Re: [PATCH V1 01/16] x86/ioreq: Prepare IOREQ feature for making it common

From: Oleksandr <olekstysh@gmail.com>
To: Jan Beulich <jbeulich@suse.com>
Cc: xen-devel@lists.xenproject.org,
	"Oleksandr Tyshchenko" <oleksandr_tyshchenko@epam.com>,
	"Paul Durrant" <paul@xen.org>,
	"Andrew Cooper" <andrew.cooper3@citrix.com>,
	"Wei Liu" <wl@xen.org>, "Roger Pau Monné" <roger.pau@citrix.com>,
	"Julien Grall" <julien@xen.org>,
	"Stefano Stabellini" <sstabellini@kernel.org>,
	"Julien Grall" <julien.grall@arm.com>
Subject: Re: [PATCH V1 01/16] x86/ioreq: Prepare IOREQ feature for making it common
Date: Mon, 21 Sep 2020 17:43:25 +0300	[thread overview]
Message-ID: <ac068d84-24cb-af37-cb89-f36fdd785d07@gmail.com> (raw)
In-Reply-To: <dfbe6c62-55f6-8d33-db25-031e36b758bc@suse.com>

On 21.09.20 16:29, Jan Beulich wrote:

Hi

> On 21.09.2020 14:47, Oleksandr wrote:
>> On 21.09.20 15:31, Jan Beulich wrote:
>>> On 21.09.2020 14:22, Oleksandr wrote:
>>>> On 14.09.20 16:52, Jan Beulich wrote:
>>>>> On 10.09.2020 22:21, Oleksandr Tyshchenko wrote:
>>>>>> @@ -1215,8 +1233,7 @@ void hvm_destroy_all_ioreq_servers(struct domain *d)
>>>>>>         struct hvm_ioreq_server *s;
>>>>>>         unsigned int id;
>>>>>>     
>>>>>> -    if ( !relocate_portio_handler(d, 0xcf8, 0xcf8, 4) )
>>>>>> -        return;
>>>>>> +    arch_hvm_ioreq_destroy(d);
>>>>> So the call to relocate_portio_handler() simply goes away. No
>>>>> replacement, no explanation?
>>>> As I understand from the review comment on that for the RFC patch, there
>>>> is no
>>>> a lot of point of keeping this. Indeed, looking at the code I got the
>>>> same opinion.
>>>> I should have added an explanation in the commit description at least.
>>>> Or shall I return the call back?
>>> If there's a reason to drop it (which I can't see, but I also
>>> don't recall seeing the discussion you're mentioning), then doing
>>> so should be a separate patch with suitable reasoning. In the
>>> patch here you definitely should only transform what's already
>>> there.
>> Sounds reasonable. Please see the comment below
>> relocate_portio_handler() here:
>> https://www.mail-archive.com/xen-devel@lists.xenproject.org/msg78512.html
>>
>> However, I might interpret the request incorrectly.
> I'm afraid you do: The way you've coded it the function was a no-op.
> But that's because you broke the caller by not bailing from
> hvm_destroy_all_ioreq_servers() if relocate_portio_handler() returned
> false. IOW you did assume that moving the "return" statement into an
> inline function would have an effect on its caller(s). For questions
> like this one it also often helps to look at the commit introducing
> the construct in question (b3344bb1cae0 in this case): Chances are
> that the description helps, albeit I agree there are many cases
> (particularly the farther you get into distant past) where it isn't
> of much help.

Hmm, now it's clear to me what I did wrong. By calling 
relocate_portio_handler() here we don't really want to relocate 
something, we just use it as a flag to indicate whether we need to 
actually release IOREQ resources down the 
hvm_destroy_all_ioreq_servers(). Thank you for the explanation, it 
wasn't obvious to me at the beginning. But, now the question is how to 
do it in a correct way and retain current behavior (to not break callers)?

I see two options here:
1. Place the check of relocate_portio_handler() in 
arch_hvm_ioreq_destroy() and make the later returning bool.
     The "common" hvm_destroy_all_ioreq_servers() will check for the 
return value and bail out if false.
2. Don't use relocate_portio_handler(), instead introduce a flag into 
struct hvm_domain's ioreq_server sub-structure.

Personally I don't like much the option 1 and option 2 is a little bit 
overhead.

What do you think?

-- 
Regards,

Oleksandr Tyshchenko