Re: [Xen-devel] Foreign VCPU register change?

[Keir Fraser <keir.xen@xxxxxxxxx>]

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 that’s pretty much what you had in the first place, except for the extra vcpu_sleep_sync() on domctl entry. That’s absolutely critical, and why your pause_nosync on vmexit, unpause after domctl doesn’t 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 it’s not actually necessary here.

 -- Keir


On 23/08/2012 19:11, "Cutter 409" <cutter409@xxxxxxxxx> 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@xxxxxxxxx> 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@xxxxxxxxx <http://keir.xen@xxxxxxxxx> > 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 that’s 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@xxxxxxxxx <http://cutter409@xxxxxxxxx> > 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@xxxxxxxxx <http://keir.xen@xxxxxxxxx> > wrote:

On 23/08/2012 18:25, "Cutter 409" <cutter409@xxxxxxxxx <http://cutter409@xxxxxxxxx> > 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!
>
>
>
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> Xen-devel@xxxxxxxxxxxxx <http://Xen-devel@xxxxxxxxxxxxx>
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