Re: [GIT pull] x86 vdso updates

[Ingo Molnar <mingo@elte.hu>]

* Andrew Lutomirski <luto@mit.edu> wrote:

> On Fri, May 27, 2011 at 7:36 AM, Andrew Lutomirski <luto@mit.edu> wrote:
> > 3. Add int 0xcc and use it from vgettimeofday.  It will SIGSEGV if
> > called from a user address (so it has no risk of ever becoming ABI)
> > and it will do gettimeofday if called from the right address.  (I like
> > 0xcc better than 0x81 because then I don't have to wonder whether any
> > syscall-like instructions start with 0x81.)  I'm not convinced that
> > the existing syscall entries are usable, because syscall itself has a
> > different calling convention and int 0x80 is a compat syscall.
> >
> 
> I started looking at what needs to be done and I wanted to get your
> opinion before I wrote a bunch of code that you'd reject.  Here are
> three ideas for how the int 0xcc / int 0x81 entry could work:
> 
> *** Idea 1 ***
> 
> Make it a real syscall but with extra constraints.  It would have the
> same calling convention as the syscall instruction, but it would turn
> into SIGKILL if the calling address isn't in the VSYSCALL page or if
> the syscall number isn't __NR_clock_gettimeofday.  It would BUG() if
> called from kernel mode.  There are two ways to implement this:
> 
> 1. Have the interrupt entry check constraints, twiddle its stack frame
> to look like a syscall instruction, and jump to the syscall entry.
> This way there's little code duplication.  (Is it safe to sysret back
> to userspace from an interrupt gate?  I don't see why not, but it
> seems to violate the spirit of the thing.)

Yeah, i think it should be safe. Lets try this? It looks like the 
simplest variant.

Thanks,

	Ingo