Patchwork [v5,4/4] i386: allow to load initrd below 4G for recent linux

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Submitter Li Zhijian
Date Jan. 11, 2019, 8:57 a.m.
Message ID <1547197071-14504-5-git-send-email-lizhijian@cn.fujitsu.com>
Download mbox | patch
Permalink /patch/697485/
State New
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Comments

Li Zhijian - Jan. 11, 2019, 8:57 a.m.
Since linux commit: cf8fa920cb42 ("i386: handle an initrd in highmem (version 2)")
linux has supported initrd up to 4 GB, but the header field
ramdisk_max is still set to 2 GB to avoid "possible bootloader bugs".

When use '-kernel vmlinux -initrd initrd.cgz' to launch a VM,
the firmware(it could be linuxboot_dma.bin) helps to read initrd
contents into guest memory(below ramdisk_max) and jump to kernel.
that's similar with what bootloader does, like grub.

In addition, initrd_max is uint32_t simply because QEMU doesn't support
the 64-bit boot protocol (specifically the ext_ramdisk_image field).

Therefore here just limit initrd_max to UINT32_MAX simply as well to
allow initrd to be loaded below 4 GB.

CC: Paolo Bonzini <pbonzini@redhat.com>
CC: Richard Henderson <rth@twiddle.net>
CC: Eduardo Habkost <ehabkost@redhat.com>
CC: "Michael S. Tsirkin" <mst@redhat.com>
CC: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>

---
V5: udpate comments and changelog
V3: correct grammar and check XLF_CAN_BE_LOADED_ABOVE_4G first (Michael S. Tsirkin)

Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
---
 hw/i386/pc.c | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)
Eduardo Habkost - Jan. 14, 2019, 5:53 p.m.
On Fri, Jan 11, 2019 at 04:57:51PM +0800, Li Zhijian wrote:
> Since linux commit: cf8fa920cb42 ("i386: handle an initrd in highmem (version 2)")
> linux has supported initrd up to 4 GB, but the header field
> ramdisk_max is still set to 2 GB to avoid "possible bootloader bugs".
> 
> When use '-kernel vmlinux -initrd initrd.cgz' to launch a VM,
> the firmware(it could be linuxboot_dma.bin) helps to read initrd
> contents into guest memory(below ramdisk_max) and jump to kernel.
> that's similar with what bootloader does, like grub.
> 
> In addition, initrd_max is uint32_t simply because QEMU doesn't support
> the 64-bit boot protocol (specifically the ext_ramdisk_image field).
> 
> Therefore here just limit initrd_max to UINT32_MAX simply as well to
> allow initrd to be loaded below 4 GB.
> 
> CC: Paolo Bonzini <pbonzini@redhat.com>
> CC: Richard Henderson <rth@twiddle.net>
> CC: Eduardo Habkost <ehabkost@redhat.com>
> CC: "Michael S. Tsirkin" <mst@redhat.com>
> CC: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
> Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
> 
> ---
> V5: udpate comments and changelog
> V3: correct grammar and check XLF_CAN_BE_LOADED_ABOVE_4G first (Michael S. Tsirkin)
> 
> Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
> ---
>  hw/i386/pc.c | 24 +++++++++++++++++++++++-
>  1 file changed, 23 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/i386/pc.c b/hw/i386/pc.c
> index 89c25b2..ea7a3c7 100644
> --- a/hw/i386/pc.c
> +++ b/hw/i386/pc.c
> @@ -904,7 +904,29 @@ static void load_linux(PCMachineState *pcms,
>  #endif
>  
>      /* highest address for loading the initrd */
> -    if (protocol >= 0x203) {
> +    if (protocol >= 0x20c &&
> +        lduw_p(header+0x236) & XLF_CAN_BE_LOADED_ABOVE_4G) {
> +        /*
> +         * Linux has supported initrd up to 4 GB for a very long time (2007,
> +         * long before XLF_CAN_BE_LOADED_ABOVE_4G which was added in 2013),
> +         * though it only sets initrd_max to 2 GB to "work around bootloader
> +         * bugs". Luckily, QEMU firmware(which does something like bootloader)
> +         * has supported this.
> +         *
> +         * It's believed that if XLF_CAN_BE_LOADED_ABOVE_4G is set, initrd can
> +         * be loaded into any address.
> +         *
> +         * In addition, initrd_max is uint32_t simply because QEMU doesn't
> +         * support the 64-bit boot protocol (specifically the ext_ramdisk_image
> +         * field).
> +         *
> +         * Therefore here just limit initrd_max to UINT32_MAX simply as well.
> +         *
> +         * FIXME: it's possible that linux protocol within [0x208, 0x20c]
> +         * supports up to 4G initrd as well.

