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Re: [Xen-devel] [PATCH v3 5/7] vpci: fix execution of long running operations

Hi Roger,

On 11/8/18 11:44 AM, Roger Pau Monné wrote:
On Thu, Nov 08, 2018 at 11:42:35AM +0000, Julien Grall wrote:

Sorry to jump in the conversation late.

On 11/8/18 11:29 AM, Roger Pau Monné wrote:
Why would that be? The do_softirq() invocation sits on the exit-
to-guest path, explicitly avoiding any such nesting unless there
was a do_softirq() invocation somewhere in a softirq handler.

It sits on an exit-to-guest path, but the following chunk:


Would prevent the path from ever reaching the exit-to-guest and
nesting on itself, unless the vCPU is marked as blocked, which
prevents it from being scheduled thus avoiding this recursion.

I can't see how the recursion could happen on Arm. So is it an x86 issue?

This is not an issue with the current code, I was just discussing with
Jan how to properly implement vPCI long running operations that need
to be preempted.

To give more context on my question, we are looking at handling preemption on Arm in some long running operations (e.g cache flush) without having to worry about returning to guest.

I am thinking something along the following on Arm in a loop.

for ( .... )
   if ( try_reschedule )

This would require to have no lock taken but I think it would work on Arm for any long operations. So I am quite interested on the result on the discussions here.


Julien Grall

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