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

[Steve Rutherford <srutherford@google.com>]

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.