linux-kernel - Re: [PATCH v2 08/19] ioasid: Add custom IOASID allocator

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Date:   Fri, 26 Apr 2019 08:19:03 -0700
From:   Jacob Pan <jacob.jun.pan@...ux.intel.com>
To:     Auger Eric <eric.auger@...hat.com>
Cc:     iommu@...ts.linux-foundation.org,
        LKML <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Jean-Philippe Brucker <jean-philippe.brucker@....com>,
        Yi Liu <yi.l.liu@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        Raj Ashok <ashok.raj@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Andriy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        jacob.jun.pan@...ux.intel.com
Subject: Re: [PATCH v2 08/19] ioasid: Add custom IOASID allocator

On Fri, 26 Apr 2019 11:06:54 +0200
Auger Eric <eric.auger@...hat.com> wrote:

> Hi Jacob,
> 
> On 4/25/19 11:29 PM, Jacob Pan wrote:
> > Hi Eric,
> > 
> > Thanks for the review.
> > 
> > On Thu, 25 Apr 2019 12:03:42 +0200
> > Auger Eric <eric.auger@...hat.com> wrote:
> >   
> >> Hi Jacob,
> >>
> >> On 4/24/19 1:31 AM, Jacob Pan wrote:  
> >>> Sometimes, IOASID allocation must be handled by platform specific
> >>> code. The use cases are guest vIOMMU and pvIOMMU where IOASIDs
> >>> need to be allocated by the host via enlightened or paravirt
> >>> interfaces.
> >>>
> >>> This patch adds an extension to the IOASID allocator APIs such
> >>> that platform drivers can register a custom allocator, possibly
> >>> at boot time, to take over the allocation. Xarray is still used
> >>> for tracking and searching purposes internal to the IOASID code.
> >>> Private data of an IOASID can also be set after the allocation.
> >>>
> >>> There can be multiple custom allocators registered but only one is
> >>> used at a time. In case of hot removal of devices that provides
> >>> the allocator, all IOASIDs must be freed prior to unregistering
> >>> the allocator. Default XArray based allocator cannot be mixed with
> >>> custom allocators, i.e. custom allocators will not be used if
> >>> there are outstanding IOASIDs allocated by the default XA
> >>> allocator.    
> >>
> >> What's the exact use case behind allowing several custom IOASID
> >> allocators to be registered?  
> > It is mainly for supporting multiple PCI segments thus multiple
> > vIOMMUs. Even though, all allocators will end up calling the host to
> > allocate PASIDs.  
> 
> Yes that was my understanding actually.
> 
> Another question is how do you handle the reserved RID_PASID
> requirement?
> 
We always use PASID 0 for request w/o PASID, so it does not go through
the allocator.
 #define PASID_RID2PASID			0x0

>  QEMU does not support multiple PCI segments/domains
> > afaik but others might.  
>  [...]  
>  [...]  
> > Yes, more clear this way.
> >   
>  [...]  
> >> is already in use?  
>  [...]  
> >> s/The reason is that custom allocator/The reason is that custom
> >> allocators  
>  [...]  
> >> This last sentence may be moved to the unregister() kerneldoc  
>  [...]  
> >> The fact the first registered custom allocator gets automatically
> >> active was not obvious to me and may deserve a comment.  
>  [...]  
>  [...]  
>  [...]  
>  [...]  
>  [...]  
>  [...]  
>  [...]  
> >> At this stage it is difficult to assess whether using a BUG_ON() is
> >> safe here. Who is responsible for freeing the IOASIDs?  
>  [...]  
>  [...]  
> >> s/data also sanitiy check/data, also sanity check  
> >>> +		 */
> >>> +		min = id;
> >>> +		max = id + 1;
> >>> +	} else
> >>> +		default_allocator_used = 1;    
> >> shouldn't default_allocator_used be protected as well?  
>  [...]  
> >> wouldn't it be possible to integrate the default io asid allocator
> >> as any custom allocator, ie. implement an alloc callback using
> >> xa_alloc. Then the active io allocator could be either a custom or
> >> a default one.  
> > That is an interesting idea. I think it is possible.
> > But since default xa allocator is internal to ioasid infrastructure,
> > why implement it as a callback?  
> 
> I mean your could directly define a static const default_allocator in
> ioasid.c and assign it by default. Do I miss something?
> 
got it, seems cleaner. let me give it a try.

Thanks

Jacob