Re: [PATCH v6 2/3] xen/arm: Enable the existing x86 virtual PCI support for ARM.

From: Bertrand Marquis <Bertrand.Marquis@arm.com>
To: Jan Beulich <jbeulich@suse.com>
Cc: "Roger Pau Monné" <roger.pau@citrix.com>,
	"Ian Jackson" <iwj@xenproject.org>,
	"Rahul Singh" <Rahul.Singh@arm.com>,
	"Stefano Stabellini" <sstabellini@kernel.org>,
	"Julien Grall" <julien@xen.org>,
	"Volodymyr Babchuk" <Volodymyr_Babchuk@epam.com>,
	"Andrew Cooper" <andrew.cooper3@citrix.com>,
	"George Dunlap" <george.dunlap@citrix.com>,
	"Wei Liu" <wl@xen.org>, "Paul Durrant" <paul@xen.org>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>
Subject: Re: [PATCH v6 2/3] xen/arm: Enable the existing x86 virtual PCI support for ARM.
Date: Fri, 15 Oct 2021 13:10:22 +0000	[thread overview]
Message-ID: <E5F78DF2-D7CA-40C3-9E98-537155DD9529@arm.com> (raw)
In-Reply-To: <08fec860-7e8a-c8c7-df70-3c277de81718@suse.com>

Hi Jan,

> On 15 Oct 2021, at 14:00, Jan Beulich <jbeulich@suse.com> wrote:
> 
> On 15.10.2021 14:28, Bertrand Marquis wrote:
>>> On 15 Oct 2021, at 13:18, Jan Beulich <jbeulich@suse.com> wrote:
>>> On 15.10.2021 14:13, Bertrand Marquis wrote:
>>>>> On 15 Oct 2021, at 12:35, Roger Pau Monné <roger.pau@citrix.com> wrote:
>>>>> On Fri, Oct 15, 2021 at 12:18:59PM +0200, Jan Beulich wrote:
>>>>>> On 15.10.2021 12:14, Ian Jackson wrote:
>>>>>>> Bertrand Marquis writes ("Re: [PATCH v6 2/3] xen/arm: Enable the existing x86 virtual PCI support for ARM."):
>>>>>>>>> On 15 Oct 2021, at 09:00, Jan Beulich <jbeulich@suse.com> wrote:
>>>>>>>>> The latter is fine to be put here (i.e. FTAOD I'm fine with it
>>>>>>>>> staying here). For the former I even question its original placement
>>>>>>>>> in asm-x86/pci.h: It's not generally correct as per the PCI spec, as
>>>>>>>>> the bus portion of the address can be anywhere from 1 to 8 bits. And
>>>>>>>>> in fact there is a reason why this macro was/is used in only a
>>>>>>>>> single place, but not e.g. in x86'es handling of physical MCFG. It
>>>>>>>>> is merely an implementation choice in vPCI that the entire segment 0
>>>>>>>>> has a linear address range covering all 256 buses. Hence I think
>>>>>>>>> this wants to move to xen/vpci.h and then perhaps also be named
>>>>>>>>> VPCI_ECAM_BDF().
>>>>>>>> 
>>>>>>>> On previous version it was request to renamed this to ECAM and agreed
>>>>>>>> to put is here. Now you want me to rename it to VPCI and move it again.
>>>>>>>> I would like to have a confirmation that this is ok and the final move if possible.
>>>>>>>> 
>>>>>>>> @Roger can you confirm this is what is wanted ?
>>>>>>> 
>>>>>>> I think Roger is not available today I'm afraid.
>>>>>>> 
>>>>>>> Bertrand, can you give me a link to the comment from Roger ?
>>>>>>> Assuming that it says what I think it will say:
>>>>>>> 
>>>>>>> I think the best thing to do will be to leave the name as it was in
>>>>>>> the most recent version of your series.  I don't think it makes sense
>>>>>>> to block this patch over a naming disagreement.  And it would be best
>>>>>>> to minimise unnecessary churn.
>>>>>>> 
>>>>>>> I would be happy to release-ack a name change (perhaps proposed bo Jan
>>>>>>> or Roger) supposing that that is the ultimate maintainer consensus.
>>>>>>> 
>>>>>>> Jan, would that approach be OK with you ?
>>>>>> 
>>>>>> Well, yes, if a subsequent name change is okay, then I could live with
>>>>>> that. I'd still find it odd to rename a function immediately after it
>>>>>> already got renamed. As expressed elsewhere, I suspect in his request
>>>>>> Roger did not pay attention to a use of the function in non-ECAM code.
>>>>> 
>>>>> Using MMCFG_BDF was original requested by Julien, not myself I think:
>>>>> 
>>>>> https://lore.kernel.org/xen-devel/a868e1e7-8400-45df-6eaa-69f1e2c99383@xen.org/
>>>>> 
>>>>> I'm slightly loss in so many messages. On x86 we subtract the MCFG
>>>>> start address from the passed one before getting the BDF, and then we
>>>>> add the startting bus address passed in the ACPI table. This is so far
>>>>> not need on Arm AFAICT because of the fixed nature of the selected
>>>>> virtual ECAM region.
>>>> 
>>>> At the end my patch will add in xen/pci.h:
>>>> #define ECAM_BDF(addr)         (((addr) & 0x0ffff000) >> 12)
>>> 
>>> Since you still make this proposal, once again: I'm not going to
>>> accept such a macro in this header, whatever the name. Its prior
>>> placement was wrong as well. Only ...
>>> 
>>>> #define ECAM_REG_OFFSET(addr)  ((addr) & 0x00000fff)
>>> 
>>> ... this one is fine to live here (and presumably it could gain uses
>>> elsewhere).
>> 
>> So you would agree if they are both moved to vpci.h with a VPCI_ prefix ?
> 
> I wouldn't object, but as said several times before I see no reason
> to also move and rename ECAM_REG_OFFSET(). If you moved it and if
> later we find a use for it outside of vPCI, we'd need to rename and
> move it again.

I will move BDF to vpci.h and had VPCI prefix and keep REG_OFFSET as
it is and where it is then.

Cheers
Bertrand

> 
> Jan
> 