Foreign VCPU register change?

Foreign VCPU register change?

[Cutter 409]

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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)

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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Re: Foreign VCPU register change?

[Keir Fraser]

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

Re: Foreign VCPU register change?

[Cutter 409]

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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
>
>
>

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Re: Foreign VCPU register change?

[Keir Fraser]

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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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Re: Foreign VCPU register change?

[Keir Fraser]

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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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Re: Foreign VCPU register change?

[Cutter 409]

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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> 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@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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Re: Foreign VCPU register change?

[Keir Fraser]

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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
>>> 
> 
> 
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xen.org
> http://lists.xen.org/xen-devel


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Re: Foreign VCPU register change?

[Cutter 409]

[-- Attachment #1.1: Type: text/plain, Size: 4995 bytes --]

That works perfectly, thank you for the help.

On Thu, Aug 23, 2012 at 2:55 PM, Keir Fraser <keir.xen@gmail.com> wrote:

>  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@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 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@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
>
>
>
>
> ------------------------------
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> http://lists.xen.org/xen-devel
>
>
>
>
> ------------------------------
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> Xen-devel@lists.xen.org
> http://lists.xen.org/xen-devel
>
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