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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 30.10.2023 08:37, Xenia Ragiadakou wrote:
> Jan would it be possible to sketch a patch of your suggested solution 
> because I 'm afraid I have not fully understood it yet and I won't be 
> able to implement it properly for a v2?

While what Roger sent looks to be sufficient, I still thought I'd send
my variant, which I think yields more overall consistent results. Like
Roger's this isn't really tested (beyond making sure it builds).

Jan

From: Xenia Ragiadakou <xenia.ragiadakou@xxxxxxx>
Subject: x86/hvm/dom0: fix PVH initrd and metadata placement

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.

This patch reorganizes the conditionals to include so far unconsidered
cases as well, uniformly returning the lowest available address.

Fixes: 73b47eea2104 ('x86/dom0: improve PVH initrd and metadata placement')
Signed-off-by: Xenia Ragiadakou <xenia.ragiadakou@xxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Contrary to my original intentions, with the function preferring lower
addresses (by walking the E820 table forwards), the new cases also
return lowest-possible addresses.
---
v2: Cover further cases of overlap.

--- a/xen/arch/x86/hvm/dom0_build.c
+++ b/xen/arch/x86/hvm/dom0_build.c
@@ -515,16 +515,23 @@ static paddr_t __init find_memory(
 
         ASSERT(IS_ALIGNED(start, PAGE_SIZE) && IS_ALIGNED(end, PAGE_SIZE));
 
+        /*
+         * NB: Even better would be to use rangesets to determine a suitable
+         * range, in particular in case a kernel requests multiple heavily
+         * discontiguous regions (which right now we fold all into one big
+         * region).
+         */
         if ( end <= kernel_start || start >= kernel_end )
-            ; /* No overlap, nothing to do. */
+        {
+            /* No overlap, just check whether the region is large enough. */
+            if ( end - start >= size )
+                return start;
+        }
         /* Deal with the kernel already being loaded in the region. */
-        else if ( kernel_start - start > end - kernel_end )
-            end = kernel_start;
-        else
-            start = kernel_end;
-
-        if ( end - start >= size )
+        else if ( kernel_start > start && kernel_start - start >= size )
             return start;
+        else if ( kernel_end < end && end - kernel_end >= size )
+            return kernel_end;
     }
 
     return INVALID_PADDR;