On 02/21/2010 02:10 AM, Joerg Roedel wrote:
On Sat, Feb 20, 2010 at 01:26:49PM -1000, Zachary Amsden wrote:
The infrastructure is already there to import / export and migrate MSR
settings. MSRs are also 64-bit, and hold "model-specific" settings, so
if you don't mind thinking of the nested feature as a model-specific
feature of the KVM-SVM CPU, it's even somewhat well defined in terms of
the architecture.
There is a lot of additional state to migrate if the vcpu is running
nested. To be architecturally correct you need to transfer 6kb of data
through MSRs only for the msr permission bitmap. The rest comes down to
the nested intercept masks and some small bits like the global interrupt
flag and the nested vmcb address. It is doable but I still think its
complicated to get this right. The simplest approach would be to
disallow migration when the vcpu is running in guest mode.
How is this a lot? The guest may have multiple gigabytes of memory and
potentially terabytes of storage. An additional 4-8k of control state
is not a lot.
Mostly the problem is figuring out what chunk of MSR space to use.
And hoping that this MSR space is not used by real hardware in the
future ;-)
There are places in MSR space that can easily be set aside for this, in
fact, I think already are being used by Hyper-V.
And further, even if it is used by real hardware, how does it matter?
These MSRs are to be used by control software to import / export data to
the virtual CPU. When being accessed by hardware virtualization, they
can be re-aliased to any meaning at all, so there is no collision.
Zach
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