Re: [Qemu-devel] [PATCH for-4.0 v4 4/4] i386: allow to load initrd below 4G for recent linux

[Eduardo Habkost]

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 <address@hidden>
> > CC: Richard Henderson <address@hidden>
> > CC: Eduardo Habkost <address@hidden>
> > CC: "Michael S. Tsirkin" <address@hidden>
> > CC: Marcel Apfelbaum <address@hidden>
> > Signed-off-by: Li Zhijian <address@hidden>
> > 
> > ---
> > V3: correct grammar and check XLF_CAN_BE_LOADED_ABOVE_4G first (Michael S. 
> > Tsirkin)
> > 
> > Signed-off-by: Li Zhijian <address@hidden>
> > ---
> >  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