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 03:58, 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

I wanted to do the work once rather than doing it per vCPU but using
KVM_{GET|SET}_MSR and extending the list of MSRs that userspace can
save/restore is certainly doable.

I'll look into that and post a v2.

Filippo

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