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Subject: Re: [Qemu-devel] [RFC/PoC PATCH 1/3] i386: set initrd_max to 4G - 1
 to allow up to 4G initrd
To:     Li Zhijian <zhijianx.li@intel.com>, Ingo Molnar <mingo@kernel.org>,
        Li Zhijian <lizhijian@cn.fujitsu.com>
Cc:     Peter Maydell <peter.maydell@linaro.org>, x86@kernel.org,
        hpa@zytor.com, bp@alien8.de, mingo@redhat.com, tglx@linutronix.de,
        QEMU Developers <qemu-devel@nongnu.org>,
        Philip Li <philip.li@intel.com>, linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Kees Cook <keescook@chromium.org>
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From:   Juergen Gross <jgross@suse.com>
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Date:   Fri, 9 Nov 2018 11:04:19 +0100
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On 09/11/2018 10:57, Li Zhijian wrote:
> On 11/9/2018 3:20 PM, Ingo Molnar wrote:
>> * Li Zhijian <lizhijian@cn.fujitsu.com> 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:
>>
>>    initrd_addr_max: .long 0x7fffffff
>>                                          # (Header version 0x0203 or
>> later)
>>                                          # The highest safe address for
>>                                          # the contents of an initrd
>>                                          # The current kernel allows
>> up to 4 GB,
>>                                          # but leave it at 2 GB to avoid
>>                                          # 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:
>>
>>   - Extend the boot protocol with a 64-bit field, named initrd_addr64_max
>>     or such.
>>   - We don't change the old field - but if the new field is set by new
>>     kernels then new bootloaders can use that as a new initrd_addr64_max
>>     value. (or reject to load the kernel if the address is too high.)
>>
>>   - The kernel build should also emit a warning when building larger than
>>     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@cn.fujitsu.com>
> Date:   Fri Nov 9 17:24:14 2018 +0800
> 
>     x86: Add header field initrd_addr64_max
>         Years ago, kernel had support load ~4GB initrd. But for some
> weird reasons (
>     avoid possible bootloader bugs), it only allow leave initrd under
> 2GB address
>     space(see initrd_addr_max fild at arch/x86/boot/header.S).
>         So modern bootloaders have not chance to load >=2G initrd
> previously.
>         To avoid the potential of bugs lurking in dozens of major and
> hundreds of
>     minor iterations of various Linux bootloaders. Ingo suggests to add
> a new field
>     initrd_addr64_max. If bootloader believes that it can load initrd to
>>=2G
>     address space, it can use initrd_addr64_max as the maximum loading
> address in
>     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:
>         # Part 2 of the header, from the old setup.S
>  
>                 .ascii  "HdrS"          # header signature
> -               .word   0x020e          # header version number (>= 0x0105)
> +               .word   0x020f          # header version number (>= 0x0105)
>                                         # or else old loadlin-1.5 will
> fail)
>                 .globl realmode_swtch
>  realmode_swtch:        .word   0, 0            # default_switch, SETUPSEG
> @@ -562,6 +562,12 @@ acpi_rsdp_addr:            .quad 0                
> # 64-bit physical pointer to the
>                                                 # ACPI RSDP table, added
> with
>                                                 # version 2.14
>  
> +#ifdef CONFIG_INITRD_SIZE_4GB
> +initrd_addr64_max:     .quad 0xffffffff        # allow ~4G initrd since
> 2.15
> +#else
> +initrd_addr64_max:     .quad 0

Shouldn't this be 0x7fffffff?

And please update Documentation/x86/boot.txt


Juergen