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

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 17/09/2009 20:13, "Nakajima, Jun" <jun.nakajima@xxxxxxxxx> wrote:

>>> A remaining hard problem is that this single
>>> "userland-accessible shared page" must be somehow
>>> made available to apps (I suggested a rdmsr emulated
>>> by Xen so that it works in userland) and must be
>>> mapped into the app address space without kernel
>>> changes.  I think someone (Keir?) suggested this
>>> problem was solveable before we got sidetracked
>>> on the need-vcpu-number-in-userland problem.
>>  I don't think mapping things into application address space is really
>> possible without guest kernel changes. The guest kernel owns and manages
>> the pte that you'd be overwriting. Just blatting the pte would not be
>> good form.
> 
> 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?

Yeah, that'd work. Doesn't even really need any Xen changes -- the
stable-TSC flag is in CPUID already, and the guest driver can concoct a
pvclock page I would think (e.g., from its own Xen pvclock TSC scaling
parameters).

 -- Keir



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