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PCI passthrough - exactly how does it work? And what low-level configuration options are there?

OK, so I have a variety of different SuperMicro motherboards (X10, X11 and X12 generations), and I'm trying to read old tapes via old Adaptec HVD and SE SCSI PCI adapters through a variety of PCIe-to-PCI bridges, some of them external, and some of them on the motherboard.

When I use Windows or Linux on the bare metal, I can read large (256k) blocks from the tapes in SCSI variable mode without any problems. But when I try doing the same thing via ESXi (I've tried 5.5, 6.0, 7.0, 8.0) with the host OSes accessing the adapters via PCI passthrough, I get "there is more data available" errors, which strongly suggests to me that ESXi is somehow limiting the request size of the block transfers from the adapter.

SO, questions:

i) Exactly how is ESXi "passing through" requests to the PCI devices? (I mean a low-level technical description)
ii) Is there a definitive list of configuration options one can specify (presumably in the .vmx file) for PCI passthrough devices?
iii) Is one of these settings a maximum data block size?

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