Re: [Xen-devel] Relocatable Xen early boot code

[Daniel Kiper <daniel.kiper@xxxxxxxxxx>]

On Fri, Jun 12, 2015 at 02:19:01PM +0100, Andrew Cooper wrote:
> On 12/06/15 12:14, Daniel Kiper wrote:
> > Hey,
> >
> > During work on multiboot2 protocol support for Xen on EFI platform
> > I discovered that we need relocatable Xen early boot code (which is
> > mostly 32-bit code). More you can find here:
> >
> > http://lists.xen.org/archives/html/xen-devel/2015-02/msg01257.html
> >
> > I would like to focus on solution #1 described above. I have working PoC
> > which implements it. We also discussed various solutions for this issue
> > during Xen Hackhaton in Shanghai. As Andrew and Jan asked I tried to
> > implement solution based on segment registers. However, after revisiting
> > this issue and further investigation I still have some doubts. You can
> > read about my findings below.
> >
> > Here I do not want to discuss GRUB2 and multiboot2 protocol support details
> > for relocatable images. It is not needed. It is sufficient to know that it
> > is able to put loaded image anywhere in available memory below 4 GiB. Loaded
> > image is informed about its base address according to multiboot2 protocol 
> > via
> > special tag. This is new feature not available in upstream GRUB2. I work on
> > upstreaming it in parallel. Relevant patches will be posted together with
> > Xen patches.
> >
> > 1) My PoC uses %ebp (last unused register available globally in Xen early
> >    boot code; there is assumption that %ds == %es == %ss) as a storage for
> >    Xen image base address. If boot protocol do not support relocatable 
> > images
> >    it is filled with static value calculated during build. Otherwise it is
> >    taken from special multiboot2 tag.
> >
> >    We need Xen image base address to access memory in Xen image area in at
> >    least three different cases:
> >      a) direct memory access, e.g.: mov 
> > %eax,sym_offset(boot_tsc_stamp)(%ebp)
> >      b) memory address calculation, e.g.: lea 
> > sym_offset(__page_tables_start)(%ebp),%edx,
> >      c) update static addresses calculated during build, e.g. prebuild page 
> > tables.
> >
> >    If we have Xen image base address in a register then all mentioned 
> > operations
> >    are quite simple.
> >
> >    This idea is based on x86_64 addressing mode which uses %rip as a 
> > reference.
> >
> > 2) Andrew and Jan suggested that we can use segment descriptor to store Xen
> >    image base address. This way all references to variables in Xen image can
> >    be easily calculated during build and they will be static. However, 
> > relevant
> >    segment descriptor must be updated during Xen start.
> >
> >    At first sight, it looks that %cs, %ds, %es, %ss should be initialized 
> > as is
> >    (start: 0, size: 4 GiB, ...). This way we will not break references to 
> > trampoline
> >    and all other variables living outside of Xen image. Additionally, we 
> > are sure
> >    that all references to variables from C code (xen/arch/x86/boot/reloc.c) 
> > are
> >    pointing always to the same place regardless of x86 instruction 
> > generated by
> >    compiler and segment register used by this instruction for addressing. 
> > So, it
> >    looks that potentially we can use %fs or %gs as segment register to 
> > access
> >    variables living in Xen image. Looks good... Case a and b from solution 
> > #1 seams
> >    easy to resolve by prefixing by chosen segment register. Case c requires 
> > an extra
> >    register to store temporarily Xen image base address. Looks quite 
> > easy... However,
> >    looking at xen/arch/x86/boot/cmdline.S (at first sight this is the worst 
> > thing)
> >    I am not so happy with segment register solution. Unfortunately all 
> > functions
> >    get as argument just displacement in segment. This means that we must 
> > rework
> >    all stuff there and pass segment(s) as additional argument(s) because 
> > most of
> >    code checks memory in and out of Xen image in parallel. Of course we can 
> > use
> >    solution similar to case c described above but I think that then whole 
> > stuff
> >    move closer to idea #1. This way we will have something which uses things
> >    from #1 and #2. I do not think that this is solution which we want.
> >
> > 3) There is a third solution which is a mixture of #1 and #2. We can use 
> > e.g.
> >    %fs:0 (e.g. located somewhere in Xen image) to store Xen image base 
> > address.
> >    If this value is needed then we can access it directly, e.g. add 
> > %fs:0,%esi,
> >    or copy to temporary register and use as required.
> >
> > So, I am still in favor of #1. It is clean and easy. It does not require a 
> > lot of work.
> > #2 has a potential but requires a lot of changes in fragile cmdline.S 
> > (maybe others
> > difficult places). Is it worth? I think that #3 is a backup solution in 
> > case we choose
> > #1 and later it will appear that we need a globally unused register.
>
> I really don't understand why we have command line parsing written asm.
> I had half a mind to rewrite it all in C, (similar to the existing reloc
> code, but have gcc just create the .s and #include it as before, so
> external references work in a rational way).  It would be far clearer
> like that.

Yep, I remember that you said that once. Do you suggest that we should
do that right now? For me it make sense. Maybe we should also remove
other crazy stuff in that way.

> As stated at the hackathon, the problem with using %ebp is that it turns
> all implicit %ds references into implicit %ss references, and tends to

I am aware of that. However, I think that this is not a problem right
now because %ds == %ss. Additionally, I do not think that it will change
in the future.

> add a SIB+imm32 to each instruction with a memory reference (ebp
> relative memory references, and r13 for that matter, have restrictions
> in the way in which they can be encoded).

I am not sure what are you talking about here. What do you mean by r13?
Register name? In 32-bit mode? Could you point me a paragraph in Intel
or AMD docs which says about this restrictions?

Daniel

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel