[Xen-devel] Re: pvclock in userland (reprise)

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

On 09/17/09 12:13, Nakajima, Jun wrote:
> Maybe you can write a device driver in the guest that sets up mapping against 
> the (virtual) physical memory, then use mmap() in the app?
>   

Dan is trying to avoid making guest kernel changes, on the grounds that
waiting for enterprise distros to catch up would take too long.

Once you're making kernel changes then you can update the pvclock
mechanism to use the xen clock algorithm, obviating the need for
usermode ABI changes.

However, if its really the case that the tsc is guaranteed synchronized,
then the guest can determine that for itself by looking at cpuid and/or
/proc/cpuinfo (and presumably doing some sanity checking) and then just
directly use rdtsc, with no need to change either Xen or the kernel.

    J

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