Re: Question about reserved_regions w/ Intel IOMMU

From: Robin Murphy <robin.murphy@arm.com>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: Alexander Duyck <alexander.duyck@gmail.com>,
	"Tian, Kevin" <kevin.tian@intel.com>,
	Alex Williamson <alex.williamson@redhat.com>,
	Baolu Lu <baolu.lu@linux.intel.com>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-pci <linux-pci@vger.kernel.org>,
	"iommu@lists.linux.dev" <iommu@lists.linux.dev>
Subject: Re: Question about reserved_regions w/ Intel IOMMU
Date: Mon, 19 Jun 2023 11:20:58 +0100	[thread overview]
Message-ID: <90823b33-1f44-8789-9a38-282407fd9f15@arm.com> (raw)
In-Reply-To: <ZIyxFpzh3WG+ifws@nvidia.com>

On 2023-06-16 19:59, Jason Gunthorpe wrote:
> On Fri, Jun 16, 2023 at 05:34:53PM +0100, Robin Murphy wrote:
>>
>> If the system has working ACS configured correctly, then this issue should
>> be moot;
> 
> Yes
> 
>> if it doesn't, then a VFIO user is going to get a whole group of
>> peer devices if they're getting anything at all, so it doesn't seem entirely
>> unreasonable to leave it up to them to check that all those devices'
>> resources play well with their expected memory map.
> 
> I think the kernel should be helping here.. 'go figure it out from
> lspci' is a very convoluted and obscure uAPI, and I don't see things
> like DPDK actually doing that.
> 
> IMHO the uAPI expectation is that the kernel informs userspace what
> the usable IOVA is, if bridge windows and lack of ACS are rendering
> address space unusable then VFIO/iommufd should return it as excluded
> as well.
> 
> If we are going to do that then all UNAMANGED domain users should
> follow the same logic.
> 
> We probably have avoided bug reports because of how rare it would be
> to see a switch and an UNMANAGED domain using scenario together -
> especially with ACS turned off.
> 
> So it is really narrow niche.. Obscure enough I'm not going to make
> patches :)

The main thing is that we've already been round this once before; we 
tried it 6 years ago and then reverted it a year later for causing more 
problems than it solved:

https://lkml.org/lkml/2018/3/2/760

Thanks,
Robin.