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

["Roger Pau Monné" <roger.pau@citrix.com>]

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.

Roger.