Re: [PATCH v11 01/34] ARM: vGIC: avoid rank lock when reading priority

[Julien Grall <julien.grall@arm.com>]

Hi Stefano,

On 06/14/2017 06:32 PM, Stefano Stabellini wrote:
> On Wed, 14 Jun 2017, Julien Grall wrote:
>>> I don't understand your explanation. There are no PV protocols under
>>> xen/, they are implemented in other repositories. I grepped for ACCESS
>>> under xen/include/public, in case you referred to the PV protocol
>>> headers, but couldn't find anything interesting.
>>
>> Have a look at the pl011 emulation from Bhupinder. It will use plain '=' for
>> updating the PV drivers. So can you explain why it is fine there and not here?
> 
> Bhupinder's series is the first PV driver in the Xen codebase. It is
> easy to forget that coding should/might have to be different compared to
> Linux, which is the codebase I usually work with for PV drivers.
> 
> It is true that updating the indexes should be done atomically,
> otherwise the other end might end up reading a wrong index value.
> 
> 
>>>> Furthermore implementation of
>>>> atomic_read/atomic_write in Linux (both ARM and x86) is based on
>>>> WRITE_ONCE/READ_ONCE, on Xen it is a simple assignment.
>>>
>>> I don't follow why you are referring to Linux constructs in this
>>> discussion about Xen atomic functions.
>>
>> My point here is Xen and Linux are very similar. Actually a lot of the atomic
>> code is been taken from Linux (have a look to our atomic_read/atomic_write).
>>
>> As the atomic code was added the code by you (likely from Linux), I don't
>> understand why you don't complain about the atomic implementation but
>> ACCESS_ONCE.
> 
> Linux and Xen are free to make different assumptions regarding
> compilers.

It is not possible to say that Xen may have different assumptions when 
you import code from Linux without deep review (yes most of the code 
taken from Linux is as it is).

> 
> FWIW I am not really complaining about either atomics or ACCESS_ONCE, I
> just don't see the advantage of using ACCESS_ONCE over read/write_atomic
> (see below).
> 
> 
>>>> In any case, all those macros does not prevent re-ordering at the
>>>> processor
>>>> level nor read/write atomicity if the variable is misaligned.
>>>
>>> My understanding is that the unwritten assumption in Xen is that
>>> variables are always aligned. You are right about processor level
>>> reordering, in fact when needed we have to have barriers
>>>
>>> I have read Andre's well written README.atomic, and he ends the
>>> document stating the following:
>>>
>>>
>>>> This makes read and write accesses to ints and longs (and their respective
>>>> unsigned counter parts) naturally atomic.
>>>> However it would be beneficial to use atomic primitives anyway to annotate
>>>> the code as being concurrent and to prevent silent breakage when changing
>>>> the code
>>>
>>> with which I completely agree
>>
>> Which means you are happy to use either ACCESS_ONCE or
>> read_atomic/write_atomic as they in-fine exactly the same on the compiler we
>> support.
> 
> I do understand that both of them will produce the same output,
> therefore, both work for this use-case.
> 
> I don't understand why anybody would prefer ACCESS_ONCE over
> read/write_atomic, given that with ACCESS_ONCE as a contributor/reviewer
> you additionally need to remember to check whether the argument is a
> native data type. Basically, I see ACCESS_ONCE as "more work" for me.
> Why do you think that ACCESS_ONCE is "better"?

Have you looked at the implementation of ACCESS_ONCE? You don't have to 
check the data type when using ACCESS_ONCE. There are safety check to 
avoid misusing it.

What I want to avoid is this split mind we currently have about atomic.
They are either all safe or not.

As Andre suggested, we should probably import a lighter version of 
WRITE_ONCE/READ_ONCE. They are first easier to understand than 
read_atomic/write_atomic that could be confused with 
atomic_read/atomic_write (IIRC Jan agreed here).

The main goal is to avoid assembly code when it is deem not necessary.

> 
> Regarding the "compiler with support": do we state clearly in any docs
> or website what are the compilers we support? I think this would be the
> right opportunity to do it.

That's a discussion to have on the README.atomics patch.

Cheers,

-- 
Julien Grall

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