Pavel Fedin <p.fe...@samsung.com> writes: > Hello! > >> > On some architectures TARGET_PAGE_ALIGN() is not enough to get the right >> > alignment. For example on ARM TARGET_PAGE_BITS is 10 because some old CPUs >> > support 1K page size, while minimum SMMU page size is 4K. >> > >> > This fixes problems like: >> > >> > 2015-11-17T07:37:42.892265Z qemu-system-aarch64: VFIO_MAP_DMA: -22 >> > 2015-11-17T07:37:42.892309Z qemu-system-aarch64: vfio_dma_map(0x223da230, >> > 0x80002f0400, >> 0x10fc00, 0x7f89b40400) = -22 (Invalid >> > argument) >> > qemu: hardware error: vfio: DMA mapping failed, unable to continue > > [skip] > >> I don't understand how this is supposed to work, if we align to a larger >> size than the processor, then there are processor size pages of RAM than >> could be handed out as DMA targets for devices, but we can't map them >> through the IOMMU. Thus if the guest tries to use them, we get IOMMU >> faults in the host and likely memory corruption in the guest because the >> device can't read or write to the page it's supposed to. This doesn't >> seem like the right solution. > > Well, this was my first try on the problem. I've got your idea. But i guess > we should discuss the proper solution then. > So, i've got this problem on ARM64. On ARM64 we actually can never have 1K > pages. This page size was supported only by old 32-bit ARM CPUs, up to ARMv5 > IIRC, then it was dropped. Linux OS never even used it. > But, since qemu can emulate those ancient CPUs, TARGET_PAGE_BITS is defined > to 10 for ARM. And, ARM64 and ARM32 is actually the same target for qemu, so > this is why we still get it. > Perhaps, TARGET_PAGE_BITS should be a variable for ARM, and we should > set it according to the actual used CPU. Then this IOMMU alignment > problem would disappear automatically. What do you think?
Yes it should be. For one thing we pay a fairly high performance penalty for using these smaller pages for no reason. What the best way to do this remains to be seen as I think there a lot of fixed sized arrays currently in the system based on various derivations of TARGET_PAGE_BITS. > Cc'ed Peter since he is the main ARM guy here. > > Kind regards, > Pavel Fedin > Expert Engineer > Samsung Electronics Research center Russia -- Alex Bennée