On Fri, Dec 21, 2018 at 11:10:30AM -0500, Michael S. Tsirkin wrote:
> On Thu, Dec 06, 2018 at 10:32:13AM +0800, Li Zhijian wrote:
> > a new field xloadflags was added to recent x86 linux, and BIT 1:
> > XLF_CAN_BE_LOADED_ABOVE_4G is used to tell bootload that where initrd can be
> > loaded safely.
> >
> > Current QEMU/BIOS always loads initrd below below_4g_mem_size which is
> > always
> > less than 4G, so here limiting initrd_max to 4G - 1 simply is enough if
> > this bit is set.
> >
> > CC: Paolo Bonzini <pbonz...@redhat.com>
> > CC: Richard Henderson <r...@twiddle.net>
> > CC: Eduardo Habkost <ehabk...@redhat.com>
> > CC: "Michael S. Tsirkin" <m...@redhat.com>
> > CC: Marcel Apfelbaum <marcel.apfelb...@gmail.com>
> > Signed-off-by: Li Zhijian <lizhij...@cn.fujitsu.com>
> >
> > ---
> > V3: correct grammar and check XLF_CAN_BE_LOADED_ABOVE_4G first (Michael S.
> > Tsirkin)
> >
> > Signed-off-by: Li Zhijian <lizhij...@cn.fujitsu.com>
> > ---
> > hw/i386/pc.c | 10 +++++++++-
> > 1 file changed, 9 insertions(+), 1 deletion(-)
> >
> > diff --git a/hw/i386/pc.c b/hw/i386/pc.c
> > index 3b10726..baa99c0 100644
> > --- a/hw/i386/pc.c
> > +++ b/hw/i386/pc.c
> > @@ -904,7 +904,15 @@ 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) {
> > + /*
> > + * Although kernel allows initrd loading to above 4G,
> > + * it just makes it as large as possible while still staying below
> > 4G
> > + * since current BIOS always loads initrd below
> > pcms->below_4g_mem_size
> > + */
> > + initrd_max = UINT32_MAX;
> > + } else if (protocol >= 0x203) {
> > initrd_max = ldl_p(header+0x22c);
> > } else {
> > initrd_max = 0x37ffffff;
>
>
> I still have trouble understanding the above.
> Anyone else wants to comment / help rephrase the comment
> and commit log so it's readable?
The comment seems to contradict what I see on the code:
| Although kernel allows initrd loading to above 4G,
Sounds correct.
| it just makes it as large as possible while still staying below 4G
I'm not a native English speaker, but I believe "it" here should
be interpreted as "the kernel", which would be incorrect. It's
this QEMU function that limits initrd_max to a uint32 value, not
the kernel.
| since current BIOS always loads initrd below pcms->below_4g_mem_size
I don't know why the BIOS is mentioned here. The
below_4g_mem_size limit comes from these 2 lines inside
load_linux():
if (initrd_max >= pcms->below_4g_mem_size - pcmc->acpi_data_size) {
initrd_max = pcms->below_4g_mem_size - pcmc->acpi_data_size - 1;
}
In addition to that, initrd_max is uint32_t simply because QEMU
doesn't support the 64-bit boot protocol (specifically the
ext_ramdisk_image field), so all talk about below_4g_mem_size
seems to be just a distraction.
All that said, I miss one piece of information here: is
XLF_CAN_BE_LOADED_ABOVE_4G really supposed to override
header+0x22c? linux/Documentation/x86/boot.txt isn't clear about
that. Is there any reference that can help us confirm this?
--
Eduardo
