Re: [PATCH] x86/hvm/dom0: fix PVH initrd and metadata placement

[Xenia Ragiadakou <xenia.ragiadakou@xxxxxxx>]

On 26/10/23 10:35, Jan Beulich wrote:

> On 26.10.2023 08:45, Xenia Ragiadakou wrote:
> > Given that start < kernel_end and end > kernel_start, the logic that
> > determines the best placement for dom0 initrd and metadata, does not
> > take into account the two cases below:
> > (1) start > kernel_start && end > kernel_end
> > (2) start < kernel_start && end < kernel_end
> >
> > In case (1), the evaluation will result in end = kernel_start
> > i.e. end < start, and will load initrd in the middle of the kernel.
> > In case (2), the evaluation will result in start = kernel_end
> > i.e. end < start, and will load initrd at kernel_end, that is out
> > of the memory region under evaluation.
> >       
> I agree there is a problem if the kernel range overlaps but is not fully
> contained in the E820 range under inspection. I'd like to ask though
> under what conditions that can happen, as it seems suspicious for the
> kernel range to span multiple E820 ranges.
>

We tried to boot Zephyr as pvh dom0 and its load address was under 1MB.

I know ... that maybe shouldn't have been permitted at all, but nevertheless we hit this issue.

> > This patch rephrases the if condition to include the above two cases
> > without affecting the behaviour for the case where
> > start < kernel_start && end > kernel_end
> >
> > Fixes: 73b47eea2104 ('x86/dom0: improve PVH initrd and metadata placement')
> > Signed-off-by: Xenia Ragiadakou <xenia.ragiadakou@xxxxxxx>
> > ---
> >  xen/arch/x86/hvm/dom0_build.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/xen/arch/x86/hvm/dom0_build.c b/xen/arch/x86/hvm/dom0_build.c
> > index c7d47d0d4c..5fc2c12f3a 100644
> > --- a/xen/arch/x86/hvm/dom0_build.c
> > +++ b/xen/arch/x86/hvm/dom0_build.c
> > @@ -518,7 +518,7 @@ static paddr_t __init find_memory(
> >          if ( end <= kernel_start || start >= kernel_end )
> >              ; /* No overlap, nothing to do. */
> >          /* Deal with the kernel already being loaded in the region. */
> > -        else if ( kernel_start - start > end - kernel_end )
> > +        else if ( kernel_start + kernel_end > start + end )
> >       
> What meaning has the sum of the start and end of either range? I can't
> figure how comparing those two values will be generally correct / useful.
> If the partial-overlap case needs handling in the first place, I think
> new conditionals need adding (and the existing one needs constraining to
> "kernel range fully contained") to use
> - as before, the larger of the non-overlapping ranges at start and end
>   if the kernel range is fully contained,
> - the tail of the range when the overlap is at the start,
> - the head of the range when the overlap is at the end.
>

Yes it is not quite straight forward to understand and is based on the
assumption that end > kernel_start and start < kernel_end, due to
the first condition failing.

Both cases:
(start < kernel_start && end < kernel_end) and
(kernel_start - start > end - kernel_end)
fall into the condition ( kernel_start + kernel_end > start + end )

And both the cases:
(start > kernel_start && end > kernel_end) and
(end - kernel_end > kernel_start - start)
fall into the condition ( kernel_start + kernel_end < start + end )

... unless of course I miss a case

> That said, in the "kernel range fully contained" case it may want
> considering to use the tail range if it is large enough, rather than
> the larger of the two ranges. In fact when switching to that model, we
> ought to be able to get away with one less conditional, as then the
> "kernel range fully contained" case doesn't need treating specially.
>
> Jan
>