On Monday 11 February 2013 04:23 PM, Jonas Bonn wrote: > On 11 February 2013 11:28, James Hogan <james.hogan@xxxxxxxxxx> wrote: >> On 11/02/13 10:13, Vineet Gupta wrote: >>> On Monday 11 February 2013 03:06 PM, Jonas Bonn wrote: >>>> On 11 February 2013 08:26, Vineet Gupta <Vineet.Gupta1@xxxxxxxxxxxx> wrote: >>>> >>>>> The only downside of this patch is that userspace signal stack grows in size, >>>>> since signal frame only cares about scratch regs (pt_regs), but has to accommodate >>>>> unused placeholder for callee regs too by virtue of using user_regs_struct. >>>> Is this really true? Don't setcontext and friends require that _all_ >>>> the registers be part of sigcontext? >>> >>> But for an ABI - callee saved regs will anyhow be saved/restored even in >>> setcontext case ! So collecting it for that purpose seems useless, or am I missing >>> something here. >> >> I think Jonas' point was that signals are asynchronous, i.e. you could >> get interrupted by a signal at virtually any time during the program's >> execution. > > No, I agree that the callee-saved regs don't need to be saved across a > signal handler invocation. It's really just the setcontext case that > wants to be able to swap out the callee-saved regs. I don't think that's needed either - and if thats mandated somewhere, it would seem a unnecessary mis-optimization IMHO. See, even a setcontext enabled control flow needs to be ABI compliant so that it plays nicely with other normal flows of execution. Thus e.g. it can't fudge a callee reg - it needs to save orig callee reg(s) and restore them in the end. And if we agree to those semantics - I don't see any value in swapping the callee reg context around usage of setcontext as it would be a wasted effort. -Vineet -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
