Re: Nested SVM and migration

Zachary Amsden
2010-02-22T16:46:23Z

On 02/21/2010 03:09 AM, Joerg Roedel wrote:
> On Sun, Feb 21, 2010 at 02:54:01PM +0200, Avi Kivity wrote:
>
>> So, if some other cpu (or the guest itself, with appropriate
>> permissions) modifies the msr permission bitmap, svm will not notice
>> this? svm loads the bitmap during entry?
>>
> Yes.
>

Ugh. So we have non-reversible architectural state all over again.
There are ways around this problem, all ugly, but the easiest is
shadowing the MSR permission bitmap.

>> I don't think you can tell, unless the host cpu modifying the vmcb is
>> synchronized with the guest (or the guest modifies its own vmcb). But
>> this is all academic.
>>
> Hmm, another thing comes to mind. We would need some redesign of the
> nested_svm code to allow userspace to put a vcpu directly into nested
> state. With the MSR approach, all userspace does is to write MSRs into
> the vcpu before the first run?
>

How does MSR_KVM_NESTED_SVM_ACTIVE not solve this problem?