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RE: [Xen-devel] [PATCH] Yield to VCPU hcall, spinlock yielding

To: "Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>
Subject: RE: [Xen-devel] [PATCH] Yield to VCPU hcall, spinlock yielding
From: Bryan S Rosenburg <rosnbrg@xxxxxxxxxx>
Date: Wed, 8 Jun 2005 11:19:11 -0400
Cc: ryanh@xxxxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxx, hohnbaum@xxxxxxxxxx, Orran Y Krieger <okrieg@xxxxxxxxxx>
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"Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx> wrote on 06/07/2005 06:38:14 AM:

> > You can only use spinlock accounting for dealing with locking
> > issues in kernel (unless you are willing to change
> > application level programs and libs).  If preemption
> > notification overhead is not prohibitive, the fact that it
> > solves the application problem as well as the kernel problem
> > seems like a compelling advantage over spinlock accounting,
> > doesn't it?
> Orran,
> Are you assuming the pre-emption notification is going to get propagated
> to user-space as a signal, and that user space applications would be
> modified to take advantage of the signal? (possibly this could be hidden
> in the pthread library?} Since the kernel doesn't know when user space
> has an application lock or not, that's going to be a lot of signals.
> Cheers,
> Ian

We're certainly *not* proposing that preemption notifications be propagated to user-space as signals.  We're not proposing any changes at all to the user/kernel interface.  Preemption notifications to the kernel will allow Linux to support its multiprocessor applications as well as (but not better than) it does today running natively.  The key point is that with kernel-level preemption notification, VCPUs are always in kernel mode when suspended, never in user mode.  Application state is always saved in Linux, not in Xen, and is available to be resumed on another VCPU if Linux so chooses.

- Bryan
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