[Xen-devel] RE: rdtsc: correctness vs performance on Xen (and KVM?)

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>>> Dan Magenheimer <dan.magenheimer@xxxxxxxxxx> 01.09.09 17:26 >>>
>> On 01/09/2009 15:53, "Dan Magenheimer" 
>> <dan.magenheimer@xxxxxxxxxx> wrote:
>> 
>> > 1) fake rdmsr (or hypercall if it works) returns a virtual
>> >    address within a range of addresses that is not "owned by"
>> >    the OS (e.g. maybe in Xen address space?).  The page is
>> >    only readable outside of ring 0, but writeable in ring 0
>> >    (by Xen).
>> > 2) All TLB misses on this page are handled directly by Xen
>> >    so the OS never sees the address/page.
>> 
>> I think these are probably possible, at least for a 64-bit 
>> hypervisor which
>> isn't playing segment limit tricks.
>
>Will it work for pv32_on_64?  (I don't care much about
>32-bit hypervisor.)

It can be made work - you just need to properly arrange this and the
compatibility p2m table.
 
>> > If these are OK, and you see other parts of the proposal
>> > that require PV kernel mods, please point them out.
>> 
>> Won't the pvclock computation be per-cpu? How will you deal with
>> that?
>
>Hmmm... is it possible for the same virtual address/page
>to map to a different physical address/page on each processor?

Not within today's Xen or Linux (which both assume a global kernel
address space, in particular non-root page table entries mapping kernel
space to be the same in all address spaces - you'd need separate entries
at all levels for this).

Jan


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