It was noticed by a patch series[1] for OpenEmbedded-core that neither barebox nor U-Boot reach their shell when tested on a native AArch64 server with Qemu running with KVM enabled. After investigation and help from the Qemu/KVM maintainers[2], it turns out that software that wants to run under KVM needs to take care what instructions it uses for MMIO accesses: Accessing a MMIO region will trap and KVM will check the exception syndrome register and extract the information it needs to emulate the access. However, not all instructions that access memory can be described by this Instruction Specific Syndrome. Notably, the pre- and post-index variants of ldr/str don't have valid syndromes on trap; this is because the instruction not only access the memory specified by the register, but also increments the address in the register and KVM would need to decode these instructions to be able to emulate that, which it currently does not. Linux doesn't suffer from this issue, because it implements readl/writel in assembly and makes sure they only use KVM-friendly instructions. This series does the same for barebox and also fixes other crashes and slow-downs related to I/O that were noticed along the way. [1]: https://lore.kernel.org/all/b77f2c6737c330ef9ecce325d50a4aaa25b3e536.camel@xxxxxxxxxxxxxxxxxxx/ [2]: https://lore.kernel.org/all/89f184d6-5b61-4c77-9f3b-c0a8f6a75d60@xxxxxxxxxxxxxx/ Ahmad Fatoum (5): ARM64: io: implement I/O accessors in assembly ARM64: board-dt-2nd: grow stack down from start of binary mtd: cfi-flash: use I/O accessors for reads/writes of MMIO regions ARM64: mmu: flush cacheable regions prior to remapping virtio: don't use DMA API unless required arch/arm/cpu/board-dt-2nd-aarch64.S | 2 +- arch/arm/cpu/mmu_64.c | 105 ++++++++++++++++++++++++++-- arch/arm/include/asm/io.h | 5 +- arch/arm/include/asm/io64.h | 99 ++++++++++++++++++++++++++ drivers/mtd/nor/cfi_flash.c | 2 +- drivers/mtd/nor/cfi_flash.h | 15 +++- drivers/virtio/virtio_ring.c | 85 +++++++++++++++++++--- include/linux/virtio_ring.h | 1 + 8 files changed, 293 insertions(+), 21 deletions(-) create mode 100644 arch/arm/include/asm/io64.h -- 2.39.5
