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Date: Fri, 9 Nov 2018 21:11:17 +0800
From: Li Zhijian <zhijianx.li@...el.com>
To: Juergen Gross <jgross@...e.com>, Ingo Molnar <mingo@...nel.org>,
Li Zhijian <lizhijian@...fujitsu.com>
Cc: Peter Maydell <peter.maydell@...aro.org>, x86@...nel.org,
hpa@...or.com, bp@...en8.de, mingo@...hat.com, tglx@...utronix.de,
QEMU Developers <qemu-devel@...gnu.org>,
Philip Li <philip.li@...el.com>, linux-kernel@...r.kernel.org,
Linus Torvalds <torvalds@...ux-foundation.org>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Kees Cook <keescook@...omium.org>
Subject: Re: [Qemu-devel] [RFC/PoC PATCH 1/3] i386: set initrd_max to 4G - 1
to allow up to 4G initrd
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@...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@...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
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