On 10/29/2012 07:45 AM, Glauber Costa wrote:
> On 10/24/2012 05:13 PM, Marcelo Tosatti wrote:
>> Allow a guest to register a second location for the VCPU time info
>>
>> structure for each vcpu (as described by MSR_KVM_SYSTEM_TIME_NEW).
>> This is intended to allow the guest kernel to map this information
>> into a usermode accessible page, so that usermode can efficiently
>> calculate system time from the TSC without having to make a syscall.
>>
>> Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
> Can you please be a bit more specific about why we need this? Why does
> the host need to provide us with two pages with the exact same data? Why
> can't just do it with mapping tricks in the guest?
In Xen the pvclock structure is embedded within a pile of other stuff
that shouldn't be mapped into guest memory, so providing for a second
location allows it to be placed whereever is convenient for the guest.
That's a restriction of the Xen ABI, but I don't know if it affects KVM.
J
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