Re: [PATCH for-4.12 v2 16/17] xen/arm: Implement Set/Way operations

From: Stefano Stabellini <sstabellini@kernel.org>
To: Julien Grall <julien.grall@arm.com>
Cc: xen-devel@lists.xenproject.org,
	Stefano Stabellini <sstabellini@kernel.org>,
	dfaggioli@suse.com
Subject: Re: [PATCH for-4.12 v2 16/17] xen/arm: Implement Set/Way operations
Date: Wed, 12 Dec 2018 09:25:43 -0800 (PST)	[thread overview]
Message-ID: <alpine.DEB.2.10.1812120925120.12259@sstabellini-ThinkPad-X260> (raw)
In-Reply-To: <851b26ab-be32-3d08-302f-3a49cb26c785@arm.com>

On Wed, 12 Dec 2018, Julien Grall wrote:
> On 07/12/2018 21:29, Stefano Stabellini wrote:
> > CC'ing Dario
> > 
> > Dario, please give a look at the preemption question below.
> > 
> > 
> > On Fri, 7 Dec 2018, Julien Grall wrote:
> > > On 06/12/2018 23:32, Stefano Stabellini wrote:
> > > > On Tue, 4 Dec 2018, Julien Grall wrote:
> > > So you may not execute them before returning to the guest introducing
> > > long delay. That's why we execute the rest of the code with interrupts
> > > masked. If sotfirq_pending() returns 0 then you know there were no
> > > more softirq pending to handle. All the new one will be signaled via
> > > an interrupt than can only come up when irq are unmasked.
> > > 
> > > The one before executing vCPU work can potentially be avoided. The reason
> > > I
> > > added it is it can take some times before p2m_flush_vm() will call
> > > softirq. As
> > > we do this on return to guest we may have already been executed for some
> > > time
> > > in the hypervisor. So this give us a chance to preempt if the vCPU
> > > consumed
> > > his sliced.
> > 
> > This one is difficult to tell whether it is important or if it would be
> > best avoided.
> > 
> > For Dario: basically we have a long running operation to perform, we
> > thought that the best place for it would be on the path returning to
> > guest (leave_hypervisor_tail). The operation can interrupt itself
> > checking sotfirq_pending() once in a while to avoid blocking the pcpu
> > for too long.
> > 
> > The question is: is it better to check sotfirq_pending() even before
> > starting? Or every so often during the operating is good enough? Does it
> > even matter?
> I am not sure to understand what is your concern here. Checking for
> softirq_pending() often is not an issue. The issue is when we happen to not
> check it. At the moment, I would prefer to be over cautious until we figure
> out whether this is a real issue.
> 
> If you are concerned about the performance impact, this is only called when a
> guest is using set/way.

Actually, I have no concerns, as I think it should make no difference,
but I just wanted a second opinion.

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