Update pxe boot flow

docs/secure-boot-keys.md

@@ -170,6 +170,44 @@ Caveats:
   CI-secret flow is a reasonable interim; the HSM flow is the invariant-#7 ideal
   (TODO #8).
 
+## PXE-only hosts: iPXE chainload and Secure Boot
+
+Firmware without UEFI HTTP boot (e.g. HP EliteBook 820 G4) falls back to PXE; the
+serving stack hands those hosts **iPXE** over TFTP (`serving/tftp/ipxe.efi`),
+which then chainloads the UKI over HTTP. dnsmasq selects per client automatically
+(HTTP-capable → UKI direct; PXE-only → iPXE → UKI), so no per-host config.
+
+Secure Boot across that chain needs **only the `db` key — the one already
+enrolled for the UKI. No extra keys, no Microsoft CA, no shim** (the fleet owns
+its keys):
+
+```
+firmware --(verify snponly.efi vs db)--> iPXE --(chain -> firmware LoadImage
+                                                  verifies UKI vs db)--> UKI
+```
+
+- The firmware verifies the TFTP-downloaded NBP (`ipxe.efi`) against `db` →
+  **the iPXE binary must be signed by `db`** (`sbsign`, same key/step as the UKI).
+  Stock/unsigned iPXE is rejected under Secure Boot.
+- iPXE must load the UKI as an **EFI image** (`imgfetch` + `boot`, or `chain`) so
+  the firmware's `LoadImage()` re-verifies it against `db` — do **not** use iPXE's
+  `kernel`/`boot` on a raw vmlinuz, which bypasses SB.
+- **cmdline gotcha (Secure Boot OFF only):** when iPXE executes the UKI it sets the
+  EFI load options to the image's cmdline, or — if empty — to the image **name**.
+  With SB off systemd-stub uses the load options *as the kernel cmdline*,
+  **replacing the UKI's embedded cmdline** (roothash/nvmf/...) → dracut emergency
+  shell. Clearing the args does NOT help (empty → iPXE falls back to the name). The
+  serving stack's rendered `boot.ipxe` therefore feeds iPXE the **real cmdline**,
+  which the dnsmasq entrypoint extracts from the UKI's `.cmdline` PE section
+  (`objcopy`, artifact mounted read-only) — see `serving/tftp/README.md`. **Secure
+  Boot ON is immune** — the stub ignores any loader-supplied cmdline — so this is
+  purely a lab/SB-off concern.
+
+**Lab posture (now): iPXE signing OFF → run the iPXE path with Secure Boot OFF.**
+`serving/tftp/ipxe.efi` is unsigned. **Production upgrade:** sign it with `db`
+and Secure Boot stays enforced end-to-end (tracked in TODO #8). The HTTP-boot path
+(firmware → UKI) is unaffected and remains SB-clean regardless.
+
 ## Key hygiene & lifecycle
 
 - **PK.key and KEK.key go offline / into an HSM and stay there.** They are only