Re: [Qemu-devel] [PATCH v5 22/45] postcopy: OS support test

[Paolo Bonzini]

On 13/03/2015 11:41, Dr. David Alan Gilbert wrote:
>>> > > +#ifdef HOST_X86_64
>>> > > +#ifndef __NR_userfaultfd
>>> > > +#define __NR_userfaultfd 323
>> > 
>> > Sholdn't this come from the kernel headers imported in the previous
>> > patch?  Rather than having an arch-specific hack.
> The header, like the rest of the kernel headers, just provides
> the constant and structure definitions for the call; the syscall numbers
> come from arch specific headers.  I guess in the final world I wouldn't
> need this at all since it'll come from the system headers; but what's
> the right way to put this in for new syscalls?
> 

You would just require new _installed_ kernel headers.  Then you can use
linux/userfaultfd.h and syscall.h (the latter from glibc, includes
asm/unistd.h to get syscall numbers).

linux-headers/ is useful for APIs that do not require system calls, or
for APIs that are extensible.  However, if a system call is required
(and mandatory) it's simpler to just use installed headers.

Paolo