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

[Oleksandr <olekstysh@xxxxxxxxx>]

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@xxxxxxxxxxxxxxxxxxxx/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