On Fri, May 17, 2013 at 10:59:25AM -0500, Anthony Liguori wrote: > Paolo Bonzini <pbonz...@redhat.com> writes: > > So you can see it in three ways: > > > > 1) It's not a QEMU problem, but a firmware problem. You could initialize > > the UMBs with 0xFF in SeaBIOS for example. > > SeaBIOS is not supposed to do any initialization. Once it's RAM, > there's no guarantee of what the contents will be. The problem is that > it's supposed to be ROM. > > I doubt it's still there, but BIOSes often had an option to disable > ROM shadowing expressly because it breaks applications that assume that > this space is ROM after BIOS loads. > > Perhaps SeaBIOS could support such an option but we can't support it in > KVM at the moment.
It would not be easy to make SeaBIOS work with a truly read-only rom. SeaBIOS is designed to store static settings in the f-segment (where it can be accessed relative to %cs at runtime), and undoing that would be quite difficult. However, it is not too difficult to make SeaBIOS turn the 0xc0000-0x100000 area into read-only memory after initialization. The biggest stumbling block is that SeaBIOS stores read/write variables in the e-segment today, but it's possible to store them at the end of the 640k region instead (I posted a patch for this at: http://www.seabios.org/pipermail/seabios/2013-February/005693.html ). Once the memory 0xc0000-0x100000 ram can be read-only, setting the pam registers to mark it as read-only and memset'ing the region to 0xff is straight forward. -Kevin
