[PATCH 0/4] RFC: split text and linear mappings using tagged pointers

[Ard Biesheuvel]

On 26 March 2015 at 02:26, Mark Rutland <mark.rutland at arm.com> wrote:
>
> Hi Ard,
>
> On Mon, Mar 23, 2015 at 03:36:52PM +0000, Ard Biesheuvel wrote:
> > This implements a split mapping of the kernel Image using the AArch64
> > tagged pointers feature. The kernel text is emitted with bit 56 cleared,
> > which can be used by the virt to phys translation to choose one translation
> > regime or another.
>
> This looks neat!
>
> I definitely want to see the linear mapping decoupled from the text mapping,
> but I'm rather worried by the prospect of tagged pointers in the kernel (there
> are a lot of edge cases for those w.r.t. the preservations of the tag bits),
> and if possible I'd like to avoid their use. I think that we can achieve the
> same effect by reorganising the VA layout within the 56 bits we have to play
> with.
>

Yes, it was nice as an experiment, but it breaks down in plenty of
unexpected places. It appears that this

-#define __virt_to_phys(x)      (((phys_addr_t)(x) - PAGE_OFFSET + PHYS_OFFSET))
+#define __virt_to_phys(x) ({                                           \
+       long __x = (long)(x) - PAGE_OFFSET;                             \
+       __x >= 0 ? (phys_addr_t)(__x + PHYS_OFFSET) :                   \
+                  (phys_addr_t)(__x + PHYS_OFFSET + image_offset); })
+

is really all we need.

>
> Nit: please s/translation regime/mapping/ (or some other wording to that
> effect) for these patches; the architectural definition of "translation regime"
> differs from what you're referring to here, and we should avoid overloading
> that terminology.
>

Ah ok. I thought it had a nice ring to it :-)


> > The purpose is to allow PHYS_OFFSET to start at an arbitrary offset below
> > the kernel image, so that memory below the kernel Image can be used, allowing
> > the Image to be loaded anywhere in physical memory.
> >
> > This series moves the kernel text right below PAGE_OFFSET, next to the modules
> > area. PHYS_OFFSET is chosen to be a suitable aligned boundary somewhere below
> > the kernel Image (1 GB or 512 MB depending on chosen page size).
> >
> > Output from a QEMU/EFI boot:
> >
> > System RAM:
> >  memory[0x0]  [0x00000040000000-0x000000bfffffff], 0x80000000 bytes flags: 0x0
> >
> > Kernel image:
> >  reserved[0x0]        [0x0000005f680000-0x0000005fe68fff], 0x7e9000 bytes flags: 0x0
> >
> > Virtual kernel memory layout:
> >     vmalloc : 0xffffff8000000000 - 0xffffffbdbfff0000   (   246 GB)
> >     vmemmap : 0xffffffbdc0000000 - 0xffffffbfc0000000   (     8 GB maximum)
> >               0xffffffbdc1000000 - 0xffffffbdc3000000   (    32 MB actual)
> >     fixed   : 0xffffffbff69fd000 - 0xffffffbff6c00000   (  2060 KB)
> >     PCI I/O : 0xffffffbff6e00000 - 0xffffffbff7e00000   (    16 MB)
> >     modules : 0xffffffbff8000000 - 0xffffffbffc000000   (    64 MB)
> >     memory  : 0xffffffc000000000 - 0xffffffc080000000   (  2048 MB)
> >       .init : 0xfeffffbffc783000 - 0xfeffffbffc7c6000   (   268 KB)
> >       .text : 0xfeffffbffc080000 - 0xfeffffbffc782344   (  7177 KB)
> >       .data : 0xfeffffbffc7ca000 - 0xfeffffbffc830c00   (   411 KB)
> >
> > Note that this time, no relocations were harmed in the making of these
> > patches. With added relocation support, it should be possible to move
> > the combined modules and kernel text anywhere in the vmalloc area (for
> > kaslr)
> >
> > There are probably some places where the cleared bit 56 in the virtual
> > address may cause trouble. Known problem is the module loader, but there
> > are surely others.
> >
> >
> > Ard Biesheuvel (4):
> >   arm64: use tagged pointers to distinguish kernel text from the linear
> >     mapping
> >   arm64: fixmap: move translation tables to dedicated region
> >   arm64: move kernel text below PAGE_OFFSET
> >   arm64: align PHYS_OFFSET to block size
> >
> >  arch/arm64/include/asm/linkage.h       |  2 ++
> >  arch/arm64/include/asm/memory.h        | 27 +++++++++++++++++--
> >  arch/arm64/include/asm/pgtable-hwdef.h |  1 +
> >  arch/arm64/kernel/head.S               | 48 +++++++++++++++++++++++++---------
> >  arch/arm64/kernel/vmlinux.lds.S        | 19 +++++++++-----
> >  arch/arm64/mm/mmu.c                    | 21 +++++++++------
> >  arch/arm64/mm/proc.S                   |  3 ++-
> >  7 files changed, 91 insertions(+), 30 deletions(-)
> >
> > --
> > 1.8.3.2
> >
> >