Re: [Qemu-devel] How does the QEMU load the binary files bios.bin and vgabios-cirrus.bin?

[Gleb Natapov <gleb@redhat.com>]

On Thu, Apr 07, 2011 at 12:03:41PM -0700, Jordan Justen wrote:
> On Thu, Apr 7, 2011 at 11:44, Gleb Natapov <gleb@redhat.com> wrote:
> > On Thu, Apr 07, 2011 at 01:37:14PM -0500, Anthony Liguori wrote:
> >> >>>>The CS base starts out at 0xf0000 and IP is 0xfff0.  That gives a
> >> >>>>real address of 0xffff0.  This is usually a trampoline to somewhere
> >> >>>>else in the space.
> >> >>>CS descriptor and CS selector don't have to be in sync (big real mode).
> >> >>Indeed.
> >> >Another place this will often be seen is SMM, as the SMBASE can easily
> >> >be>  1MB, but the SMM entry is in 16 bit mode.
> >>
> >> KVM doesn't support SMM although that's not because of this.  KVM
> >> doesn't allow execution of ROM memory which makes it difficult to
> >> implement PAM in the way it's intended to be implemented.  This
> >> makes SMM a bit tricky to make work.  Since there's never really
> >> been a pressing need to support SMM, to my knowledge, noone has even
> >> tried.
> >>
> > KVM allows to execute ROM memory (BIOS and option roms run this way). It
> > just makes it indistinguishable from RAM, but read only memory slot
> > support shouldn't be too hard. Why ability to execute ROM memory is
> > needed to support SMM though?
> 
> True.
> 
> The only possible requirement is to 'hide SMRAM' when not in SMM mode.
>  Even this is chipset specific, and arguable depending on the goals of
> SMM support in that system.  (Although, generally, hiding SMRAM is a
> requirement. :)
> 
> But, Anthony's point ('there's never really been a pressing need to
> support SMM') is probably the most important here, as I can't see a
> compelling use for SMM in QEMU.
> 
Yeah, that is probably the main reason. Although lately there was a
proposition to use SMM in seabios to access MMIO bar of USB device from
16bit mode. Do not remember details exactly. But I, personally, will be
very glad to not implement SMM support for KVM ;)

--
			Gleb.