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

Madhavan T. Venkataraman
2020-07-28T18:52:24Z

On 7/28/20 12:16 PM, Andy Lutomirski wrote:
> On Tue, Jul 28, 2020 at 9:32 AM Madhavan T. Venkataraman
> <madvenka@linux.microsoft.com> 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 sent a response to this. But the mail was returned to me.
I am resending.

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():

if (unlikely(access_error(hw_error_code, vma))) {
/*
* If it is a user execute fault, it could be a trampoline
* invocation.
*/
if ((hw_error_code & tflags) == tflags &&
trampfd_fault(vma, regs)) {
up_read(&mm->mmap_sem);
return;
}
bad_area_access_error(regs, hw_error_code, address, vma);
return;
}
...
fault = handle_mm_fault(vma, address, flags);

trampfd faults are instruction faults that go through a different code path than
the one that calls handle_mm_fault(). Perhaps, it is the handle_mm_fault() that
is time consuming. Could you clarify?

Thanks.

Madhavan