RE: Discussion of Xenheap problems on AArch64

[Henry Wang <Henry.Wang@xxxxxxx>]

Hi Julien,

> -----Original Message-----
> From: Julien Grall <julien@xxxxxxx>
> Sent: Wednesday, April 21, 2021 5:04 PM
> To: Henry Wang <Henry.Wang@xxxxxxx>; sstabellini@xxxxxxxxxx; xen-
> devel@xxxxxxxxxxxxxxxxxxxx
> Cc: Wei Chen <Wei.Chen@xxxxxxx>; Penny Zheng
> <Penny.Zheng@xxxxxxx>; Bertrand Marquis <Bertrand.Marquis@xxxxxxx>
> Subject: Re: Discussion of Xenheap problems on AArch64
> 
> 
> 
> On 21/04/2021 07:28, Henry Wang wrote:
> > Hi,
> 
> Hi Henry,
> 
> >
> > We are trying to implement the static memory allocation on AArch64. Part
> of
> > this feature is the reserved heap memory allocation, where a specific range
> of
> > memory is reserved only for heap. In the development process, we found a
> > pitfall in current AArch64 setup_xenheap_mappings() function.
> >
> > According to a previous discussion in community
> > https://lore.kernel.org/xen-devel/20190216134456.10681-1-
> peng.fan@xxxxxxx/,
> > on AArch64, bootmem is initialized after setup_xenheap_mappings(),
> > setup_xenheap_mappings() may try to allocate memory before memory
> has been
> > handed over to the boot allocator. If the reserved heap memory allocation
> is
> > introduced, either of below 2 cases will trigger a crash:
> >
> > 1. If the reserved heap memory is at the end of the memory block list and
> the
> > gap between reserved and unreserved memory is bigger than 512GB, when
> we setup
> > mappings from the beginning of the memory block list, we will get OOM
> caused
> > by lack of pages in boot allocator. This is because the memory that is
> reserved
> > for heap has not been mapped and added to the boot allocator.
> >
> > 2. If we add the memory that is reserved for heap to boot allocator first,
> and
> > then setup mappings for banks in the memory block list, we may get a page
> which
> > has not been setup mapping, causing a data abort.
> 
> There are a few issues with setup_xenheap_mappings(). I have been
> reworking the code on my spare time and started to upstream bits of it.
> A PoC can be found here:
> 
> https://xenbits.xen.org/gitweb/?p=people/julieng/xen-
> unstable.git;a=shortlog;h=refs/heads/pt/dev
> 

Really great news! Thanks you very much for the information and your hard
work on the PoC :) I will start to go through your PoC code then.

Kind regards,
Henry

> Cheers,
> 
> --
> Julien Grall