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Bisected with cherry-picked firecracker-microvm/firecracker@4e01ed8 gave firecracker-microvm/firecracker@b2a4081:

b2a40810174213d74f6470744e59f74031597195 is the first bad commit
commit b2a40810174213d74f6470744e59f74031597195
Author: Adam Jensen <[email protected]>
Date:   Wed Dec 27 20:26:41 2023 +0000

    Populate the Entry Count field in MP configuration table
    
    Guest operating systems that comply with the Intel MP spec, such as
    FreeBSD, fail to boot because the Entry Count field in the
    MultiProcessor Configuration Table header is unset (defaults to 0).
    With the field set to a reasonable value, those guests are able to
    discover the MP configuration.
    
    Signed-off-by: Adam Jensen <[email protected]>

 src/vmm/src/arch/x86_64/mptable.rs | 42 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)