@Tom-Elliott Sorry for not seeing this sooner. PAGE_SIZE is defined as 4096, so the mask is being set to 4095, which is the same value that iscsi_iser.c uses (~MASK_4K).
From the notes in LIS, I suspect that setting blk_queue_virt_boundary is supposed to insure that there are no gaps in th sg list, but they are still present, so the bounce buffer needs to be put back in place or the gaps need to be eliminated elsewhere.
The patch author responded this morning and is looking into the slowdown report. I’ll post any updates as I hear them.