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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 26.10.2023 10:34, Xenia Ragiadakou wrote:
> 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.

How can they expect to have a contiguous, large enough range there?

>>> 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

Well, mathematically (i.e. ignoring the potential for overflow) the
original expression and your replacement are identical anyway. But
overflow needs to be taken into consideration, and hence there is a
(theoretical only at this point) risk with the replacement expression
as well. As a result I still think that ...

>> 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.

... this alternative approach may want considering (provided we need
to make a change in the first place, which I continue to be
unconvinced of).

Jan