Re: [Xen-devel] [PATCH RFC 1/3] x86/pvclock: add setter for pvclock_pvti_cpu0_va

[Andy Lutomirski <luto@xxxxxxxxxxxxxx>]

On Tue, Dec 29, 2015 at 4:50 AM, Joao Martins <joao.m.martins@xxxxxxxxxx> wrote:
> On 12/28/2015 11:45 PM, Andy Lutomirski wrote:
>> On Mon, Dec 28, 2015 at 1:52 PM, Joao Martins <joao.m.martins@xxxxxxxxxx> 
>> wrote:
>>> Right now there is only a pvclock_pvti_cpu0_va() which is defined on
>>> kvmclock since:
>>>
>>> commit dac16fba6fc5
>>> ("x86/vdso: Get pvclock data from the vvar VMA instead of the fixmap")
>>>
>>> The only user of this interface so far is kvm. This commit adds a setter
>>> function for the pvti page and moves pvclock_pvti_cpu0_va to pvclock, which
>>> is a more generic place to have it; and would allow other PV clocksources
>>> to use it, such as Xen.
>>>
>>
>>> +
>>> +void pvclock_set_pvti_cpu0_va(struct pvclock_vsyscall_time_info *pvti)
>>> +{
>>> +       pvti_cpu0_va = pvti;
>>> +}
>>
>> IMO this either wants to be __init or wants a
>> WARN_ON(vclock_was_used(VCLOCK_PVCLOCK)).  The latter hasn't landed in
>> -tip yet, but I think it'll land next week unless the merge window
>> opens early.
> OK, I will add those two once it lands in -tip.
>
> I had a silly mistake in this patch as I bindly ommited the parameter name to
> keep checkpatch happy, but didn't compile check when built without PARAVIRT.
> Apologies for that and will fix that also on the next version.
>
>>
>> It may pay to actually separate out the kvm-clock clocksource and
>> rename it rather than partially duplicating it, assuming the result
>> wouldn't be messy.
>>
> Not sure if I follow but I moved out pvclock_pvti_cpu0_va from kvm-clock or do
> you mean to separate out kvm-clock in it's enterity, or something else within
> kvm-clock is that is common to both (such as kvm_setup_vsyscall_timeinfo) ?

I meant literally using the same clocksource.  I don't know whether
the Xen and KVM variants are similar enough for that to make sense.

>
>> Can you CC me on the rest of the series for new versions?
>>
> Sure! Thanks for the prompt reply.
>
>> BTW, since this seems to require hypervisor changes to be useful, it
>> might make sense to rethink the interface a bit.  Are you actually
>> planning to support per-cpu pvti for this in any useful way?  If not,
>> I think that this would work a whole lot better and be considerably
>> less code if you had a single global pvti that lived in
>> hypervisor-allocated memory instead of an array that lives in guest
>> memory.  I'd be happy to discuss next week in more detail (currently
>> on vacation).
> Initially I had this series using per-cpu pvti's based on Linux 4.4 but since
> that was removed in favor of vdso using solely cpu0 pvti, then I ended up just
> registering the cpu 0 page. I don't intend to add per-cpu pvti's since it 
> would
> only be used for this case: (unless the reviewers think it should be done)
> meaning I would register pvti's for the other CPUs without having them used.
> Having a global pvti as you suggest it would get a lot simpler for the guest,
> but I guess this would only work assuming PVCLOCK_TSC_STABLE_BIT is there?
> Looking forward to discuss it next week.

Sounds good.

--Andy

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