Re: Foreign VCPU register change?

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

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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> 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> 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> wrote:
>>> On 23/08/2012 18:25, "Cutter 409" <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://lists.xen.org/xen-devel
>>> 
>>> 
>> 
>> 
>> 
>> _______________________________________________
>> Xen-devel mailing list
>> Xen-devel@lists.xen.org
>> http://lists.xen.org/xen-devel
> 


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