Re: [Qemu-devel] [PATCH] linuxboot optionrom: do not jump into loaded kernel in a big real mode

[Gleb Natapov]

On Sun, Jan 06, 2013 at 01:47:17PM +0100, Alexander Graf wrote:
> 
> On 06.01.2013, at 13:36, Gleb Natapov wrote:
> 
> > Big real mode is fully emulated by KVM now, so if control is passed to
> > the loaded kernel while one of the segment registers is in big real
> > mode all the real mode part of the Linux start up is emulated. This
> > slows boot process down. Fix that by resetting ES limit to 0xffff before
> > jumping into the kernel.
> > 
> > The patch also removes unused code segment definition from GDT and changes
> > ES register to be 16bit in protected mode since CS stays 16bit too and
> > it is CS segment that determines effective operands and addresses length.
> > 
> > Signed-off-by: Gleb Natapov <address@hidden>
> 
> I assume you tested that things still work, so the changes look ok to me.
> 
Loaded kernel with kvm and tcg using -kernel/-initrd.

> However, this patch should also include a binary patch to 
> pc-bios/linuxboot.bin, since linuxboot.S doesn't get compiled into the .bin 
> form on every machine.
> 
> 
> Alex
OK, something like this?:


diff --git a/pc-bios/linuxboot.bin b/pc-bios/linuxboot.bin
index 
e7c36694f997c3c34f7f4af3c2923bd2ef6094e7..435cac4ebff3fcd83cab4bf74de11f7071ab5aa6
 100644
GIT binary patch
delta 72
zcmZqRXyBNj#oWTwIZ^izW6s78X^grY3=9l?2hw%`DF%L}13f4D4!pei7sLaB|Nnu+
PBpBVlxtghlk#QmbUy&EH

delta 68
zcmZqRXyBNj#azSGI8pZyW6H)4X^g52K)|nbpyx#2ftL^ef_NbC|38qJbsj=bI={J@
NsfKa#1||;1e*hia7ytkO



> 
> > diff --git a/pc-bios/optionrom/linuxboot.S b/pc-bios/optionrom/linuxboot.S
> > index 748c831..afe39a5 100644
> > --- a/pc-bios/optionrom/linuxboot.S
> > +++ b/pc-bios/optionrom/linuxboot.S
> > @@ -101,18 +101,20 @@ copy_kernel:
> >     mov             $1, %eax
> >     mov             %eax, %cr0
> > 
> > -   /* So we can set ES to a 32-bit segment */
> > +   /* So we can enlarge ES segment limit */
> >     mov             $0x10, %eax
> >     mov             %eax, %es
> > 
> > -   /* We're now running in 16-bit CS, but 32-bit ES! */
> > -
> >     /* Load kernel and initrd */
> >     read_fw_blob_addr32(FW_CFG_KERNEL)
> >     read_fw_blob_addr32(FW_CFG_INITRD)
> >     read_fw_blob_addr32(FW_CFG_CMDLINE)
> >     read_fw_blob_addr32(FW_CFG_SETUP)
> > 
> > +   /* Do not leave ES in big real mode  */
> > +   mov             $0x08, %eax
> > +   mov             %eax, %es
> > +
> >     /* And now jump into Linux! */
> >     mov             $0, %eax
> >     mov             %eax, %cr0
> > @@ -130,10 +132,10 @@ gdt:
> >     /* 0x00 */
> > .byte       0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
> > 
> > -   /* 0x08: code segment (base=0, limit=0xfffff, type=32bit code 
> > exec/read, DPL=0, 4k) */
> > -.byte      0xff, 0xff, 0x00, 0x00, 0x00, 0x9a, 0xcf, 0x00
> > +   /* 0x08: data segment (base=0, limit=0xffff, type=16bit data 
> > read/write, DPL=0, 4k) */
> > +.byte      0xff, 0xff, 0x00, 0x00, 0x00, 0x92, 0x00, 0x00
> > 
> > -   /* 0x10: data segment (base=0, limit=0xfffff, type=32bit data 
> > read/write, DPL=0, 4k) */
> > -.byte      0xff, 0xff, 0x00, 0x00, 0x00, 0x92, 0xcf, 0x00
> > +   /* 0x10: data segment (base=0, limit=0xfffff, type=16bit data 
> > read/write, DPL=0, 4k) */
> > +.byte      0xff, 0xff, 0x00, 0x00, 0x00, 0x92, 0x8f, 0x00
> > 
> > BOOT_ROM_END
> > --
> >                     Gleb.

--
                        Gleb.