Re: [RFC v1 4/8] x86/init: add linker table support

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 01/21/2016 03:38 AM, Roger Pau Monné wrote:
> El 20/01/16 a les 22.33, Luis R. Rodriguez ha escrit:
>> On Wed, Jan 20, 2016 at 1:00 PM, Konrad Rzeszutek Wilk
>> <konrad.wilk@oracle.com> wrote:
>>>> +static bool x86_init_fn_supports_subarch(struct x86_init_fn *fn)
>>>> +{
>>>> +     if (!fn->supp_hardware_subarch) {
>>>> +             pr_err("Init sequence fails to declares any supported subarchs: %pF\n", fn->early_init);
>>>> +             WARN_ON(1);
>>>> +     }
>>>> +     if (BIT(boot_params.hdr.hardware_subarch) & fn->supp_hardware_subarch)
>>>> +             return true;
>>>> +     return false;
>>>> +}
>>> So the logic for this working is that boot_params.hdr.hardware_subarch
>>>
>>> And the macros define two: BIT(X86_SUBARCH_PC) or BIT(X86_SUBARCH_XEN).
>>>
>>> But hardware_subarch by default is set to zero. Which means if GRUB2, PXELinux, Xen multiboot1
>>> don't set it - then the X86_SUBARCH_PC is choosen right?
>>>
>>>   1 << 0 & 1 << X86_SUBARCH_PC (which is zero).
>>>
>>> For this to nicely work with Xen it ought to do this:
>>>
>>> diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
>>> index 993b7a7..6cf9afd 100644
>>> --- a/arch/x86/xen/enlighten.c
>>> +++ b/arch/x86/xen/enlighten.c
>>> @@ -1676,6 +1676,7 @@ asmlinkage __visible void __init xen_start_kernel(void)
>>>          boot_params.hdr.ramdisk_image = initrd_start;
>>>          boot_params.hdr.ramdisk_size = xen_start_info->mod_len;
>>>          boot_params.hdr.cmd_line_ptr = __pa(xen_start_info->cmd_line);
>>> +       boot_params.hdr.hardware_subarch = X86_SUBARCH_XEN;
>>>
>>>          if (!xen_initial_domain()) {
>>>                  add_preferred_console("xenboot", 0, NULL);
>>>
>>>
>>> ?
>> That's correct for PV and PVH, likewise when qemu is required for HVM
>> qemu could set it. I have the qemu change done but that should only
>> cover HVM. A common place to set this as well could be the hypervisor,
>> but currently the hypervisor doesn't set any boot_params, instead a
>> generic struct is passed and the kernel code (for any OS) is expected
>> to interpret this and then set the required values for the OS in the
>> init path. Long term though if we wanted to merge init further one way
>> could be to have the hypervisor just set the zero page cleanly for the
>> different modes. If we needed more data other than the
>> hardware_subarch we also have the hardware_subarch_data, that's a u64
>> , and how that is used would be up to the subarch. In Xen's case it
>> could do what it wants with it. That would still mean perhaps defining
>> as part of a Xen boot protocol a place where xen specific code can
>> count on finding more Xen data passed by the hypervisor, the
>> xen_start_info. That is, if we wanted to merge init paths this is
>> something to consider.
>>
>> One thing I considered on the question of who should set the zero page
>> for Xen with the prospect of merging inits, or at least this subarch
>> for both short term and long term are the obvious implications in
>> terms of hypervisor / kernel / qemu combination requirements if the
>> subarch is needed. Having it set in the kernel is an obvious immediate
>> choice for PV / PVH but it means we can't merge init paths completely
>> (down to asm inits), we'd still be able to merge some C init paths
>> though, the first entry would still be different. Having the zero page
>> set on the hypervisor would go long ways but it would mean a
>> hypervisor change required.
> I don't think the hypervisor should be setting Linux specific boot
> related parameters, the boot ABI should be OS agnostic. IMHO, a small
> shim should be added to Linux in order to set what Linux requires when
> entering from a Xen entry point.

And that's exactly what HVMlite does. Most of this shim layer is setting 
up boot_params, after which we jump to standard x86 boot path (i.e. 
startup_{32|64}). With hardware_subarch set to zero.

-boris