Re: [PATCH v3 2/9] x86/PV: properly set shadow allocation for Dom0

[Jan Beulich <jbeulich@xxxxxxxx>]

On 22.09.2021 16:25, Roger Pau Monné wrote:
> On Wed, Sep 22, 2021 at 03:50:25PM +0200, Jan Beulich wrote:
>> On 22.09.2021 15:31, Andrew Cooper wrote:
>>> On 21/09/2021 08:17, Jan Beulich wrote:
>>>> @@ -933,7 +934,18 @@ int __init dom0_construct_pv(struct doma
>>>>  #ifdef CONFIG_SHADOW_PAGING
>>>>      if ( opt_dom0_shadow )
>>>>      {
>>>> +        bool preempted;
>>>> +
>>>>          printk("Switching dom0 to using shadow paging\n");
>>>> +
>>>> +        nr_pt_pages = dom0_paging_pages(d, nr_pages);
>>>> +
>>>> +        do {
>>>> +            preempted = false;
>>>> +            shadow_set_allocation(d, nr_pt_pages, &preempted);
>>>> +            process_pending_softirqs();
>>>> +        } while ( preempted );
>>>
>>> This is still broken.
>>>
>>> The loop setting the shadow allocation needs to be outside of this
>>> conditional, because it is not related to early activation of the l1tf
>>> tasklet.
>>
>> Well, I'm not sure what to say. On v1 you already said so. But then you
>> didn't care to reply to me responding:
>>
>> "Are you suggesting to set up a (perhaps large) shadow pool just in
>>  case we need to enable shadow mode on Dom0? And all of this memory
>>  to then remain unused in the majority of cases?
>>
>>  Plus even if so, I'd view this as a 2nd, independent step, largely
>>  orthogonal to the handling of "dom0=shadow". If somebody really
>>  wanted that, I think this should be driven by an explicit setting
>>  of the shadow pool size, indicating the admin is willing to waste
>>  the memory.
> 
> Maybe an acceptable compromise would be to allocate the pool if
> opt_dom0_shadow || opt_pv_l1tf_hwdom?
> 
> opt_pv_l1tf_hwdom is not enabled by default, so an admin opting to
> enable it should also be willing to reserve the memory it requires in
> case it needs activating during runtime.

I'd be fine making that change. Andrew - would that be sufficient to
address your concern?

Jan