Re: [Qemu-devel] [RFC/PoC PATCH 1/3] i386: set initrd_max to 4G - 1 to allow up to 4G initrd

[Li Zhijian]

Just noticed that there is a field xloadflags at recent protocol
  60 Protocol 2.12:  (Kernel 3.8) Added the xloadflags field and extension fields
  61                 to struct boot_params for loading bzImage and ramdisk
  62                 above 4G in 64bit.
[snip]
 617 Field name:     xloadflags
 618 Type:           read
 619 Offset/size:    0x236/2
 620 Protocol:       2.12+
 621
 622   This field is a bitmask.
 623
 624   Bit 0 (read): XLF_KERNEL_64
 625         - If 1, this kernel has the legacy 64-bit entry point at 0x200.
 626
 627   Bit 1 (read): XLF_CAN_BE_LOADED_ABOVE_4G
 628         - If 1, kernel/boot_params/cmdline/ramdisk can be above 4G.
 629

maybe we can reuse this field and append a new Bit 5 XLF_INITRD_SIZE_4G or such


thanks
Zhijian

 

On 11/9/2018 6:04 PM, Juergen Gross wrote:
> On 09/11/2018 10:57, Li Zhijian wrote:
> > On 11/9/2018 3:20 PM, Ingo Molnar wrote:
> > > * Li Zhijian <lizhijian@xxxxxxxxxxxxxx> wrote:
> > >
> > > > > If the kernel initrd creation process creates an initrd which
> > > > > is larger than 2GB and also claims that it can't be placed
> > > > > with any part of it above 2GB, then that sounds like a bug
> > > > > in the initrd creation process...
> > > > Exactly, it's a real problem.
> > > >
> > > > Add x86 maintainers and LKML:
> > > >
> > > > The background is that QEMU want to support up to 4G initrd. but
> > > > linux header (
> > > > initrd_addr_max field) only allow 2G-1.
> > > > Is one of the below approaches reasonable:
> > > > 1) change initrd_addr_max to 4G-1 directly
> > > > simply(arch/x86/boot/header.S)?
> > > > 2) lie QEMU bootloader the initrd_addr_max is 4G-1 even though header
> > > > said 2G-1
> > > > 3) any else
> > > A 10 years old comment from hpa says:
> > >
> > >  ÂÂ initrd_addr_max: .long 0x7fffffff
> > >  ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ # (Header version 0x0203 or
> > > later)
> > >  ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ # The highest safe address for
> > >  ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ # the contents of an initrd
> > >  ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ # The current kernel allows
> > > up to 4 GB,
> > >  ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ # but leave it at 2 GB to avoid
> > >  ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ # possible bootloader bugs.
> > >
> > > To avoid the potential of bugs lurking in dozens of major and hundreds of
> > > minor iterations of various Linux bootloaders I'd prefer a real solution
> > > and extend it - because if there's a 2GB initrd for some weird reason
> > > today there might be a 4GB one in two years.
> > thank a lots. that's amazing.
> >
> >
> > > The real solution would be to:
> > >
> > >  Â - Extend the boot protocol with a 64-bit field, named initrd_addr64_max
> > >  ÂÂÂ or such.
> > >  Â - We don't change the old field - but if the new field is set by new
> > >  ÂÂÂ kernels then new bootloaders can use that as a new initrd_addr64_max
> > >  ÂÂÂ value. (or reject to load the kernel if the address is too high.)
> > >
> > >  Â - The kernel build should also emit a warning when building larger than
> > >  ÂÂÂ 2GB initrds, with a list of bootloaders that support the new
> > > protocol.
> > Actually i just knew QEMU(Seabios + optionrom(linuxboot_dma.bin)) can
> > support ~4GB initrd so far.
> >
> > i just drafted at patch to add this field. could you have a look.
> > another patch which is to document initrd_addr64_max is ongoing.
> >
> > commit db463ac9c1975f115d1ce2acb82d530c2b63b888
> > Author: Li Zhijian <lizhijian@xxxxxxxxxxxxxx>
> > Date:ÂÂ Fri Nov 9 17:24:14 2018 +0800
> >
> >  ÂÂÂ x86: Add header field initrd_addr64_max
> >  ÂÂÂ ÂÂÂ Years ago, kernel had support load ~4GB initrd. But for some
> > weird reasons (
> >  ÂÂÂ avoid possible bootloader bugs), it only allow leave initrd under
> > 2GB address
> >  ÂÂÂ space(see initrd_addr_max fild at arch/x86/boot/header.S).
> >  ÂÂÂ ÂÂÂ So modern bootloaders have not chance to load >=2G initrd
> > previously.
> >  ÂÂÂ ÂÂÂ To avoid the potential of bugs lurking in dozens of major and
> > hundreds of
> >  ÂÂÂ minor iterations of various Linux bootloaders. Ingo suggests to add
> > a new field
> >  ÂÂÂ initrd_addr64_max. If bootloader believes that it can load initrd to
> > > =2G
> >  ÂÂÂ address space, it can use initrd_addr64_max as the maximum loading
> > address in
> >  ÂÂÂ stead of the old field initrd_addr_max.
> >
> > diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
> > index 4c881c8..5fc3ebe 100644
> > --- a/arch/x86/boot/header.S
> > +++ b/arch/x86/boot/header.S
> > @@ -300,7 +300,7 @@ _start:
> >  ÂÂÂÂÂÂÂ # Part 2 of the header, from the old setup.S
> >   
> >   .ascii "HdrS" # header signature
> > -ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ .wordÂÂ 0x020eÂÂÂÂÂÂÂÂÂ # header version number (>= 0x0105)
> > +ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ .wordÂÂ 0x020fÂÂÂÂÂÂÂÂÂ # header version number (>= 0x0105)
> >  ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ # or else old loadlin-1.5 will
> > fail)
> >  ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ .globl realmode_swtch
> >  Ârealmode_swtch:ÂÂÂÂÂÂÂ .wordÂÂ 0, 0ÂÂÂÂÂÂÂÂÂÂÂ # default_switch, SETUPSEG
> > @@ -562,6 +562,12 @@ acpi_rsdp_addr:ÂÂÂÂÂÂÂÂÂÂÂ .quad 0
> > # 64-bit physical pointer to the
> >  ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ # ACPI RSDP table, added
> > with
> >  ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ # version 2.14
> >   
> > +#ifdef CONFIG_INITRD_SIZE_4GB
> > +initrd_addr64_max:ÂÂÂÂ .quad 0xffffffffÂÂÂÂÂÂÂ # allow ~4G initrd since
> > 2.15
> > +#else
> > +initrd_addr64_max:ÂÂÂÂ .quad 0
> Shouldn't this be 0x7fffffff?
>
> And please update Documentation/x86/boot.txt
>
>
> Juergen