Re: [Xen-devel] [PATCH 17/17] xen/arm64: Zero BSS after the MMU and D-cache is turned on

[Julien Grall <julien.grall@xxxxxxx>]

Hi Stefano,

On 26/06/2019 22:08, Stefano Stabellini wrote:
> On Wed, 26 Jun 2019, Julien Grall wrote:
> > Hi Stefano,
> >
> > On 6/26/19 8:29 PM, Stefano Stabellini wrote:
> > > On Mon, 10 Jun 2019, Julien Grall wrote:
> > > > At the moment BSS is zeroed before the MMU and D-Cache is turned on.
> > > > In other words, the cache will be bypassed when zeroing the BSS section.
> > > >
> > > > Per the Image protocol [1], the state of the cache for BSS region is not
> > > > known because it is not part of the "loaded kernel image".
> > > >
> > > > This means that the cache will need to be invalidated twice for the BSS
> > > > region:
> > > >       1) Before zeroing to remove any dirty cache line. Otherwise they may
> > > >       get evicted while zeroing and therefore overriding the value.
> > > >       2) After zeroing to remove any cache line that may have been
> > > >       speculated. Otherwise when turning on MMU and D-Cache, the CPU may
> > > >       see old values.
> > > >
> > > > However, the only reason to have the BSS zeroed early is because the
> > > > boot page tables are part of BSS. To avoid the two cache invalidations,
> > > > it is possible to move the page tables in the section .data.page_aligned.
> > > I am not following the last part. How is moving the boot page tables to
> > > .data.page_aligned solving the problem? Do we need to zero
> > > .data.page_aligned early or can we skip it because it is guaranteed to
> > > already be zero?
> > Global variables are initialized to zero by default regardless the section
> > they reside. Usually they are stored in BSS because it saves space in the
> > binary.
> >
> > With the Image protocol, BSS is not initialized by the bootloader so we have
> > to do ourself.
> >
> > The section .data.page_aligned is always part of the binary. So the compiler
> > will write zero in the binary for any global variables part of that section
> > and therefore the corresponding memory will be zeroed when loading the binary.
> >
> > If it makes sense, I can try to incorporate that in the commit message.
> So basically it is really only BSS the problem. All right, looks good to
> me.
Yes that's correct.

> Acked-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Thank you!

Cheers,

--
Julien Grall

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel