Re: [Xen-devel] [PATCH v2] tools: implement initial ramdisk support for ARM.

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2014-04-09 at 10:10 +0100, Julien Grall wrote:
> 
> On 09/04/14 09:37, Ian Campbell wrote:
> > On Wed, 2014-04-09 at 09:17 +0100, Julien Grall wrote:
> >>
> >> On 09/04/14 09:12, Ian Campbell wrote:
> >>>> This small changes fix boot of the guest with RAM < 128Mb:
> >>>>
> >>>> diff --git a/tools/libxc/xc_dom_arm.c b/tools/libxc/xc_dom_arm.c
> >>>> index f051515..2228ba5 100644
> >>>> --- a/tools/libxc/xc_dom_arm.c
> >>>> +++ b/tools/libxc/xc_dom_arm.c
> >>>> @@ -300,7 +300,7 @@ int arch_setup_meminit(struct xc_dom_image *dom)
> >>>>        if ( ramend >= ram128mb + modsize && kernend < ram128mb )
> >>>>            modbase = ram128mb;
> >>>>        else if ( ramend >= kernend + modsize )
> >>>> -        modbase = kernend;
> >>>> +        modbase = ramend - modsize;
> >>>>        else
> >>>>            return -1;
> >>>>
> >>>> I guess this is because the kernel is extracting on it. I think we
> >>>> should follow the same "algorithm" as Xen (see place_modules) to decide
> >>>> where the modules should be loaded.
> >>>
> >>> Yes, this fix is correct. The existing code is just bogus, it makes no
> >>> sense to place the modules exactly at the end of RAM since they will
> >>> spin over the end.
> >>
> >> I'm not sure to understand why you are talking about RAM... currently
> >> the modules are placed just after the kernel (which in this case is
> >> always before ramend).
> >
> > I misread the change.
> >
> > I think the current code matches the dom0 place_modules call (modulo the
> > condition being expressed differently). Perhaps that case is also wrong?
> 
> The 2 if of libxc  express the first if of dom0 place_modules, right?
> 
> If dom0 as less than 128MB of RAM, we are trying to load the modules as 
> high as possible.

In Xen we have:
    if ( kernel_end < MIN(mem_start + MB(128), mem_end - total) )
        addr = MIN(mem_start + MB(128), mem_end - total);
    else if ( mem_end - ROUNDUP(kernel_end, MB(2)) >= total )
        addr = ROUNDUP(kernel_end, MB(2));
    else if ( kernel_start - mem_start >= total )
        addr = kernel_start - total;
    else
    {
        panic("Unable to find suitable location for dtb+initrd");

In libxc we have:
    if ( ramend >= ram128mb + modsize && kernend < ram128mb )
        modbase = ram128mb;
    else if ( ramend >= kernend + modsize )
        modbase = kernend;
    else
        return -1;

I think the second if and assignments are equivalent. I think the
problem comes from the differences in the first if which are in fact
different. They also differ in that userspace won't try and place the
modules before the kernel as a last resort.

Switching the Xen side to use the  libxc variable names makes it less
confusing to compare IMO:

    if ( kernend < MIN(ram128mb, ramend - modsize) )
        addr = MIN(ram128mb, ramend - modsize);
    else if ( ramend - kernend >= modsize )
        addr = kernend;
    else if ( kernel_start - mem_start >= total )
        addr = kernel_start - total;
    else
    {
        panic("Unable to find suitable location for dtb+initrd");

I think we need to use that MIN(ram128mb, ramend - modsize) calculation
in libxc too.

I'm also wondering if the second statement actually make sense with that
change. If kernelend >= ramend - modsize (so the first test fails) then 
I'm not convinced that ramend - kernend >= modsize can ever be true.

Ian.


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