On Fri, Feb 18, 2022 at 1:30 AM David Laight <David.Laight@xxxxxxxxxx> wrote: > > From: Andy Lutomirski > > Sent: 17 February 2022 19:15 > ... > > This isn't actually optimal. On x86, TASK_SIZE_MAX is a bizarre > > constant that has a very specific value to work around a bug^Wdesign > > error^Wfeature of Intel CPUs. TASK_SIZE_MAX is the maximum address at > > which userspace is permitted to allocate memory, but there is a huge > > gap between user and kernel addresses, and any value in the gap would > > be adequate for the comparison. If we wanted to optimize this, simply > > checking the high bit (which x86 can do without any immediate > > constants at all) would be sufficient and, for an access known to fit > > in 32 bits, one could get even fancier and completely ignore the size > > of the access. (For accesses not known to fit in 32 bits, I suspect > > some creativity could still come up with a construction that's > > substantially faster than the one in your patch.) > > > > So there's plenty of room for optimization here. > > > > (This is not in any respect a NAK -- it's just an observation that > > this could be even better.) > > For 64bit arch that use the top bit to separate user/kernel > you can test '(addr | size) >> 62)'. > The compiler optimises out constant sizes. > > This has all been mentioned a lot of times. > You do get different fault types. > > OTOH an explicit check for constant size (less than something big) > can use the cheaper test of the sign bit. > Big constant sizes could be compile time errors. The different fault type issue may well be a real problem. Right now the core x86 fault code reserves the right to grouch if we get #GP instead of #PF. We could change that.
