Back when I was looking at fw_cfg support for -kernel in OVMF, I noted that it took a while to read the kernel. We improved the perf substantially by using a 'rep insb' instruction, which I think kvm special cases to minimize VM traps.
Nevertheless, I thought that it would be good to implement a DMA interface to fw_cfg. It's great to see that Marc made that happen. One complication I thought of was that it might be tricky to deal with the implications of allowing this DMA to specify any old address to fill with fw_cfg data. So, for example, since Red Hat is working on SMM. Would a DMA to SMRAM be protected? I haven't watched the fw_cfg DMA discussion too closely, but has this been thought about? One idea I had was that near the end of the firmware boot, the firmware could trigger fw_cfg in QEMU to stop supporting DMA until a reset. -Jordan
