On Tue, May 19, 2015 at 11:03:22AM +0100, Andrew Jones wrote: > On Mon, May 18, 2015 at 04:53:03PM +0100, Catalin Marinas wrote: > > Another way would be to split the vma containing the non-cacheable > > memory so that you get a single vma with the vm_page_prot as > > Non-cacheable. > > This sounds interesting. Actually, it even crossed my mind once when I > first saw that the vma would overwrite the attributes, but then, sigh, > I let my brain take a stupidity bath. > > > > > Yet another approach could be for KVM to mmap the necessary memory for > > Qemu via a file_operations.mmap call (but that's only for ranges outside > > the guest "RAM"). > > I guess I prefer the vma splitting, rather than this (the vma creating > with mmap), as it keeps the KVM interface from changing (as you point out > below). Well, unless there are other advantages to this that are worth > considering? The advantage is that you don't need to deal with the mm internals in the KVM code. But you can probably add such code directly to mm/ and reuse some of the existing code in there already as part of change_protection(), mprotect_fixup(), sys_mprotect(). Actually, once you split the vma and set the new protection (something similar to mprotect_fixup), it looks to me like you can just call change_protection(vma->vm_page_prot). > > I didn't have time to follow these threads in details, but just to > > recap my understanding, we have two main use-cases: > > > > 1. Qemu handling guest I/O to device (e.g. PCIe BARs) > > 2. Qemu emulating device DMA > > > > For (1), I guess Qemu uses an anonymous mmap() and then tells KVM about > > this memory slot. The memory attributes in this case could be Device > > because that's how the guest would normally map it. The > > file_operations.mmap trick would work in this case but this means > > expanding the KVM ABI beyond just an ioctl(). > > > > For (2), since Qemu is writing to the guest "RAM" (e.g. video > > framebuffer allocated by the guest), I still think the simplest is to > > tell the guest (via DT) that such device is cache coherent rather than > > trying to remap the Qemu mapping as non-cacheable. > > If we need a solution for (1), then I'd prefer that it work and be > applied to (2) as well. Anyway, I'm still not 100% sure we can count on > all guest types (booloaders, different OSes) to listen to us. They may > assume non-cacheable is typical and safe, and thus just do that always. > We can certainly change some of those bootloaders and OSes, but probably > not all of them. That's fine by me. Once you get the vma splitting and attributes changing done, I think you get the second one for free. Do we want to differentiate between Device and Normal Non-cacheable memory? Something like KVM_MEMSLOT_DEVICE? Nitpick: I'm not sure whether "uncached" is clear enough. In Linux, pgprot_noncached() returns Strongly Ordered memory. For Normal Non-cachable we used pgprot_writecombine (e.g. a video framebuffer). Maybe something like KVM_MEMSLOT_COHERENT meaning a request to KVM to ensure that guest and host access it coherently (which would mean writecombine for ARM). That's similar naming to functions like dma_alloc_coherent() that return cacheable or non-cacheable memory based on what the device supports. Anyway, I'm not to bothered with the naming. -- Catalin _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
