On Tue, Jul 28, 2020 at 9:32 AM Madhavan T. Venkataraman <madvenka@xxxxxxxxxxxxxxxxxxx> wrote: > > Thanks. See inline.. > > On 7/28/20 10:13 AM, David Laight wrote: > > From: madvenka@xxxxxxxxxxxxxxxxxxx > >> Sent: 28 July 2020 14:11 > > ... > >> The kernel creates the trampoline mapping without any permissions. When > >> the trampoline is executed by user code, a page fault happens and the > >> kernel gets control. The kernel recognizes that this is a trampoline > >> invocation. It sets up the user registers based on the specified > >> register context, and/or pushes values on the user stack based on the > >> specified stack context, and sets the user PC to the requested target > >> PC. When the kernel returns, execution continues at the target PC. > >> So, the kernel does the work of the trampoline on behalf of the > >> application. > > Isn't the performance of this going to be horrid? > > It takes about the same amount of time as getpid(). So, it is > one quick trip into the kernel. I expect that applications will > typically not care about this extra overhead as long as > they are able to run. What did you test this on? A page fault on any modern x86_64 system is much, much, much, much slower than a syscall. --Andy
