Re: [Xen-devel] [PATCH 1/3] PAD helper for native and paravirt platform

["Liu, Jinsong" <jinsong.liu@xxxxxxxxx>]

Konrad Rzeszutek Wilk wrote:
> On Wed, Mar 28, 2012 at 10:48:53AM +0000, Liu, Jinsong wrote:
>>>>> 
>>>>> Liu,
>>>>> 
>>>>> With this patch: "  xen/enlighten: Expose MWAIT and MWAIT_LEAF if
>>>>> hypervisor OKs it." which is now in 3.4-rc0:
>>>>> (http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=blobdiff;f=arch/x86/xen/enlighten.c;h=b132ade26f778f2cfec7c2d5c7b6db48afe424d5;hp=4172af8ceeb363d06912af15bf89e8508752b794;hb=d4c6fa73fe984e504d52f3d6bba291fd76fe49f7;hpb=aab008db8063364dc3c8ccf4981c21124866b395)
>>>>> it means that now that the drivers/acpi/acpi_pad.c can run
>>>>> as is under Xen (as the MWAIT_LEAF is exposed) What is the impact
>>>>> of that? Is the monitor call causing a trap to the hypervisor
>>>>> which will ignore the call? Or will it have some more worrysome
>>>>> consequences? 
>>>>> 
>>>> 
>>>> IMO this patch doesn't affect acpi_pad logic (both native and xen
>>>> acpi_pad).
>>> 
>>> You are sure? The acpi_pad logic will now be activated so the
>>> native driver will run under Xen. My question is - what is the
>>> impact of that? 
>> 
>> I know what you mean now. What I mean is, w/ xen_acpi_pad patches,
>> native acpi_pad only work under baremetal and xen_acpi_pad work
>> under Xen (so no problem exposing mwait). What you mean is, w/o
>> xen_acpi_pad patches, native acpi_pad will be actived under Xen and
>> then risk occur ... I agree.    
> 
> Can you test that? And see what happens please? I don't have the
> hardware 
> with _PUD.

I don't have this hardware neither. We test core parking by software simulation.

> 
>> 
>> But just curious, what's the purpose and benefit of exposing mwait
>> to dom0? I remember xen against doing so before. 
> 
> To expose deeper C-states to cstate.c so that xen-acpi-processor can
> then upload said states to the hypervisor.
> 
>> 
>>> 
>>> My assumption is that the __monitor call will trap and we end up in
>>> the hypervisor - so that is not so bad, but not sure.
>> 
>> Have you added code to hypervisor side (do_invalid_op)? if not, I
>> think it would be problem (break dom0). Dom0 __monitor would trigger
>> UD, then not handled by hypervisor, and bounce back to dom0 kernel,
>> and kill itself.   
> 
> No, that is why I am asking you.
>> 
>> But the point is, if exposing mwait, it would be risk for all logic
>> which executed __monitor. So need add native_monitor/ xen_monitor. 
> 
> Argh.
>> 
>>> 
>>> But what I wonder is if what is the impact of the _OST call by the
>>> native driver? 
>>> 
>>> Say the firmware tells us - please offline 4 CPUS (we have eight).
>>> We enter 'acpi_pad_handle_notify' -  create four threads, and each
>>> thread calls __monitor (which ends up in the hypervisor - and the
>>> hypervisor might not persue the __monitor call).
>>> 
>>> During this time, the Linux kernel calls the _OST with 4 CPUs and ..
>>> 
>>> what then? What happens if the _OST values are actually ignored (as
>>> it seems it would be in this case?) Is that OK? Or is that going to
>>> lead to the firmware turning off some of the cores anyhow?
>> 
>> Hmm, if __monitor was tolerated silently as you assume, it would
>> bring problem for _OST. 
> 
> What kind of problems?

_OST report that there are X cpus take action but in fact not guaranteed.

Thanks,
Jinsong
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