Re: [PATCH 1/3] Update BIOS INT15-E820 to allow a larger BIOS image

Jordan Justen
2009-07-28T16:51:27Z

On Tue, Jul 28, 2009 at 7:40 AM, Marcelo Tosatti<mtosatti@redhat.com> wrote:
> On Sun, Jul 26, 2009 at 05:23:51PM -0700, Jordan Justen wrote:
>> The bios will now reserve more memory via the E820 functions.
>>
>> Note that the standard KVM BIOS will most likely not make use of
>> this expanded BIOS region.  This change will synchronize
>> the BIOS INT15-E820 reservations to match other changes that
>> will allow alternate BIOS images to be larger in size.
>>
>> Previously the BIOS reserved:
>>   0xfffbc000-0xfffbcfff -   4KB - EPT identity mapping pages
>>   0xfffbd000-0xfffbffff -  12KB - TSS pages
>>   0xfffc0000-0xffffffff - 256KB - Max bios.bin (usually top 128KB is used)
>>
>> Now the BIOS will reserve:
>>   0xfeffc000-0xfeffcfff -   4KB - EPT identity mapping pages
>>   0xfeffd000-0xfeffffff -  12KB - TSS Pages
>>   0xff000000-0xffffffff -  16MB - Max bios.bin
>>
>> Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
>> ---
>>  kvm/bios/rombios.c |    8 ++++----
>>  1 files changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/kvm/bios/rombios.c b/kvm/bios/rombios.c
>> index 6186199..2d0c153 100644
>> --- a/kvm/bios/rombios.c
>> +++ b/kvm/bios/rombios.c
>> @@ -4596,14 +4596,14 @@ ASM_END
>>                      case 5:
>>                          /* 4 pages before the bios, 3 pages for vmx tss pages,
>>                        * the other page for EPT real mode pagetable */
>> -                        set_e820_range(ES, regs.u.r16.di, 0xfffbc000L,
>> -                                       0xfffc0000L, 0, 0, 2);
>> +                        set_e820_range(ES, regs.u.r16.di, 0xfeffc000L,
>> +                                       0xff000000L, 0, 0, 2);
>>                          regs.u.r32.ebx = 6;
>
> So if you use an older kernel, and the kvm_set_identity_map_addr fails,
> you get the e820 entry wrong right? Perhaps you should use the hw/fw_cfg.c
> interface to communicate with the BIOS.
>

If you use this newer BIOS code with the older kernel code, the
expanded E820 BIOS region of the will cover the older region where the
EPT page tables are at. So, the OS will still know to keep away from
this region.

There should be no impact if someone uses a newer qemu-kvm with and
older kvm module. Since the normal legacy kvm BIOS is only 128KB, it
will be able to boot fine. (There will be no conflict with reserving
the memory region for the small BIOS.) If a bios.bin is used that is
larger than 256KB, then it will fail in the same way as today, since
there will be a conflict while trying to reserve the 0xfffbc000 -
0xfffbcfff region.

The only difference in this case would be that E820 reserves a larger
chunk of memory space, but I can't see how this could cause a problem.
(Previously kvm-bios would reserve 256KB while the BIOS was normally
only 128KB in size.)

>
>>                          break;
>>                      case 6:
>> -                        /* 256KB BIOS area at the end of 4 GB */
>> +                        /* 16MB BIOS area at the end of 4 GB */
>>                          set_e820_range(ES, regs.u.r16.di,
>> -                                       0xfffc0000L, 0x00000000L ,0, 0, 2);
>> +                                       0xff000000L, 0x00000000L ,0, 0, 2);
>>                          if (extra_highbits_memory_size || extra_lowbits_memory_size)
>>                              regs.u.r32.ebx = 7;
>>                          else
>> --
>> 1.6.0.4
>>
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