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Re: [Xen-devel] [PATCH] xen: privcmd: schedule() after private hypercall when non CONFIG_PREEMPT

On 12/03/2014 03:28 AM, Luis R. Rodriguez wrote:
On Tue, Dec 02, 2014 at 11:11:18AM +0000, David Vrabel wrote:
On 01/12/14 22:36, Luis R. Rodriguez wrote:

Then I do agree its a fair analogy (and find this obviously odd that how
widespread cond_resched() is), we just don't have an equivalent for IRQ
context, why not avoid the special check then and use this all the time in the
middle of a hypercall on the return from an interrupt (e.g., the timer


OK thanks! That explains why we need some asm code but in that submission you
still also had used is_preemptible_hypercall(regs) and in the new
implementation you use a CPU variable xen_in_preemptible_hcall prior to calling
preempt_schedule_irq(). I believe you added the CPU variable because
preempt_schedule_irq() will preempt first without any checks if it should, I'm
asking why not do something like cond_resched_irq() where we check with
should_resched() prior to preempting and that way we can avoid having to use
the CPU variable?

Because that could preempt at any asynchronous interrupt making the
no-preempt kernel fully preemptive. How would you know you are just
doing a critical hypercall which should be preempted?


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