On 5/6/07, Arnd Bergmann <[EMAIL PROTECTED]> wrote:
> On Sunday 06 May 2007, Wink Saville wrote:
> > +Atomic Code Execution (ACE) allows code to execute as if it was
> > +surrounded by spin_lock_irqsave and spin_unlock_irqrestore without
> > +requiring actual access to the processor flags register.
>
> I guess you mean spin_{un,}lock_irq here, right? The save/restore
> part had me confused for quite a while when trying to understand
> what you are doing.Sorry, I should have said spin_unlock_irqrestore. > > > Thus code > > +executing within the ACE area can also be executed from user space or > > +kernel space. This is accomplished by using spin locks when executing > > +within the ACE area and changes to arch/x86_64/kernel/entry.S such that > > +when an interrupt occurs while executing code in the ACE area that code > > +will be completed before the interrupt is dispatched. > > I don't understand how you can write to the spinlock when coming from > user space. If the page is writable, how do you make sure the user can't > write malicious code or data into it? Trusted code should only be allowed access to the feature, at the moment it is enforced by requiring the applications to have root permissions to open the character device driver. > > > +The modifications to entry.S starts at the label ace_common plus the > > +macro HANDLE_ACE immediately following. HANDLE_ACE invokes ace_common > > +at the beginning of each interrupt, if it finds that code in the ACE > > +area was interrupted it causes the ACE code to resume execution prior > > +to handling the interrupt. > > You add eight instruction to every interrupt entry, have you measured > whether that makes a difference in timing? I haven't measured this, but assume the cost is negligible relative to the shortest interrupt path. I welcome any suggestions on improvements, which no doubt are possible. > > +The code for the ACE area is in the file arch/x86_64/kernel/ace.S. This > > +file contains all of the code that maybe executed by the kernel or > > +user-space. To be able to share this code with user space the code is > > +also compiled by user space, but this is only to generate the jump > > +table. Other techniques are possible, but this is the current > > +implementation and can be seen in the directory test/ace. > > Can't you put this into the vdso? Calling into the right place sounds > like a problem that is already solved. Possibly, but it isn't universally available, I hope to use this technique on other architectures. > > +The ACE area is initialized by calling > > +drivers/ace/ace_device.c/ace_init() from init/main.c/start_kernel(). > > I don't understand what you even want the character device for, > shouldn't the memory just always be there? The initial reason was just as a place to test the ideas and the implementation. But, as noted above, I also use it to enforce which applications can access the feature. Some technique will probably be required to do this and the driver can certainly be removed if it is unnecessary. > > > +4. ACE area API Reference > > + > > +ACE supports the following API: > > + > > +uint64_t ace_inc_counter(void) > > + > > + Increment the counter > > + > > + > > +uint64_t ace_inc_timer_counter(void) > > + > > + Increment the timer counter > > + > > + > > +void ace_get_counters(uint64_t *counter, uint64_t *timer_counter) > > + > > + Return the counter and timer counter > > None of these functions sound like you need such a complex infrastructure > to do them, setting single values can be done with atomic operations from > user space. Is there anything more complex that you plan to add as the > next step? > These were only used for testing, and in fact there are different now and I should have updated the documentation. Thank you for the feed back, Wink ------------------------------------------------------------------------- This SF.net email is sponsored by DB2 Express Download DB2 Express C - the FREE version of DB2 express and take control of your XML. No limits. Just data. Click to get it now. http://sourceforge.net/powerbar/db2/ _______________________________________________ kvm-devel mailing list kvm-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/kvm-devel
