Re: [PATCH v1 0/4] [RFC] Implement Trampoline File Descriptor

From: Andy Lutomirski
Date: Tue Jul 28 2020 - 13:16:46 EST

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.