RE: [Xen-devel] Windows SMP

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

>From: James Harper
>Sent: Monday, December 29, 2008 4:48 PM
>
>> On 29/12/2008 08:20, "James Harper" <james.harper@xxxxxxxxxxxxxxxx>
>wrote:
>> 
>> >>  3. Run Citrix drivers which patch Windows to avoid TPR writes.
>> 
>> > Can you elaborate on that last point? Does that pass WHQL?
>> 
>> The WHQL tests are oblivious to it. It's just a patching of mmio
>writes to
>> the APIC TPR register.
>> 
>
>Looking at the way KVM does it, it appears to detect writes to the TPR
>register when they are trapped, and then give the DomU (or whatever KVM
>calls it) the address of the instruction so that the DomU driver can
>then patch it. Is that what Citrix is doing? Does the current xensource
>tree have such a mechanism in it?
>

IIRC, based on my understanding months ago when KVM PTR
patching was introduced, it only works for UP windows guest.
One tricky issue for dynamic instruction patching is how to encode
vcpu ID in register, since vcpu ID is the index into per-vcpu shadow
vTPR value. Use memory accesses to retrieve vcpu ID would lower
down the performance gain. KVM borrows only one bit from vTR (TI?)
as vcpu id.

Thanks,
Kevin
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