[SeaBIOS] [RFC 2/3] Transitions to and from 64 bits

[Kevin O'Connor]

On Mon, Sep 21, 2015 at 04:38:00PM +0200, Marc Marí wrote:
> On Mon, 21 Sep 2015 15:57:56 +0200
> Gerd Hoffmann <kraxel at redhat.com> wrote:
> > > +/****************************************************************
> > > + * Page table
> > > + ****************************************************************/
> > > +void *gen_identity_page_table(u64 max_addr)
> > > +{
> > > +    /* Map directly all the addresses */
> > > +    u32 pt_entries = (max_addr + 0xFFF) >> 12;
> > > +    u32 pdt_entries = (pt_entries + 0x1FF) >> 9;
> > > +    u32 pdpt_entries = (pdt_entries + 0x1FF) >> 9;
> > > +    u32 pml4_entries = (pdpt_entries + 0x1FF) >> 9;
> > 
> > You can use hugepages.  Then you'll need _alot_ less memory for the
> > pagetables, and the copying probably goes faster too.
> 
> True. Tried with 2M. The memory used went down from 8M to 24K more or
> less, and the time for the copying went down by 4ms (from 15ms to
> 11ms). The other option is 1GB. I'll test later if it's enabled in QEMU
> CPUs.
> 
> > You might consider mapping the nvdimm below 4G instead of entering
> > long mode.
> 
> The QEMU NVDIMM driver maps it directly above 4G to avoid the trouble
> of looking for empty spaces in the below 4G section. I didn't want to
> rework the driver or add more complexity to it.
> 
> If you were talking about just enabling paging and map the NVDIMM to
> below 4G, it would work for just a few gigs above 4G (don't remember
> exactly the number). So better give it all.

According to my Intel docs, PAE supports the same 52bit physical
address range that x86_64 supports.

Switching to/from PAE looks like it would be notably simpler (it's
just four 4KB page directories loaded with 2MB pages) and I think it
may be possible to do the switch entirely in C code.  But it does
require the pain of "bank switching" to access the high memory.

> And SeaBIOS may need 64 bit support for other things in the future, so
> this might be useful later.

If that's the case, those things will only work on 64bit CPUs, and
SeaBIOS currently supports machines as old as i386.  (BTW, your patch
should check the cpuid stuff before jumping to 64bit mode.)

Also, your code seems to run regular 32bit code when in "long mode" -
is that valid?

-Kevin