I don't understand what exactly this FIXME comment is
documenting.  What exactly needs to be fixed?


> +         */
> +        initrd_max = UINT32_MAX;
> +    } else if (protocol >= 0x203) {
>          initrd_max = ldl_p(header+0x22c);
>      } else {
>          initrd_max = 0x37ffffff;
> -- 
> 2.7.4
>
Li Zhijian - Jan. 15, 2019, 1:35 a.m.
Hi Eduardo


On 1/15/19 01:53, Eduardo Habkost wrote:
>> +    if (protocol >= 0x20c &&
>> +        lduw_p(header+0x236) & XLF_CAN_BE_LOADED_ABOVE_4G) {
>> +        /*
>> +         * Linux has supported initrd up to 4 GB for a very long time (2007,
>> +         * long before XLF_CAN_BE_LOADED_ABOVE_4G which was added in 2013),
>> +         * though it only sets initrd_max to 2 GB to "work around bootloader
>> +         * bugs". Luckily, QEMU firmware(which does something like bootloader)
>> +         * has supported this.
>> +         *
>> +         * It's believed that if XLF_CAN_BE_LOADED_ABOVE_4G is set, initrd can
>> +         * be loaded into any address.
>> +         *
>> +         * In addition, initrd_max is uint32_t simply because QEMU doesn't
>> +         * support the 64-bit boot protocol (specifically the ext_ramdisk_image
>> +         * field).
>> +         *
>> +         * Therefore here just limit initrd_max to UINT32_MAX simply as well.
>> +         *
>> +         * FIXME: it's possible that linux protocol within [0x208, 0x20c]
>> +         * supports up to 4G initrd as well.
> I don't understand what exactly this FIXME comment is
> documenting.  What exactly needs to be fixed?
>
XLF_CAN_BE_LOADED_ABOVE_4G is one of the indicators, actually as comments said,
linux has supported up to 4 GB initrd since linux-2.26(protocol version 0x208).


I just want to comment that linux with protocol within [0x208, 0x20c] supports up to 4 GB initrd as well.

Is documenting with FIXME appropriate?


Thanks
Michael S. Tsirkin - Jan. 15, 2019, 1:46 a.m.
On Tue, Jan 15, 2019 at 09:35:09AM +0800, Li Zhijian wrote:
> Hi Eduardo
> 
> 
> On 1/15/19 01:53, Eduardo Habkost wrote:
> 
>         +    if (protocol >= 0x20c &&
>         +        lduw_p(header+0x236) & XLF_CAN_BE_LOADED_ABOVE_4G) {
>         +        /*
>         +         * Linux has supported initrd up to 4 GB for a very long time (2007,
>         +         * long before XLF_CAN_BE_LOADED_ABOVE_4G which was added in 2013),
>         +         * though it only sets initrd_max to 2 GB to "work around bootloader
>         +         * bugs". Luckily, QEMU firmware(which does something like bootloader)
>         +         * has supported this.
>         +         *
>         +         * It's believed that if XLF_CAN_BE_LOADED_ABOVE_4G is set, initrd can
>         +         * be loaded into any address.
>         +         *
>         +         * In addition, initrd_max is uint32_t simply because QEMU doesn't
>         +         * support the 64-bit boot protocol (specifically the ext_ramdisk_image
>         +         * field).
>         +         *
>         +         * Therefore here just limit initrd_max to UINT32_MAX simply as well.
>         +         *
>         +         * FIXME: it's possible that linux protocol within [0x208, 0x20c]
>         +         * supports up to 4G initrd as well.
> 
>     I don't understand what exactly this FIXME comment is
>     documenting.  What exactly needs to be fixed?
> 
> 
> XLF_CAN_BE_LOADED_ABOVE_4G is one of the indicators, actually as comments said,
> linux has supported up to 4 GB initrd since linux-2.26(protocol version 0x208).
> 
> 
> I just want to comment that linux with protocol within [0x208, 0x20c] supports up to 4 GB initrd as well.
> 
> Is documenting with FIXME appropriate?
> 
> 
> Thanks
> 
> 


Fixme should say what is missing in the qemu implementation.
E.g.

