On Tue, Oct 11, 2022 at 01:12:43AM +0000, Oliver Upton wrote: > The VMM must know something about the architecture it is running on, as > it calls KVM_DEV_ARM_ITS_SAVE_TABLES after all... IIUC this is still a kernel impl detail to flush data into guest pages within this ioctl, or am I wrong? For example, I'm assuming it's safe to change KVM_DEV_ARM_ITS_SAVE_TABLES impl one day to not flush data to guest memories, then the kernel should also disable the ALLOW_BITMAP cap in the same patch, so that any old qemu binary that supports arm64 dirty ring will naturally skip all the bitmap ops and becoming the same as what it does with x86 when running on that new kernel. With implicit approach suggested, we need to modify QEMU. Changing impl of KVM_DEV_ARM_ITS_SAVE_TABLES is probably not a good example.. but just want to show what I meant. Fundamentally it sounds cleaner if it's the kernel that tells the user "okay you collected the ring, but that's not enough; you need to collect the bitmap too", rather than assuming the user app will always know what kvm did in details. No strong opinion though, as I could also have misunderstood how arm works. -- Peter Xu
