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charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org > > > I'm also running out of ideas here. I don't think a sysfs interface would > > > be any different to your earlier ioctl interface or the the /dev/msr approach, > > > they all share the same problem that they expose low-level access to > > > platform specific registers in a way that is neither portable nor safe to > > > use for general-purpose applications outside the very narrow scope > > > of running highly optimized HPC applications. > > > > Ok, but ARM architecture permits implementation defined registers at the > > first place. So can we provide some method/interface to access them as > > CPU feature if these registers do not at least affect kernel operations (like > > this barrier) and only root can access them? Library could offer portable way > > for user applications (under root permission) to access them. > > The kernel is meant to provide an abstraction for any differences between the > CPUs, including implementation defined registers. While any such abstraction > will be leaky, just passing through the raw registers is generally not a helpful > abstraction at all, as seen from the x86 MSR discussion you pointed to. > > One problem with having a root-only register level interface is that this > can break the boundary between kernel mode and root user space, and > this is something that a lot of people would like to strengthen for security > reasons (e.g. a root user should not be able to break secure boot). > > Another problem is that exposing the raw registers from kernel space > creates an ABI, and if it turns out to be a bad idea later on, this is hard to > take back without breaking existing applications. Not breaking things that > used to work is the primary rule for the Linux kernel. Ok, thanks for the thorough explanations. It helps my understandings. > In order to merge anything into the mainline kernel, I think the requirement > would be that it does provide a sensible abstraction inside of the kernel > that can directly be used from applications without having to go through > another library that abstracts it, and that has a good chance of being > supportable forever. As you mentioned an idea of process-based approach earlier, I will reconsider the possibility of general abstraction interface in that way. Regards, Tomohiro _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel