Re: [Minios-devel] [UNIKRAFT PATCH 16/22] plat/kvm: Disable D-Cache before doing any change for Arm64

["Wei Chen (Arm Technology China)" <Wei.Chen@xxxxxxx>]

> -----Original Message-----
> From: Julien Grall <julien.grall@xxxxxxx>
> Sent: 2018年11月9日 18:17
> To: Wei Chen (Arm Technology China) <Wei.Chen@xxxxxxx>; minios-
> devel@xxxxxxxxxxxxxxxxxxxx; simon.kuenzer@xxxxxxxxx
> Cc: Kaly Xin (Arm Technology China) <Kaly.Xin@xxxxxxx>; nd <nd@xxxxxxx>
> Subject: Re: [Minios-devel] [UNIKRAFT PATCH 16/22] plat/kvm: Disable D-Cache
> before doing any change for Arm64
> 
> 
> 
> On 09/11/2018 07:36, Wei Chen (Arm Technology China) wrote:
> > Hi Julien,
> 
> Hi,
> 
> >
> >> -----Original Message-----
> >> From: Julien Grall <julien.grall@xxxxxxx>
> >> Sent: 2018年11月8日 20:20
> >> To: Wei Chen (Arm Technology China) <Wei.Chen@xxxxxxx>; minios-
> >> devel@xxxxxxxxxxxxxxxxxxxx; simon.kuenzer@xxxxxxxxx
> >> Cc: Kaly Xin (Arm Technology China) <Kaly.Xin@xxxxxxx>; nd <nd@xxxxxxx>
> >> Subject: Re: [Minios-devel] [UNIKRAFT PATCH 16/22] plat/kvm: Disable D-
> Cache
> >> before doing any change for Arm64
> >>
> >> Hi,
> >>
> >> On 11/8/18 9:51 AM, Wei Chen wrote:
> >>> Disable the D-Cache before doing anything change, This will
> >>> help us to avoid doing clean the D-Cache for changed area.
> >>
> >> NAcked-by: Julien Grall <julien.grall@xxxxxxx>
> >>
> >> I am pretty sure we discussed about it in the previous series. D-Cache
> >> disable means the cache will be bypassed. The processor may still
> >> speculatively load cache line behind your back.
> >>
> >> So if you change memory with cache disable, then you need to invalidate
> >> the cache to ensure there are no stall data in it.
> >>
> >> Furthermore, for data you write with cache disabled,  you need to know
> >> whether the cache was cleaned to PoC for the region you modify. The
> >> protocol you use should tell you what is the guarantee here.
> >>
> >> If there are no guarantee, then you need to clean to PoC the region you
> >> are about to modify.
> >>
> >> For instance, the Linux Image protocol will guarantee that the DTB,
> >> Kernel, Initramfs will be clean to PoC. The rest of the memory is unknown.
> >>
> >> As you use ELF, I invite you to look at what are the guarantees here.
> >>
> >
> > I think I had mis-understood your previous comment. So I call
> > clean_and_invalidate_dcache_range at the very beginning, should I cover the
> > whole RAM area? Because I don't know the areas that had been cached.
> 
> You don't know the size of the RAM either. But it would be expensive to flush
> the whole cache. In your case, you only need to cover what you are going to
> use
> (i.e the kernel, BSS, bootstack and page-tables).
> 

That makes sense.

> > And I do clean_and_invalidate_dcache_range at the very beginning, should I
> > Keep the clean_and_invalidate_dcache_range operation in start MMU function?
> 
> Yes. Note that you need to do the clean & invalidate before re-enabling the
> cache.
> 
> Cheers,
> 
> --
> Julien Grall
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