On Tuesday, December 19, 2017 Jarkko Sakkinen wrote: > On Tue, 2017-12-19 at 18:52 +0000, Christopherson, Sean J wrote: > > > We can cache tokens in future in the kernel space, can't we? > > > > Yes, but why? Deferring to userspace is less complex and likely > > more performant. > > That's quite strong argument especially if you are making that for > systems running multiple independent workloads and not just a single > application. > > > Tokens are large enough that there would need to be some form of > > limit on the number of tokens, which brings up questions about > > how to account tokens, the cache eviction scheme, whether or not > > the size of the cache should be controllable from userspace, etc... > > Leaving caching decisions to the kernel also gives more freedoms to > do global decisions. > > > Userspace caching can likely provide better performance because > > the user/application knows the usage model and life expectancy of > > its tokens, i.e. userspace can make informed decisions about when > > to discard a token, how much memory to dedicate to caching tokens, > > etc... And in the case of VMs, userspace can reuse tokens across > > reboots (of the VM), e.g. by saving tokens to disk. > > I'm not really convinced that your argument is sound if you consider the > whole range of x86 systems that can run enclaves especially if the > system is running multiple irrelated applications. > > And you are ignoring everything else but the performance, which is does > not make any sense. The current design governs the Linux kernel to have > the ultimate power, which enclaves to run with the minimized proprietary > risk. I think that is something worth of emphasizing too. Exposing the token generated by the in-kernel LE doesn't affect the kernel's power in the slightest, e.g. the kernel doesn't need a LE to refuse to run an enclave and a privileged user can always load an out-of-tree driver if they really want to circumvent the kernel's policies, which is probably easier than stealing the LE's private key. > > Whether the token caching is left to kernel or user space will most > definitely introduce some non-trivial performance problems to solve > with some unexpected workloads that we cannot imagine right now. That's > why the governance should be the driver. Not the performance. Those > issues can and must be sorted out in any case. >
