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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Nov 08, 2018 at 06:04:11AM -0700, Jan Beulich wrote:
> >>> On 08.11.18 at 13:47, <roger.pau@xxxxxxxxxx> wrote:
> > My point would be that on x86 I think the only way to have preemptible
> > long-running operations inside of Xen is to block the guest vCPU and
> > run them in a tasklet, or at least this seems the less invasive one.
> > 
> > Do you still have objections to this patch/approach?
> 
> Well, I still don't understand why you think you need to introduce
> a tasklet in the first place. That's because I still don't understand
> what you think is wrong with the current approach (leaving aside
> the exact placement of where the vpci hook needs to be called).

The current approach doesn't prevent the vCPU from returning to guest
context with pending work.

Placing the pending work hook (vpci_process_pending) in hvm_do_resume
is not going to solve this unless we raise and process a scheduler
softirq, and then this leads to the recursion problem.

So overall I don't see how it's possible to have this long running
preemptible operations inside the hypervisor if it's not using a
tasklet.

Sorry for making this discussion so long, hope this is more clear than
my previous replies.

Thanks, Roger.

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