Re: [Xen-devel] [PATCH v1 04/12] xen/hvmlite: Bootstrap HVMlite guest

[Boris Ostrovsky]

On 01/25/2016 05:19 PM, Luis R. Rodriguez wrote:
> On Sat, Jan 23, 2016 at 02:49:36PM +0000, Andrew Cooper wrote:
>
>
> > it causes inappropriate linkage between the
> > toolstack and the version of Linux wishing to be booted.
> There are ways to do it in such a way that it does not create dependency issues
> on Linux specific code.
>
>
> 0) The Xen way and EFI way
>
> A generic data structure is passed to the entry point on the kernel to
> load the kernel. The kernel in turn must parse this generic specific struct
> and interprets it and translate it to the kernel specific values.
>
> 1) The qemu way:
>
> One way is to simply not refer to the boot_params data structures but IMHO that
> produces really ugly code. The qemu folks did it this way, so qemu does not use
> arch/x86/include/uapi/asm/bootparam.h in any way, instead it uses offsets from
> a char *header. Refer to load_linux() in hw/i386/pc.c, for instance:
>
> header[0x211] |= 0x80;  /* CAN_USE_HEAP */
>
> 2) The grub way:
>
> Another way, which grub uses, is to define their own data structures based
> on arch/x86/include/uapi/asm/bootparam.h, with their own data types, and refer
> to them in code, for instance refer to grub_cmd_linux() on the grub file
> grub-core/loader/i386/pc/linux.c and its copy definition of the header definition
> in include/grub/i386/linux.h.
>
> lh.loadflags |= GRUB_LINUX_FLAG_CAN_USE_HEAP;
>
> The way lguest does it in the lguest launcher is IMHO the cleanest of course,
> but it includes asm/bootparam.h and uses that in code:
>
> /* Boot protocol version: 2.07 supports the fields for lguest. */
> boot->hdr.version = 0x207;
>
> But that does mean copying over or using the bootparam.h file and using
> linux data types.
>
> 3) Merge of xen way and using subarch_data
>
> Only set the subarch and subarch_data pointer, and have the kernel then
> read the generic data structure and parse it as it used to, the benefit
> is sharing a common entry point.
>
> No one uses this yet. But I think its a reasonable compromise.
>
> Perhaps there are other ways as well.
>
> > > If true, then by no means, no matter how hard we try, and no matter
> > > what we do on the Linux front to help clean things up will we be able
> > > to have a unified bare metal / Xen entry. I'm noting it could be
> > > possible though provided we do just set the zero page, the subarch to
> > > Xen and subarch_data to the Xen custom data structure.
> > All you need is a very small stub which starts per the DMLite ABI, sets
> > up an appropriate zero_page, and jumps to the native entrypoint.
> Right.


I am trying to understand what your objection is to what is proposed in 
this patch. It is the size of the stub? If yes -- what would you like to 
leave out to be done later?


-boris

> > However, this stub belongs in Linux, not in the Xen toolstack.  That
> > way, when the Linux boot protocol is modified, both sides can be updated
> > accordingly.
> Makes sense the different entry points just seems best avoided if possible.
>
>    Luis