Jeremy Fitzhardinge wrote
> This all makes me think it would be more awkward than helpful to have
> the Xen boot path go through the normal startup_32 path.
>
If you move the CPUID code out of head.S into C, it gets a lot cleaner:
/* CPUID is virtualized. Now get vendor info. */
cpuid(0, &new_cpu_data.cpuid_level,
(int *)&new_cpu_data.x86_vendor_id[0],
(int *)&new_cpu_data.x86_vendor_id[8],
(int *)&new_cpu_data.x86_vendor_id[4]);
/* Get feature bits. */
cpuid(1, &eax, &ebx, &ecx, &edx);
new_cpu_data.x86 = (eax >> 8) & 0xf;
new_cpu_data.x86_model = (eax >> 4) & 0xf;
new_cpu_data.x86_mask = eax & 0xf;
new_cpu_data.x86_capability[0] = edx;
new_cpu_data.hard_math = 1;
> Zach proposed a change to the beginning of startup_32 to see if its
> running in ring != 0 or if paging is already enabled, and then jumping
> to a startup_paravirt entrypoint. That's workable, but it essentially
> means we're creating a distinct hypervisor boot protocol. That's not
> necessarily a bad thing - and it could be made to look more like the
> normal boot protocol - but because the setup code is so simple there
> doesn't seem to be a lot to be gained from it. In the Xen case, it
> makes more sense to simply have a separate Xen-specific entrypoint to do
> a little bit of setup before jumping into start_kernel.
>
Why do you insist that Xen have a separate kernel entry point?
Publishing and maintaining multiple entry points is messy. Please come
up with a convincing argument that can't be thwarted with a couple lines
of code. The only thing you need to do is stash %esi to save the Xen
start info. It might not be a bad idea to preserve the full BSP
register state on entry, even for native hardware. And we can certainly
accomodate stashing %esi away. You'll get to run the hypervisor
specific code a few instructions later, when we call out to fill in the
paravirt-ops table based on %ebx. And then you can write paravirt_ops
hooks for the things Xen has changed - command line, physical e820 map,
and do everything in C-code along the standard Linux startup path.
Zach
