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x86/mem_sharing: let fork_complete leave a child lazy - #54
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azenla
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alexandermerritt,
bleggett,
kaniini and
tycho
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October 18, 2026 02:52
antoineco
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Oct 18, 2026
kaniini
previously approved these changes
Oct 18, 2026
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lgtm but needs rebase |
XENMEM_FORK_COMPLETE_LAZY makes the child runnable the way fork_complete does, by unmapping the vcpu areas it inherited and lifting the interrupt block, but skips materializing the physmap and keeps the parent link and its pause, so the child keeps faulting pages in from a parent that has to stay paused.
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Description
Adds
XENMEM_FORK_COMPLETE_LAZYtoXENMEM_sharing_op_fork_complete. A lazy completion makes the child runnable the way a normal completion does: it unmaps the vcpu areas the child inherited and lifts the interrupt block. It skips the walk of the parent's physmap and keeps the parent link and the parent's pause, so the child faults pages in from the parent through the existingmem_sharing_fork_page()path.The parent has to stay paused and alive for as long as the child lives. A hypervisor without this change rejects the flag with
EINVAL. A hypervisor with it answersENOSYSwhen the flag is sent to a domain that is not a fork, so a toolstack can probe for support without completing anything.Measured with a 512 MiB PVH zone, forking a suspended parent:
Four lazy children of one parent ran and answered ping, and destroying the children and then the parent did not panic.