On Tue, Jul 28, 2020 at 10:40 AM Madhavan T. Venkataraman
[off-list ref] wrote:

On 7/28/20 12:16 PM, Andy Lutomirski wrote:

On Tue, Jul 28, 2020 at 9:32 AM Madhavan T. Venkataraman
[off-list ref] wrote:

Thanks. See inline..

On 7/28/20 10:13 AM, David Laight wrote:

From:  madvenka@linux.microsoft.com

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.


I tested it in on a KVM guest running Ubuntu. So, when you say
that a page fault is much slower, do you mean a regular page
fault that is handled through the VM layer? Here is the relevant code
in do_user_addr_fault():

I mean that x86 CPUs have reasonably SYSCALL and SYSRET instructions
(the former is used for 64-bit system calls on Linux and the latter is
mostly used to return from system calls), but hardware page fault
delivery and IRET (used to return from page faults) are very slow.