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Date: Mon, 14 Jan 2013 12:20:45 +0100
From: Borislav Petkov <bp@...en8.de>
To: Yinghai Lu <yinghai@...nel.org>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...e.hu>,
"H. Peter Anvin" <hpa@...or.com>,
"Eric W. Biederman" <ebiederm@...ssion.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Jan Kiszka <jan.kiszka@....de>,
Jason Wessel <jason.wessel@...driver.com>,
linux-kernel@...r.kernel.org, Rob Landley <rob@...dley.net>,
Matt Fleming <matt.fleming@...el.com>
Subject: Re: [PATCH v7u1 23/31] x86, boot: update comments about entries for
64bit image
On Thu, Jan 03, 2013 at 04:48:43PM -0800, Yinghai Lu wrote:
> Now 64bit entry is fixed on 0x200, can not be changed anymore.
>
> Update the comments to reflect that.
>
> Also put info about it in boot.txt
>
> Signed-off-by: Yinghai Lu <yinghai@...nel.org>
> Cc: Rob Landley <rob@...dley.net>
> Cc: Matt Fleming <matt.fleming@...el.com>
> ---
> Documentation/x86/boot.txt | 38 ++++++++++++++++++++++++++++++++++++
> arch/x86/boot/compressed/head_64.S | 22 ++++++++++++---------
> 2 files changed, 51 insertions(+), 9 deletions(-)
>
> diff --git a/Documentation/x86/boot.txt b/Documentation/x86/boot.txt
> index 18ca9fb..24cc542 100644
> --- a/Documentation/x86/boot.txt
> +++ b/Documentation/x86/boot.txt
> @@ -1042,6 +1042,44 @@ must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
> must be __BOOT_DS; interrupt must be disabled; %esi must hold the base
> address of the struct boot_params; %ebp, %edi and %ebx must be zero.
>
> +**** 64-bit BOOT PROTOCOL
> +
> +For machine with 64bit cpus and 64bit kernel, we could use 64bit bootloader
> +We need a 64-bit boot protocol.
Make that:
"64-bit kernels using 64-bit bootloaders use this protocol for booting."
> +
> +In 64-bit boot protocol, the first step in loading a Linux kernel
> +should be to setup the boot parameters (struct boot_params,
> +traditionally known as "zero page"). The memory for struct boot_params
> +should be allocated under or above 4G and initialized to all zero.
"Memory for struct boot_params may be allocated anywhere (even above
4G). This memory must be zeroed out."
Also, add a \n here.
> +Then the setup header from offset 0x01f1 of kernel image on should be
"Then, the setup header at offset 0x01f1 of the kernel image should be..."
> +loaded into struct boot_params and examined. The end of setup header
> +can be calculated as follow:
"follows:"
> +
> + 0x0202 + byte value at offset 0x0201
What is that value at 0x201? What's its name? Maybe it is called "magic" :-)
> +
> +In addition to read/modify/write the setup header of the struct
> +boot_params as that of 16-bit boot protocol,
Hmm, do you mean:
"In addition to modifying struct setup_header in boot_params as part of
the 16-bit boot protocol, the boot loader..."
> the boot loader should
> +also fill the additional fields of the struct boot_params as that
remove "that"
> +described in zero-page.txt.
Btw, you could document the sentinel mechanism here or in zero-page.txt,
for example.
> +
> +After setting up the struct boot_params, the boot loader can load the
s/the//
> +64-bit kernel in the same way as that of 16-bit boot protocol, but
> +kernel could be above 4G.
"... the boot loader can load a 64-bit kernel the same way as with the
16-bit boot protocol with the additional advantage that the kernel can
be placed above the 4Gb barrier."
> +
> +In 64-bit boot protocol, the kernel is started by jumping to the
"In the 64-bit... "
> +64-bit kernel entry point, which is the start address of loaded
no comma:
"... entry point which is the start address of the loaded..."
> +64-bit kernel plus 0x200.
Again, what does the 0x200 value mean?
> +
> +At entry, the CPU must be in 64-bit mode with paging enabled.
> +The range with setup_header.init_size from start address of loaded
> +kernel and zero page and command line buffer get ident mapping;
Hmm, maybe:
"The ranges from the start address of the loaded kernel and with size
setup_header.init_size, the zero page and the command line buffer are
ident-mapped."
Newline here.
Then enumerate the further steps:
> +a GDT must be loaded with the descriptors for selectors
> +__BOOT_CS(0x10) and __BOOT_DS(0x18); both descriptors must be 4G flat
> +segment; __BOOT_CS must have execute/read permission, and __BOOT_DS
> +must have read/write permission; CS must be __BOOT_CS and DS, ES, SS
> +must be __BOOT_DS; interrupt must be disabled; %rsi must hold the base
> +address of the struct boot_params.
"Then:
* a GDT must be loaded with the descriptors for selectors
__BOOT_CS(0x10) and __BOOT_DS(0x18)
* both descriptors must describe a 4G, flat segment
* __BOOT_CS must have execute/read permissions, and __BOOT_DS must have
read/write permissions
* CS must be __BOOT_CS and DS, ES, SS must be __BOOT_DS
* interrupts must be disabled
* %rsi must hold the base address of the struct boot_params."
> +
> **** EFI HANDOVER PROTOCOL
>
> This protocol allows boot loaders to defer initialisation to the EFI
> diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> index 5c80b94..aaafd4e 100644
> --- a/arch/x86/boot/compressed/head_64.S
> +++ b/arch/x86/boot/compressed/head_64.S
> @@ -37,6 +37,12 @@
> __HEAD
> .code32
> ENTRY(startup_32)
> + /*
> + * 32bit entry is 0, could not be changed!
What does that mean? Did we try to change it or what?
> + * If we come here directly from a bootloader,
> + * kernel(text+data+bss+brk) ramdisk, zero_page, command line
> + * all need to be under 4G limit.
"under the"
> + */
> cld
> /*
> * Test KEEP_SEGMENTS flag to see if the bootloader is asking
> @@ -182,20 +188,18 @@ ENTRY(startup_32)
> lret
> ENDPROC(startup_32)
>
> - /*
> - * Be careful here startup_64 needs to be at a predictable
> - * address so I can export it in an ELF header. Bootloaders
> - * should look at the ELF header to find this address, as
> - * it may change in the future.
> - */
> .code64
> .org 0x200
> ENTRY(startup_64)
> /*
> + * 64bit entry is 0x200, could not be changed!
Ah, I see what you mean:
"64-bit entry point is 0x200 and it is ABI so immutable!"
Ditto for startup_32 above.
> * We come here either from startup_32 or directly from a
> - * 64bit bootloader. If we come here from a bootloader we depend on
> - * an identity mapped page table being provied that maps our
> - * entire text+data+bss and hopefully all of memory.
> + * 64bit bootloader.
> + * If we come here from a bootloader, kernel(text+data+bss+brk),
> + * ramdisk, zero_page, command line could be above 4G.
> + * We depend on an identity mapped page table being provided
> + * that maps our entire kernel(text+data+bss+brk), zero page
> + * and command line.
Heey, this one is good! :-)
Thanks.
--
Regards/Gruss,
Boris.
Sent from a fat crate under my desk. Formatting is fine.
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