Queue tweaks

[ShadowCurse]

Changes

• Update calculations in the queue to use std::mem::size_of instead of hard-coded values
• Add ability to reload IoVecBuffMut the same way as it can be done with non Mut variant

Reason

These were part of #4658, but because these changes seem generic, move into separate PR.

License Acceptance

By submitting this pull request, I confirm that my contribution is made under
the terms of the Apache 2.0 license. For more information on following Developer
Certificate of Origin and signing off your commits, please check
CONTRIBUTING.md.

PR Checklist

• If a specific issue led to this PR, this PR closes the issue.
• The description of changes is clear and encompassing.
• Any required documentation changes (code and docs) are included in this
  PR.
• API changes follow the Runbook for Firecracker API changes.
• User-facing changes are mentioned in CHANGELOG.md.
• All added/changed functionality is tested.
• New TODOs link to an issue.
• Commits meet
  contribution quality standards.

---

• This functionality cannot be added in rust-vmm.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.34%. Comparing base (167b1b0) to head (25b0b15).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4726      +/-   ##
==========================================
- Coverage   84.37%   84.34%   -0.03%     
==========================================
  Files         249      249              
  Lines       27452    27461       +9     
==========================================
  Hits        23162    23162              
- Misses       4290     4299       +9     

Flag	Coverage Δ
5.10-c5n.metal	84.57% <100.00%> (-0.04%)	⬇️
5.10-m5n.metal	84.55% <100.00%> (-0.04%)	⬇️
5.10-m6a.metal	83.84% <100.00%> (-0.04%)	⬇️
5.10-m6g.metal	80.91% <100.00%> (-0.04%)	⬇️
5.10-m6i.metal	84.54% <100.00%> (-0.04%)	⬇️
5.10-m7g.metal	80.91% <100.00%> (-0.04%)	⬇️
6.1-c5n.metal	84.57% <100.00%> (-0.04%)	⬇️
6.1-m5n.metal	84.55% <100.00%> (-0.04%)	⬇️
6.1-m6a.metal	83.84% <100.00%> (-0.04%)	⬇️
6.1-m6g.metal	80.91% <100.00%> (-0.04%)	⬇️
6.1-m6i.metal	84.54% <100.00%> (-0.04%)	⬇️
6.1-m7g.metal	80.91% <100.00%> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[bchalios]

I got some comments, mainly regarding documentation of the changes in the commit messages. Please be wordy in these. It might be obvious now why we did this change, but months/years from now it won't.

Also, have you checked whether the new functions added here affect kani proofs somehow, or whether we need to add new proofs? (cc @roypat).

Finally, I will be approving this only after I take a look at the mergeable buffers PR. The usefulness of these commits depends on that other PR. Please do NOT merge until then.

[bchalios]

Actually, kani does break :/

[roypat]

Also, have you checked whether the new functions added here affect kani proofs somehow, or whether we need to add new proofs? (cc @roypat).

On this part, I think we are fine because we're only moving code around inside functions that are covered (and get_desc_chain has all possible inputs covered because avail_idx is non-deterministic in the proof for do_pop_unchecked).

The timeout simply means that we wandered off the happy path of "verification finishes in a reasonable amount of time" 😭

[bchalios]

On this part, I think we are fine because we're only moving code around inside functions that are covered (and get_desc_chain has all possible inputs covered because avail_idx is non-deterministic in the proof for do_pop_unchecked).

You are right, today! However, this is all preparatory work for the next PR which will actually make direct use of these new functions. This is what I'm worried about.