Re: [Xen-devel] [RFC 1/6] xen/arm: Re-enable interrupt later in the trap path

[Julien Grall <julien.grall@xxxxxxx>]

Hi,

On 01/08/2019 07:45, Andrii Anisov wrote:
> On 30.07.19 23:10, Julien Grall wrote:
>
> > > In this series I think I need interrupts locked until I start time accounting 
> > > for hypervisor. Time accounting is started by `tacc_head()` function. I 
> > > prefer to have it called from C, because it is more convenient and obvious 
> > > for those who are less familiar with the ARM code.
> Here is the question to you: what is the best place (and way) to start 
> hypervisor time tracking?
Looking at the patch, hypervisor time accounting is for:
    1) softirqs
    2) hardirqs

For hardirqs, you always enter in C with interrupt disabled. So this can be 
called directly from there.
For softirqs, they are quite a few places where do_sofirq() is called. So you 
either want to track the time in the function directly or on each callers.
I am not sure which one is the best way.

> > > > Resending the patch without things addressed is only going to make it worst.
> > > I'm still convinced the patch would improve interrupt latency for high 
> > > interrupt rate use cases.
> > > But I understand that I have no experiment to prove the effect, so I'm not 
> > > willing to push through the patch.
> > The only thing I ask is justification in your commit message rather than 
> > throwing things and expecting the reviewer to understand why you do that. I 
> > would recommend to refresh yourself how to submit a patch series [1].
> I'll follow you recommendation.
>
> > > Also, I have a question to you about another aspect of this patch. In the 
> > > function `enter_hypervisor_head()` there is a check for a disabled workaround 
> > > and turning it on. If we have the interrupts enabled until there, we have 
> > > good chances to go through the interrupt processing `do_IRQ()` before WA 
> > > enabled. Is it still OK?
> > Hmmm, that's correct.
> Sorry I did not get your point. What exactly is correct? My observation of the 
> scenario where we can go through the big piece of the hypervisor without WA 
> enabled? Or processing IRQs without WA enabled is the correct way to do?
"big piece" is somewhat half-correct.... All the hypercalls will be correctly 
protected, so the problem is only if you receive an interrupt before SSBD is 
enabled.
I would move the enablement in assembly code as part of entry.

Cheers,

--
Julien Grall

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