Subject: Re: Summary of LPC guest MSI discussion in Santa Fe
To: Joerg Roedel <joro@8bytes.org>,
        Alex Williamson <alex.williamson@redhat.com>
References: <20161109151709.74927f83@t450s.home>
 <20161109222522.GS17771@arm.com> <20161109162458.39594fdb@t450s.home>
 <20161109233847.GT17771@arm.com> <20161109165957.62c1eb61@t450s.home>
 <83b6440a-31eb-c1b4-642c-a4c311f37ef2@redhat.com>
 <20161109175517.174e7803@t450s.home> <20161110020130.GA19108@arm.com>
 <ddd8af9d-ad8f-78d8-3048-3d640b74470e@redhat.com>
 <20161110104601.0939ba9a@t450s.home> <20161111111944.GO2078@8bytes.org>
Cc: Auger Eric <eric.auger@redhat.com>, Will Deacon <will.deacon@arm.com>,
        drjones@redhat.com, Christoffer Dall <christoffer.dall@linaro.org>,
        jason@lakedaemon.net, kvm@vger.kernel.org, marc.zyngier@arm.com,
        benh@kernel.crashing.org, punit.agrawal@arm.com,
        linux-kernel@vger.kernel.org, diana.craciun@nxp.com,
        iommu@lists.linux-foundation.org, pranav.sawargaonkar@gmail.com,
        arnd@arndb.de, dwmw@amazon.co.uk, jcm@redhat.com, tglx@linutronix.de,
        robin.murphy@arm.com, linux-arm-kernel@lists.infradead.org,
        eric.auger.pro@gmail.com
From: Don Dutile <ddutile@redhat.com>
Message-ID: <5825EB29.3040805@redhat.com>
Date: Fri, 11 Nov 2016 11:00:41 -0500
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On 11/11/2016 06:19 AM, Joerg Roedel wrote:
> On Thu, Nov 10, 2016 at 10:46:01AM -0700, Alex Williamson wrote:
>> In the case of x86, we know that DMA mappings overlapping the MSI
>> doorbells won't be translated correctly, it's not a valid mapping for
>> that range, and therefore the iommu driver backing the IOMMU API
>> should describe that reserved range and reject mappings to it.
>
> The drivers actually allow mappings to the MSI region via the IOMMU-API,
> and I think it should stay this way also for other reserved ranges.
> Address space management is done by the IOMMU-API user already (and has
> to be done there nowadays), be it a DMA-API implementation which just
> reserves these regions in its address space allocator or be it VFIO with
> QEMU, which don't map RAM there anyway. So there is no point of checking
> this again in the IOMMU drivers and we can keep that out of the
> mapping/unmapping fast-path.
>
>> For PCI devices userspace can examine the topology of the iommu group
>> and exclude MMIO ranges of peer devices based on the BARs, which are
>> exposed in various places, pci-sysfs as well as /proc/iomem.  For
>> non-PCI or MSI controllers... ???
>
> Right, the hardware resources can be examined. But maybe this can be
> extended to also cover RMRR ranges? Then we would be able to assign
> devices with RMRR mappings to guests.
>
eh gads no!

Assigning devices w/RMRR's is a security issue waiting to happen, if
it doesn't crash the system before the guest even gets the device --
reset the device before assignment; part of device is gathering system
environmental data; if BIOS/SMM support doesn't get env. data update,
it NMI's the system..... in fear that it may overheat ...


>
>
> 	Joerg
>