xen/arch/x86/hvm/dom0_build.c | 21 ++++++++++++++------- 1 file changed, 14 insertions(+), 7 deletions(-)
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 three cases below:
(1) start > kernel_start && end > kernel_end
(2) start < kernel_start && end < kernel_end
(3) 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.
In case (3), the evaluation will result in either end = kernel_start
or start = kernel_end but in both cases will be end < start, and
will either load initrd in the middle of the image, or arbitrarily
at kernel_end.
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@amd.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
---
Changes in v2:
- cover further cases of overlap (Jan)
- mention with an in-code comment that a proper, more fine-grained
solution can be implemented using a rangeset (Roger)
xen/arch/x86/hvm/dom0_build.c | 21 ++++++++++++++-------
1 file changed, 14 insertions(+), 7 deletions(-)
diff --git a/xen/arch/x86/hvm/dom0_build.c b/xen/arch/x86/hvm/dom0_build.c
index c7d47d0d4c..62debc7415 100644
--- 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;
--
2.34.1
On Mon, Oct 30, 2023 at 03:32:40PM +0200, 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 three cases below: > (1) start > kernel_start && end > kernel_end > (2) start < kernel_start && end < kernel_end > (3) 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. > In case (3), the evaluation will result in either end = kernel_start > or start = kernel_end but in both cases will be end < start, and > will either load initrd in the middle of the image, or arbitrarily > at kernel_end. > > This patch reorganizes the conditionals to include so far unconsidered > cases as well, uniformly returning the lowest available address. It would be good to mention that this was discovered because Zephyr has multiple loaded segments in non-contiguous ranges, so that we know what triggered the change and how we could test further improvements (like the usage of a rangeset). > > Fixes: 73b47eea2104 ('x86/dom0: improve PVH initrd and metadata placement') > Signed-off-by: Xenia Ragiadakou <xenia.ragiadakou@amd.com> > Signed-off-by: Jan Beulich <jbeulich@suse.com> Reviewed-by: Roger Pau Monné <roger.pau@citrix.com> With the commit message adjusted preferably. Thanks, Roger.
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