@r1 Quick note to say thanks for your earlier input but don't worry about this.
We've abandoned trying to get the Broadcom 57800 cards to run under XCP-NG or Xen 7.5; replaced them with Dell Intel i350&X540 Quad Port cards.
As a quick recap for anyone else with this issue, we had a set of 3 Dell R630 servers fitted with Dell's Qlogic/Broadcom 57800 based quad-port daughter network cards (2 x 10GBe RJ45's with 2 x 1GBe RJ45s). Hardware is rev 10 apparently and they identify within the OS as
Ethernet controller: Broadcom Limited NetXtreme II BCM57800 1/10 Gigabit Ethernet (rev 10)
Under Xen 7.1 and earlier these ran perfectly, using the BNX2X driver v 1.714.1 along with the latest firmware from Dell. Oddly most of the reported issues seem to concern this version ?
The Broadcom/QLogic 57800 doesn't seem to be on Citrix's HCL any longer for newer releases so presumably someone's decided that the year old NICs are now obsolete?
Attempting to upgrade to Xen 7.2 or indeed XCP-NG 7.5 resulted in installation of a newer BNX2X driver (1.714.6 if memory serves), which for whatever reason was unable to bring the 10GBe links up or detect connected cables.
We attempted to update the driver further using the appropriate Citrix driver disks along with attempting firmware rollbacks to see if that was the problem.
I had been hoping to try a fresh build of the driver as above, but have neither the build environment nor knowledge to do that!
After burning through 3-4 days troubleshooting this, we've given up on the Broadcom cards and bought replacement Intel NICs - Dell i350&X540 quad port daughter cards, again offering 2 x 10GBe & 2 x 1GBe RJ45 ports. Swapped the cards over, booted the machines and upgraded successfully to XCP-NG on first attempt.
Zero hassle; zero issues.
I can only assume there's a fix to be had somewhere but we've not got the time to keep experimenting in the hope something eventually results in a previously working system resuming working!
If it helps anyone else, we bought the Intel cards from here: https://www.etb-tech.com/dell-intel-i350-x540-1-10gb-rj-45-sfp-quad-port-rndc-98493.html#.W5Dt5-hKi70