/*
 * Bar 2010 and up can actually be supported using foo.
 * FIXME: make use of foo to support bar.
 */
Li Zhijian - Jan. 16, 2019, 10:19 a.m.
Hi Michael, Eduardo

On 1/15/19 09:46, Michael S. Tsirkin wrote:
> On Tue, Jan 15, 2019 at 09:35:09AM +0800, Li Zhijian wrote:
>> Hi Eduardo
>>
>>
>> On 1/15/19 01:53, Eduardo Habkost wrote:
>>
>>          +    if (protocol >= 0x20c &&
>>          +        lduw_p(header+0x236) & XLF_CAN_BE_LOADED_ABOVE_4G) {
>>          +        /*
>>          +         * Linux has supported initrd up to 4 GB for a very long time (2007,
>>          +         * long before XLF_CAN_BE_LOADED_ABOVE_4G which was added in 2013),
>>          +         * though it only sets initrd_max to 2 GB to "work around bootloader
>>          +         * bugs". Luckily, QEMU firmware(which does something like bootloader)
>>          +         * has supported this.
>>          +         *
>>          +         * It's believed that if XLF_CAN_BE_LOADED_ABOVE_4G is set, initrd can
>>          +         * be loaded into any address.
>>          +         *
>>          +         * In addition, initrd_max is uint32_t simply because QEMU doesn't
>>          +         * support the 64-bit boot protocol (specifically the ext_ramdisk_image
>>          +         * field).
>>          +         *
>>          +         * Therefore here just limit initrd_max to UINT32_MAX simply as well.
>>          +         *
>>          +         * FIXME: it's possible that linux protocol within [0x208, 0x20c]
>>          +         * supports up to 4G initrd as well.
>>
>>      I don't understand what exactly this FIXME comment is
>>      documenting.  What exactly needs to be fixed?
>>
>>
>> XLF_CAN_BE_LOADED_ABOVE_4G is one of the indicators, actually as comments said,
>> linux has supported up to 4 GB initrd since linux-2.26(protocol version 0x208).
>>
>>
>> I just want to comment that linux with protocol within [0x208, 0x20c] supports up to 4 GB initrd as well.
>>
>> Is documenting with FIXME appropriate?
>>
>>
>> Thanks
>>
>>
>
> Fixme should say what is missing in the qemu implementation.



thanks for your explanation @Michael
I'd like to update "FIXME" to "NOTE" and move it into git-commit-log if no objection
and it's okay to delete it simply if it confuses others :)

BTW: any other comments for the others

Thanks
Zhijian




> E.g.
>
> /*
>   * Bar 2010 and up can actually be supported using foo.
>   * FIXME: make use of foo to support bar.
>   */
>
>

Patch

diff --git a/hw/i386/pc.c b/hw/i386/pc.c
index 89c25b2..ea7a3c7 100644
--- a/hw/i386/pc.c
+++ b/hw/i386/pc.c
@@ -904,7 +904,29 @@  static void load_linux(PCMachineState *pcms,
 #endif
 
     /* highest address for loading the initrd */
-    if (protocol >= 0x203) {
+    if (protocol >= 0x20c &&
+        lduw_p(header+0x236) & XLF_CAN_BE_LOADED_ABOVE_4G) {
+        /*
+         * Linux has supported initrd up to 4 GB for a very long time (2007,
+         * long before XLF_CAN_BE_LOADED_ABOVE_4G which was added in 2013),
+         * though it only sets initrd_max to 2 GB to "work around bootloader
+         * bugs". Luckily, QEMU firmware(which does something like bootloader)
+         * has supported this.
+         *
+         * It's believed that if XLF_CAN_BE_LOADED_ABOVE_4G is set, initrd can
+         * be loaded into any address.
+         *
+         * In addition, initrd_max is uint32_t simply because QEMU doesn't
+         * support the 64-bit boot protocol (specifically the ext_ramdisk_image
+         * field).
+         *
+         * Therefore here just limit initrd_max to UINT32_MAX simply as well.
+         *
+         * FIXME: it's possible that linux protocol within [0x208, 0x20c]
+         * supports up to 4G initrd as well.
+         */
+        initrd_max = UINT32_MAX;
+    } else if (protocol >= 0x203) {
         initrd_max = ldl_p(header+0x22c);
     } else {
         initrd_max = 0x37ffffff;