In Thu, Jan 25, 2018 at 2:16 PM, Linus Torvalds <torva...@linux-foundation.org> wrote: > On Thu, Jan 25, 2018 at 10:48 AM, Linus Torvalds > <torva...@linux-foundation.org> wrote: >> >> So the biggest impact of this is the extra register saves > > Actually, the other noticeable part is the reloading of the argument > registers from ptregs. Together with just the extra level of > 'call/ret' and the stack setup, I'm guessing we're talking maybe 20 > cycles or so. > > So there's the extra register saves, and simply the fact that the > fastpath had a flatter calling structure. > > It still feels worth it. And if we do decide that we want to do the > register clearing on kernel entry for some paranoid mode, we'd pretty > much have to do this anyway. > > Linus
Another extra step the slow path does is checking to see if ptregs is safe for SYSRET. I think that can be mitigated by moving the check to the places that do modify ptregs (ptrace, sigreturn, and exec) which would set a flag to force return with IRET if the modified regs do not satisfy the criteria for SYSRET. -- Brian Gerst
