Re: [Qemu-devel] [PATCH] linuxboot: compute initrd loading address

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Oct 06, 2014 at 04:49:57PM +0200, Paolo Bonzini wrote:
> Even though hw/i386/pc.c tries to compute a valid loading address for the
> initrd, close to the top of RAM, this does not take into account other
> data that is malloced into that memory by SeaBIOS.
> 
> Luckily we can easily look at the memory map to find out how much memory is
> used up there.  This patch places the initrd in the first four gigabytes,
> below the first hole (as returned by INT 15h, AX=e801h).
> 
> Without this patch:
> [    0.000000] init_memory_mapping: [mem 0x07000000-0x07fdffff]
> [    0.000000] RAMDISK: [mem 0x0710a000-0x07fd7fff]
> 
> With this patch:
> [    0.000000] init_memory_mapping: [mem 0x07000000-0x07fdffff]
> [    0.000000] RAMDISK: [mem 0x07112000-0x07fdffff]
> 
> So linuxboot is able to use the 64k that were added as padding for
> QEMU <= 2.1.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Acked-by: Michael S. Tsirkin <mst@redhat.com>

OK, and we need something similar for multiboot.S?

Also - can we drop FW_CFG_INITRD_ADDR and the patching
in load_linux now (unless running a compat machine type)?




