RE: [PATCH v2 11/40] xen/mpu: build up start-of-day Xen MPU memory region map

[Penny Zheng <Penny.Zheng@xxxxxxx>]

Hi, Julien

> -----Original Message-----
> From: Julien Grall <julien@xxxxxxx>
> Sent: Thursday, February 2, 2023 6:58 PM
> To: Penny Zheng <Penny.Zheng@xxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx;
> Wei Chen <Wei.Chen@xxxxxxx>; Stefano Stabellini
> <sstabellini@xxxxxxxxxx>; Bertrand Marquis <Bertrand.Marquis@xxxxxxx>;
> ayan.kumar.halder@xxxxxxxxxx
> Cc: Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>
> Subject: Re: [PATCH v2 11/40] xen/mpu: build up start-of-day Xen MPU
> memory region map
> 
> 
> 
> On 02/02/2023 10:53, Penny Zheng wrote:
> > Hi Julien,
> 
> Hi,
> 
> >> -----Original Message-----
> >> From: Julien Grall <julien@xxxxxxx>
> >> Sent: Thursday, February 2, 2023 2:57 AM
> >> To: Penny Zheng <Penny.Zheng@xxxxxxx>;
> >> xen-devel@xxxxxxxxxxxxxxxxxxxx; Wei Chen <Wei.Chen@xxxxxxx>;
> Stefano
> >> Stabellini <sstabellini@xxxxxxxxxx>; Bertrand Marquis
> >> <Bertrand.Marquis@xxxxxxx>; ayan.kumar.halder@xxxxxxxxxx
> >> Cc: Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>
> >> Subject: Re: [PATCH v2 11/40] xen/mpu: build up start-of-day Xen MPU
> >> memory region map
> >>
> >> Hi Penny,
> >>
> >> On 01/02/2023 05:39, Penny Zheng wrote:
> >>>>> If we are adding MPU regions in sequence as you suggested, while
> >>>>> using bitmap at the same time to record used entry.
> >>>>> TBH, this is how I designed at the very beginning internally. We
> >>>>> found that if we don't do reorg late-boot to keep fixed in front
> >>>>> and switching ones after, each time when we do vcpu context
> >>>>> switch, not only we need to hunt down switching ones to disable,
> >>>>> while we add new switch-in regions, using bitmap to find free
> >>>>> entry is saying that the
> >>>> process is unpredictable. Uncertainty is what we want to avoid in
> >>>> Armv8-R architecture.
> >>>>
> >>>> I don't understand why it would be unpredictable. For a given
> >>>> combination of platform/device-tree, the bitmap will always look
> >>>> the same. So the number of cycles/instructions will always be the
> same.
> >>>>
> >>>
> >>> In boot-time, it will be always the same. But if we still use bitmap
> >>> to find free entry(for switching MPU regions) on runtime, hmmm, I
> >>> thought this part will be unpredictable.
> >>
> >> I know this point is now moot as we agreed on not using a bitmap but
> >> I wanted to answer on the unpredictability part.
> >>
> >> It depends on whether you decide to allocate more entry at runtime.
> >> My assumption is you won't and therefore the the time to walk the
> >> bitmap will always be consistent.
> >>
> >
> > In MPU, we don't have something like vttbr_el2 in MMU, to store stage
> > 2
> > EL1/EL0 translation table. Xen stage 1 EL2 mapping and stage 2 EL1/EL0
> > mapping are both sharing one table.
> > So when context switching into different guest, the current design is
> > to disable DOM1's guest RAM mapping firstly, then enable DOM2's guest
> > RAM mapping, to ensure isolation and safety.
> 
> I understood that but I don't understand how this is related to my point
> here. The entries you are replacing are always going to be the same after
> boot.
> 
> So if you have a bitmap indicate the fixed entries and you don't add more
> fixed one at runtime, then it will always take the same time to walk it.
> 

Ah, sorry for taking so long to understand ;/. True, the fixed entries will 
never
change after boot-time, each time when switching to guest vcpu, we always choose
the same entry.

> Cheers,
> 
> --
> Julien Grall

Cheers,

--
Penny Zheng