[Barrelfish-users] Fwd: Interrupt Handlers

[Piyus Kedia]

Thanks, that helped a lot.



On Fri, Aug 2, 2013 at 6:34 AM, Timothy Roscoe <troscoe at inf.ethz.ch> wrote:

> Piyus,
>
> It sounds like you're working within a Unix model of per-process kernel
> stacks, call-and-return patterns for exceptions, and kernel preemption.
>
> The Barrelfish kernel is different: it is more of a set of short event
> handlers which run with interrupts disabled, do not generate exceptions, do
> not return (the stack is discarded, not necessarily unwound) and instead
> jump at a continuation when they're done.   They execute serially.  The
> kernel cannot take a page fault, as it never accesses virtual memory that
> might be paged out, and cannot be handed a user reference to non-existent
> memory.
>
> When the kernel is entered from user mode (whether due to an interrupt or
> a user trap), the userspace execution context (including the PC) is saved
> in the dispatcher (think: process) control block.  This PC value *is* (in
> some versions of Barrelfish, I think) inspected by the kernel to determine
> the dispatcher's state - within a particular range of addresses.
>
> In the common case, this PC address is not returned to when the process is
> rescheduled; instead the kernel initiates an upcall to a different
> continuation in the dispatcher, and the thread can then be resumed in user
> space.   Consequently, it's pretty important that this value isn't changed
> by the kernel, but it is always the address of a userspace instruction.
>
> It might help us to answer your question if we understood the assumptions
> (independent of OS design choices) that you want to verify hold in
> Barrelfish.
>
> Of course, the source (and documentation) is always available from the web
> site...
>
> Best regards,
>
>  -- Mothy
>
>
> On 13-08-01 01:46 PM, Piyus Kedia wrote:
>
>> Hi,
>>
>> I am working on Kernel Binary Translator which pushes the translated PC
>> instead of the original PC on the stack when an interrupt or exception
>> occurs while executing in kernel mode. There could be some problem if
>> the interrupt or exception handler uses the PC value pushed on the
>> stack. Since interrupts are disabled in kernel mode this problem won't
>> exist. But still some exceptions like page fault can occur in kernel
>> mode. I just want to make sure that these handlers don't use the PC
>> value from the stack or don't overwrite the return address on the stack.
>>
>> Thanks,
>> Piyus
>>
>>
>> On Fri, Aug 2, 2013 at 4:20 AM, Timothy Roscoe <troscoe at inf.ethz.ch
>> <mailto:troscoe at inf.ethz.ch>> wrote:
>>
>>
>>     As Andrew says, the Barrelfish CPU driver processes all exceptions
>>     (whether hardware interrupts or traps) serially with interrupts
>>     disabled, unless it's idling.
>>
>>     Note that (like K42 and Psyche) Barrelfish uses upcall dispatch most
>>     of the time (think Scheduler Activations), so exit from an exception
>>     handler is typically a jump to a completely different IP/PC value
>>     from when that (or any other) exception occurred.
>>
>>     Note also that Barrelfish doesn't have any kernel threads - each CPU
>>     driver has a single, static, dedicated kernel stack.
>>
>>     Could you say a little more about what you mean?
>>
>>       -- Mothy
>>
>>
>>     On 13-08-01 05:32 AM, Piyus Kedia wrote:
>>
>>         Hi All,
>>
>>         I am working on dynamic binary translator. Please tell me if
>> anybody
>>         know if in Barrelfish interrupted kernel PC is ever used in
>>         interrupt
>>         handlers?
>>
>>         Thanks and regards,
>>         Piyus
>>
>>
>>
>>
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>>
>>
>>
>>
>
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