>> +     case KVM_GET_MICROCODE_VERSION: {
>>> +             r = -EFAULT;
>>> +             if (copy_to_user(argp,
>>> +                              &kvm->arch.microcode_version,
>>> +                              sizeof(kvm->arch.microcode_version)))
>>> +                     goto out;
>>> +             break;
>>> +     }
>>> +     case KVM_SET_MICROCODE_VERSION: {
>>> +             u32 microcode_version;
>>> +
>>> +             r = -EFAULT;
>>> +             if (copy_from_user(&microcode_version,
>>> +                                argp,
>>> +                                sizeof(microcode_version)))
>>> +                     goto out;
>>> +             r = -EINVAL;
>>> +             if (!microcode_version)
>>> +                     goto out;
>>> +             kvm->arch.microcode_version = microcode_version;
>>> +             r = 0;
>>> +             break;
>>> +     }
>> 
>> Also, there's no need to define new ioctls, instead you can just place
>> it in the vcpu and use KVM_GET_MSR/KVM_SET_MSR.  I'd agree that's
>> slightly less polished, but it matches what we do already for e.g.
>> nested VMX model specific registers.  And it spares you for writing the
>> documentation that you didn't include in this patch. :)
>> 
>> Paolo
> 
> This feels good time to mention Peter Hornyack's old MSR KVM_EXIT
> patches. With something like them, there would be no need to push this
> into the kernel at all.

That's one of the solution we discussed internally (at Amazon) but we
didn't pursue yet given the need to release a quick fix for customers.
I was thinking about implementing a mechanism to selectively go back to
userspace to emulate MSRs; something that's not limited to KVM unhandled
MSRs but that instead could even override KVM's handling.

Filippo

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