> ---
>  pc-bios/linuxboot.bin         | Bin 1024 -> 1024 bytes
>  pc-bios/optionrom/linuxboot.S |  47 ++++++++++++++++++++++++++++++++++++++----
>  pc-bios/optionrom/optionrom.h |  21 ++++++++++++++++---
>  3 files changed, 61 insertions(+), 7 deletions(-)
> 
> diff --git a/pc-bios/linuxboot.bin b/pc-bios/linuxboot.bin
> index e7c36694f997c3c34f7f4af3c2923bd2ef6094e7..130103fb739228a6869aaf1b174b9d20c13378fc 100644
> GIT binary patch
> delta 168
> zcmZqRXyBNj#e9V6<V4+-#yc2a7@jn|bXJt}WGM&drBas8gPpg4G+*OE29`Ab?LX5F
> zKIeMP(|Cx15y<-m<Oxh}WRz3ZJf7D0oZ-X&|7o31)0*E19C!O5&XCq~0;uRf+QA1b
> zX{>7|eo$aa3kRw;nk>i|IC(Q;0c%?4;T_@=t7IoTF$qbirKj~bOE57or0rk;0)C|f
> SJtz7Oyqvi?nJI*kF&F^X7ev$m
> 
> delta 107
> zcmZqRXyBNj#azSGI8k@yWCKP?#+1okj0#LU*e5$O$xYtNXvD|`VlnOD22!$<yBUQi
> zzh^99+93|&DjwV+!H~8~fR%ya{VqY)Kk1)y(snQa0l(6Lo)disUOwEsnkj^F@&_gl
> G#(w~}wj;0r
> 
> diff --git a/pc-bios/optionrom/linuxboot.S b/pc-bios/optionrom/linuxboot.S
> index 748c831..5bc0af0 100644
> --- a/pc-bios/optionrom/linuxboot.S
> +++ b/pc-bios/optionrom/linuxboot.S
> @@ -76,14 +76,45 @@ boot_kernel:
>  
>  
>  copy_kernel:
> +	/* Compute initrd address */
> +	mov		$0xe801, %ax
> +	xor		%cx, %cx
> +	xor		%dx, %dx
> +	int		$0x15
> +
> +	/* Output could be in AX/BX or CX/DX */
> +	or		%cx, %cx
> +	jnz		1f
> +	or		%dx, %dx
> +	jnz		1f
> +	mov		%ax, %cx
> +	mov		%bx, %dx
> +1:
> +
> +	or		%dx, %dx
> +	jnz		2f
> +	addw		$1024, %cx            /* add 1 MB */
> +	movzwl		%cx, %edi
> +	shll		$10, %edi             /* convert to bytes */
> +	jmp		3f
> +
> +2:
> +	addw		$16777216 >> 16, %dx  /* add 16 MB */
> +	movzwl		%dx, %edi
> +	shll		$16, %edi             /* convert to bytes */
> +
> +3:
> +	read_fw         FW_CFG_INITRD_SIZE
> +	subl            %eax, %edi
> +	andl            $-4096, %edi          /* EDI = start of initrd */
>  
>  	/* We need to load the kernel into memory we can't access in 16 bit
>  	   mode, so let's get into 32 bit mode, write the kernel and jump
>  	   back again. */
>  
>  	/* Reserve space on the stack for our GDT descriptor. */
> -	mov		%esp, %ebp
> -	sub		$16, %esp
> +	mov             %esp, %ebp
> +	sub             $16, %esp
>  
>  	/* Now create the GDT descriptor */
>  	movw		$((3 * 8) - 1), -16(%bp)
> @@ -108,10 +139,18 @@ copy_kernel:
>  	/* We're now running in 16-bit CS, but 32-bit ES! */
>  
>  	/* Load kernel and initrd */
> +	pushl		%edi
> +	read_fw_blob_addr32_edi(FW_CFG_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)
> +
> +	read_fw		FW_CFG_SETUP_ADDR
> +	mov		%eax, %edi
> +	mov		%eax, %ebx
> +	read_fw_blob_addr32_edi(FW_CFG_SETUP)
> +
> +	/* Update the header with the initrd address we chose above */
> +	popl		%es:0x218(%ebx)
>  
>  	/* And now jump into Linux! */
>  	mov		$0, %eax
> diff --git a/pc-bios/optionrom/optionrom.h b/pc-bios/optionrom/optionrom.h
> index ce43608..f1a9021 100644
> --- a/pc-bios/optionrom/optionrom.h
> +++ b/pc-bios/optionrom/optionrom.h
> @@ -51,8 +51,6 @@
>  .endm
>  
>  #define read_fw_blob_pre(var)				\
> -	read_fw		var ## _ADDR;			\
> -	mov		%eax, %edi;			\
>  	read_fw		var ## _SIZE;			\
>  	mov		%eax, %ecx;			\
>  	mov		$var ## _DATA, %ax;		\
> @@ -68,6 +66,8 @@
>   * Clobbers:	%eax, %edx, %es, %ecx, %edi
>   */
>  #define read_fw_blob(var)				\
> +	read_fw		var ## _ADDR;			\
> +	mov		%eax, %edi;			\
>  	read_fw_blob_pre(var);				\
>  	/* old as(1) doesn't like this insn so emit the bytes instead: \
>  	rep insb	(%dx), %es:(%edi);		\
> @@ -80,7 +80,22 @@
>   *
>   * Clobbers:	%eax, %edx, %es, %ecx, %edi
>   */
> -#define read_fw_blob_addr32(var)				\
> +#define read_fw_blob_addr32(var)			\
> +	read_fw		var ## _ADDR;			\
> +	mov		%eax, %edi;			\
> +	read_fw_blob_pre(var);				\
> +	/* old as(1) doesn't like this insn so emit the bytes instead: \
> +	addr32 rep insb	(%dx), %es:(%edi);		\
> +	*/						\
> +	.dc.b		0x67,0xf3,0x6c
> +
> +/*
> + * Read a blob from the fw_cfg device in forced addr32 mode, address is in %edi.
> + * Requires _SIZE and _DATA values for the parameter.
> + *
> + * Clobbers:	%eax, %edx, %edi, %es, %ecx
> + */
> +#define read_fw_blob_addr32_edi(var)			\
>  	read_fw_blob_pre(var);				\
>  	/* old as(1) doesn't like this insn so emit the bytes instead: \
>  	addr32 rep insb	(%dx), %es:(%edi);		\
> -- 
> 2.1.0