Re: [PATCH for-4.19] x86/mtrr: avoid system wide rendezvous when setting AP MTRRs

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.05.2024 15:10, Roger Pau Monné wrote:
> On Tue, May 14, 2024 at 01:57:13PM +0200, Jan Beulich wrote:
>> On 13.05.2024 10:59, Roger Pau Monne wrote:
>>> --- a/xen/arch/x86/cpu/mtrr/main.c
>>> +++ b/xen/arch/x86/cpu/mtrr/main.c
>>> @@ -573,14 +573,15 @@ void mtrr_ap_init(void)
>>>     if (!mtrr_if || hold_mtrr_updates_on_aps)
>>>             return;
>>>     /*
>>> -    * Ideally we should hold mtrr_mutex here to avoid mtrr entries changed,
>>> -    * but this routine will be called in cpu boot time, holding the lock
>>> -    * breaks it. This routine is called in two cases: 1.very earily time
>>> -    * of software resume, when there absolutely isn't mtrr entry changes;
>>> -    * 2.cpu hotadd time. We let mtrr_add/del_page hold cpuhotplug lock to
>>> -    * prevent mtrr entry changes
>>> +    * hold_mtrr_updates_on_aps takes care of preventing unnecessary MTRR
>>> +    * updates when batch starting the CPUs (see
>>> +    * mtrr_aps_sync_{begin,end}()).
>>> +    *
>>> +    * Otherwise just apply the current system wide MTRR values to this AP.
>>> +    * Note this doesn't require synchronization with the other CPUs, as
>>> +    * there are strictly no modifications of the current MTRR values.
>>>      */
>>> -   set_mtrr(~0U, 0, 0, 0);
>>> +   mtrr_set_all();
>>>  }
>>
>> While I agree with the change here, it doesn't go quite far enough. 
>> Originally
>> I meant to ask that, with this (supposedly) sole use of ~0U gone, you please
>> also drop the handling of that special case from set_mtrr(). But another
>> similar call exist in mtrr_aps_sync_end(). Yet while that's "fine" for the
>> boot case (watchdog being started only slightly later), it doesn't look to be
>> for the S3 resume one: The watchdog is re-enabled quite a bit earlier there.
>> I actually wonder whether mtrr_aps_sync_{begin,end}() wouldn't better
>> themselves invoke watchdog_{dis,en}able(), thus also making the boot case
>> explicitly safe, not just safe because of ordering.
> 
> Hm, I don't like disabling the watchdog, I guess it could be
> acceptable here because both usages of mtrr_aps_sync_end() are limited
> to specific scenarios (boot or resume from suspension).  I can prepare
> a separate patch, but I don't think the watchdog disabling should be
> part of this patch.

Not sure (as to being part of this patch). Of course it would be okay to
address the S3 side separately, whichever approach we use. Yet imo it
would also be okay to address both in one go, again whichever approach we
use. If you prefer a separate one, so be it.

Jan