Re: Foreign VCPU register change?

[Keir Fraser <keir.xen@gmail.com>]

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If you have your own flag in v->pause_flags then indeed you do not need to
vcpu_pause_nosync()  in the vmexit handler.

The best sequence would be:
 - vmexit handler: set flag in v->pause_flags, then vcpu_sleep_nosync()
 - domctl entry: vcpu_sleep_sync()
 - domctl exit: clear flag in v->pause_flags, then vcpu_wake()

So that¹s pretty much what you had in the first place, except for the extra
vcpu_sleep_sync() on domctl entry. That¹s absolutely critical, and why your
pause_nosync on vmexit, unpause after domctl doesn¹t work ‹ something is
needed on domctl entry to be sure that the vcpu is descheduled and its state
is synchronised. Of course the extra machinery of vcpu_pause/unpause is
harmless enough, but it¹s not actually necessary here.

 -- Keir


On 23/08/2012 19:11, "Cutter 409" <cutter409@gmail.com> wrote:

> Thanks, Keir!
> 
> I've spent so much time trying to track down this problem, even before I
> realized the registers weren't actually changing. You have no idea how helpful
> that was.
> 
> Before I tried your example, I just wrapped the code to change the register in
> vcpu_pause() and vcpu_unpause(), which worked.
> 
> Everything seems fine at the moment, is there any reason I should still change
> the vcpu_sleep_nosync() to vcpu_pause_nosync()? It seems to actually work as
> is; I'm setting a bit in v->pause_flags before I call it, then clear the bit
> before I wake it. I also tried pause_nosync on vmexit, unpause after domctl,
> but that didn't work.
> 
> Thanks again!
> 
> 
> On Thu, Aug 23, 2012 at 1:54 PM, Keir Fraser <keir.xen@gmail.com> wrote:
>> So, for example, one possibly-valid scheme would be:
>> 
>>  - vcpu_pause_nosync() from the vmexit handler
>> 
>>  - vcpu_sleep_sync() at the start of the domctl
>>  - vcpu_unpause() at the end of the domctl
>> 
>> HTH,
>>  Keir
>> 
>> 
>> 
>> On 23/08/2012 18:49, "Keir Fraser" <keir.xen@gmail.com
>> <http://keir.xen@gmail.com> > wrote:
>> 
>>> FWIW I would expect your approach to basically work.
>>> 
>>> Except... Does your domctl do a vcpu_pause()/vcpu_unpause() on the vcpu?
>>> This will ensure that the vcpu is both fully de-scheduled, and all of its
>>> register state is synced back into its vcpu structure.
>>> 
>>> Otherwise you race the vcpu_sleep_nosync() -- and that¹s assuming you also
>>> have a reason for that vcpu to sleep (e.g., non-zero pause counter), else
>>> vcpu_sleep_*() operations do nothing!
>>> 
>>> In short, your problems are almost certainly something to do with the
>>> subtleties of actually putting a vcpu properly to sleep.
>>> 
>>>  -- Keir
>>> 
>>> 
>>> On 23/08/2012 18:37, "Cutter 409" <cutter409@gmail.com
>>> <http://cutter409@gmail.com> > wrote:
>>> 
>>>> I'm making the register change directly from the hypervisor, inside of the
>>>> domctl code. 
>>>> 
>>>> It's a custom domctl that I've added. I'll look into what setcontext does
>>>> after it modifies the register values, though.
>>>> 
>>>> Thank you!
>>>> 
>>>> On Thu, Aug 23, 2012 at 1:34 PM, Keir Fraser <keir.xen@gmail.com
>>>> <http://keir.xen@gmail.com> > wrote:
>>>>> On 23/08/2012 18:25, "Cutter 409" <cutter409@gmail.com
>>>>> <http://cutter409@gmail.com> > wrote:
>>>>> 
>>>>>> > With Xen-4.1.2:
>>>>>> >
>>>>>> > I'm trying to change a register value in a paused vmx vcpu. The general
>>>>>> > process looks like this:
>>>>>> >
>>>>>> > 1. Some vmexit calls vcpu_sleep_nosync(v) on the vcpu
>>>>>> > 2. From dom0, I issue a domctl to change a register via
>>>>>> > v->arch.guest_context.user_reg, then vcpu_wake(v)
>>>>> 
>>>>> Which domctl? From dom0 userspace you can use the libxc functions
>>>>> xc_vcpu_getcontext() and xc_vcpu_setcontext() to read/modify register
>>>>> state.
>>>>> 
>>>>> You can read the libxc sources to see what hypercall these map to, if you
>>>>> don't want to use libxc for any reason.
>>>>> 
>>>>>  -- Keir
>>>>> 
>>>>>> > However, the guest register does not seem to be changed when I do it
>>>>>> this way.
>>>>>> > Is there something I need to do to mark the registers as "dirty" ? Is
>>>>>> there a
>>>>>> > way to force the foreign vcpu to update the changed registers? Or maybe
>>>>>> I just
>>>>>> > have to change the registers somewhere else?
>>>>>> >
>>>>>> > I've tried directly using vmcs_enter(v), __vmwrite(), vmcs_exit(v)
>>>>>> also, but
>>>>>> > that doesn't seem to make a change either.
>>>>>> >
>>>>>> > Thanks!
>>>>>> >
>>>>>> >
>>>>>> >
>>>>>> > _______________________________________________
>>>>>> > Xen-devel mailing list
>>>>>> > Xen-devel@lists.xen.org <http://Xen-devel@lists.xen.org>
>>>>>> > http://lists.xen.org/xen-devel
>>>>> 
>>>>> 
>>>> 
>>>> 
>>>> 
>>>> _______________________________________________
>>>> Xen-devel mailing list
>>>> Xen-devel@lists.xen.org <http://Xen-devel@lists.xen.org>
>>>> http://lists.xen.org/xen-devel
>>> 
> 
> 
> 
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