vexpress: Horribly slow MMC emulation on ARM host

Hi,

this basically concludes my problems of getting KVM running on the Jetson TK1 board with QEMU: all fine now, provided I switch from

qemu-system-arm -machine vexpress-a15 -sd disk.img ...

to

  qemu-system-arm -machine vexpress-a15 \
    -drive file=disk.img,if=none,id=disk \
    -device virtio-blk-device,drive=disk ...

This applies to both emulated and KVM accelerated mode. If I run the same image (and guest kernel) emulated on my x86 box, there is still a difference between both disk modes, but it's not that excessive. On ARM the system requires minutes to boot from MMC - if it doesn't run into timeouts earlier. It's seconds with virtio.

Known problem?

Jan

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MMC is slow for several reasons, which boil down to:
* we aren't doing async I/O on the block device layer
(the SD/MMC card code is old and probably a long way from current best practice)
* we model the mmc-controller-to-card interface fairly closely, which is not a very efficient way of transferring data
* the MMC card interface in the vexpress board doesn't do DMA, which means that all data transferred is via (at best) word at a time MMIO register accesses
* MMIO register access is particularly bad with KVM because it requires us to come out of EL1 guest to EL2 which then switches to EL1 host (host kernel) which then drops out to EL0 (QEMU), and then on return we do the reverse transition EL0->EL1->EL2->EL1.

Some of this is likely fixable (and possibly there are bugs which make it slower than it need be as well as the issues we actually know about). But in general we've taken the approach of saying "if you want sensible IO performance use virtio", because some of the above list (no DMA, for instance) is unavoidable.

-- PMM