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Re: [Qemu-devel] [PATCH v4 6/7] Make the kernel image in the fw_cfg DMA
From: |
Laszlo Ersek |
Subject: |
Re: [Qemu-devel] [PATCH v4 6/7] Make the kernel image in the fw_cfg DMA interface bootable |
Date: |
Thu, 1 Oct 2015 17:25:11 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.2.0 |
On 10/01/15 14:16, Marc Marí wrote:
> Add an entry to the bootorder file with name "vmlinux".
> Give this entry more priority than the romfile.
>
> Signed-off-by: Marc Marí <address@hidden>
> ---
> hw/i386/pc.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/hw/i386/pc.c b/hw/i386/pc.c
> index 81d93b4..c4c51f7 100644
> --- a/hw/i386/pc.c
> +++ b/hw/i386/pc.c
> @@ -1012,8 +1012,10 @@ static void load_linux(PCMachineState *pcms,
> fw_cfg_add_bytes(fw_cfg, FW_CFG_SETUP_DATA, setup, setup_size);
>
> option_rom[nb_option_roms].name = "linuxboot.bin";
> - option_rom[nb_option_roms].bootindex = 0;
> + option_rom[nb_option_roms].bootindex = 1;
> nb_option_roms++;
> +
> + add_boot_device_path(0, NULL, "vmlinux");
> }
>
> #define NE2000_NB_MAX 6
>
Where does this idea come from?
This will yet again break the invariant that the bootorder fw_cfg file
is a list of OpenFirmware device paths.
(The other annoying offender being "HALT", which caused me huge grief in
the OVMF OpenFirmware devpath parser parser, when libvirt decided that
"-boot strict=on" would become default.)
OVMF (and AAVMF) have been able to boot kernels directly from fw_cfg for
quite some time now, without the above change. They look at the fw_cfg
key 0x0008 (FW_CFG_KERNEL_SIZE). Direct kernel boot is being requested
iff the (little endian encoded) uint32 value is nonzero.
In QEMU, this role of FW_CFG_KERNEL_SIZE is true for:
- arm_load_kernel_notify() [hw/arm/boot.c], relied upon by AAVMF,
- load_multiboot() [hw/i386/multiboot.c] and
load_linux() [hw/i386/pc.c], relied upon by OVMF,
- "hw/ppc/mac_newworld.c", "hw/ppc/mac_oldworld.c", "hw/sparc/sun4m.c",
and "hw/sparc64/sun4u.c", relied upon by whatever boot firmware they
have.
Why is this necessary for SeaBIOS?
... I can see the function bootprio_find_vmlinux(), in SeaBIOS patch
[PATCH v4 2/2] Boot Linux using QEMU fw_cfg DMA interface
Given that direct kernel boot is always expected to take priority over
anything else (which is ensured by this QEMU patch too), can
bootprio_find_vmlinux() in SeaBIOS just look at the same fw_cfg key
(0x0008)?
I checked the QEMU_CFG_* macros in "src/fw/paravirt.c", and I think when
SeaBIOS boots an fw_cfg kernel *now*, it doesn't do it with its own
implementation; it probably launches the "linuxboot.bin" oprom (from
QEMU -- "pc-bios/optionrom/linuxboot.S").
I vaguely recall that this assembly code has been deemed unwieldy for
implementing the DMA interface (and I fully agree), which is why the
above-referenced SeaBIOS patch adds the capability to SeaBIOS itself. I
agree with that too.
But, instead of messing up the "bootorder" fw_cfg file, can
bootprio_find_vmlinux() look at the non-nullity of the
QEMU_CFG_KERNEL_SIZE key? Such as:
- read four bytes from under the fw_cfg selector QEMU_CFG_KERNEL_SIZE
(0x0008),
- if it is zero, return -1 --> no kernel boot requested,
- if it is nonzero, return 0 --> which means "top priority".
In other words, I agree with:
> - option_rom[nb_option_roms].bootindex = 0;
> + option_rom[nb_option_roms].bootindex = 1;
in this patch, but I disagree with:
> + add_boot_device_path(0, NULL, "vmlinux");
Thank you
Laszlo
- Re: [Qemu-devel] [PATCH v4 3/7] Implement fw_cfg DMA interface, (continued)
- Re: [Qemu-devel] [PATCH v4 3/7] Implement fw_cfg DMA interface, Marc Marí, 2015/10/08
- Re: [Qemu-devel] [PATCH v4 3/7] Implement fw_cfg DMA interface, Stefan Hajnoczi, 2015/10/08
- Re: [Qemu-devel] [PATCH v4 3/7] Implement fw_cfg DMA interface, Peter Maydell, 2015/10/08
- Re: [Qemu-devel] [PATCH v4 3/7] Implement fw_cfg DMA interface, Stefan Hajnoczi, 2015/10/09
- Re: [Qemu-devel] [PATCH v4 3/7] Implement fw_cfg DMA interface, Marc Marí, 2015/10/09
- Re: [Qemu-devel] [PATCH v4 3/7] Implement fw_cfg DMA interface, Marc Marí, 2015/10/08
[Qemu-devel] [PATCH v4 2/7] fw_cfg DMA interface documentation, Marc Marí, 2015/10/08
[Qemu-devel] [PATCH v4 1/7] fw_cfg: document fw_cfg_modify_iXX() update functions, Marc Marí, 2015/10/08
[Qemu-devel] [PATCH v4 6/7] Make the kernel image in the fw_cfg DMA interface bootable, Marc Marí, 2015/10/08
- Re: [Qemu-devel] [PATCH v4 6/7] Make the kernel image in the fw_cfg DMA interface bootable,
Laszlo Ersek <=
- Re: [Qemu-devel] [PATCH v4 6/7] Make the kernel image in the fw_cfg DMA interface bootable, Gerd Hoffmann, 2015/10/08
- Re: [Qemu-devel] [PATCH v4 6/7] Make the kernel image in the fw_cfg DMA interface bootable, Kevin O'Connor, 2015/10/08
- Re: [Qemu-devel] [PATCH v4 6/7] Make the kernel image in the fw_cfg DMA interface bootable, Laszlo Ersek, 2015/10/08
- Re: [Qemu-devel] [PATCH v4 6/7] Make the kernel image in the fw_cfg DMA interface bootable, Gerd Hoffmann, 2015/10/08
- Re: [Qemu-devel] [PATCH v4 6/7] Make the kernel image in the fw_cfg DMA interface bootable, Daniel P. Berrange, 2015/10/08
Re: [Qemu-devel] [PATCH v4 6/7] Make the kernel image in the fw_cfg DMA interface bootable, Kevin O'Connor, 2015/10/08
Re: [Qemu-devel] [PATCH v4 6/7] Make the kernel image in the fw_cfg DMA interface bootable, Laszlo Ersek, 2015/10/08
Re: [Qemu-devel] [PATCH v4 6/7] Make the kernel image in the fw_cfg DMA interface bootable, Marc Marí, 2015/10/08
Re: [Qemu-devel] [PATCH v4 6/7] Make the kernel image in the fw_cfg DMA interface bootable, Gerd Hoffmann, 2015/10/08
Re: [Qemu-devel] [PATCH v4 6/7] Make the kernel image in the fw_cfg DMA interface bootable, Marc Marí, 2015/10/08