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

From: Julien Grall <julien.grall@arm.com>
To: Andrii Anisov <andrii.anisov@gmail.com>, xen-devel@lists.xenproject.org
Cc: Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>,
	Stefano Stabellini <sstabellini@kernel.org>,
	Andrii Anisov <andrii_anisov@epam.com>
Subject: Re: [Xen-devel] [RFC 1/6] xen/arm: Re-enable interrupt later in the trap path
Date: Thu, 1 Aug 2019 10:37:16 +0100	[thread overview]
Message-ID: <096ebef8-0bc5-8b49-9b59-10fdb7e6c1de@arm.com> (raw)
In-Reply-To: <be1d31db-60d6-6e05-e50d-d5bc6963d9e7@gmail.com>

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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