RE: [PATCH v6 0/7] vfio/type1: Add support for valid iova list management

[Shameerali Kolothum Thodi]

> -----Original Message-----
> From: Alex Williamson [mailto:alex.williamson@xxxxxxxxxx]
> Sent: 05 June 2018 18:03
> To: Shameerali Kolothum Thodi <shameerali.kolothum.thodi@xxxxxxxxxx>
> Cc: eric.auger@xxxxxxxxxx; pmorel@xxxxxxxxxxxxxxxxxx;
> kvm@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; iommu@xxxxxxxxxxxx
> foundation.org; Linuxarm <linuxarm@xxxxxxxxxx>; John Garry
> <john.garry@xxxxxxxxxx>; xuwei (O) <xuwei5@xxxxxxxxxx>; Joerg Roedel
> <joro@xxxxxxxxxx>; David Woodhouse <dwmw2@xxxxxxxxxxxxx>
> Subject: Re: [PATCH v6 0/7] vfio/type1: Add support for valid iova list
> management
> 
> [Cc +dwmw2]
> 
> On Fri, 25 May 2018 14:54:08 -0600
> Alex Williamson <alex.williamson@xxxxxxxxxx> wrote:
> 
> > On Fri, 25 May 2018 08:45:36 +0000
> > Shameerali Kolothum Thodi <shameerali.kolothum.thodi@xxxxxxxxxx>
> wrote:
> >
> > > Yes, the above changes related to list empty cases looks fine to me.
> >
> > Thanks Shameer, applied to my next branch with the discussed fixes for
> > v4.18 (modulo patch 4/7 which Joerg already pushed for v4.17).  Thanks,
> 
> Hi Shameer,
> 
> We're still hitting issues with this.  VT-d marks reserved regions for
> any RMRR ranges, which are IOVA ranges that the BIOS requests to be
> identity mapped for a device.  These are indicated by these sorts of
> log entries on boot:
> 
>  DMAR: Setting RMRR:
>  DMAR: Setting identity map for device 0000:00:02.0 [0xbf800000 - 0xcf9fffff]
>  DMAR: Setting identity map for device 0000:00:1a.0 [0xbe8d1000 - 0xbe8dffff]
>  DMAR: Setting identity map for device 0000:00:1d.0 [0xbe8d1000 - 0xbe8dffff]
> 
> So while for an unaffected device, I see very usable ranges for QEMU,
> such as:
> 
> 	00: 0000000000000000 - 00000000fedfffff
> 	01: 00000000fef00000 - 01ffffffffffffff
> 
> If I try to assign the previously assignable 00:02.0 IGD graphics
> device, I get:
> 
> 	00: 0000000000000000 - 00000000bf7fffff
> 	01: 00000000cfa00000 - 00000000fedfffff
> 	02: 00000000fef00000 - 01ffffffffffffff
> 
> And we get a fault when QEMU tries the following mapping:
> 
> vfio_dma_map(0x55f790421a20, 0x100000, 0xbff00000, 0x7ff163f00000)
> 
> bff00000 clearly extends into the gap starting at bf800000.  VT-d is
> rather split-brained about RMRRs, typically we'd exclude devices from
> assignment at all for relying on RMRRs and these reserved ranges
> would be a welcome mechanism to avoid conflicts with those ranges, but
> for RMRR ranges covering IGD and USB devices we've decided that we
> don't need to honor the RMRR (see device_is_rmrr_locked()), but it's
> still listed as a reserved range and bites us here.

Ah..:(. This looks similar to the pci window range reporting issue faced in
the arm world. I see the RFC you have sent out to ignore these "known" 
RMRRs. Hope we will be able to resolve this soon.

Thanks,
Shameer

> Unless we can get VT-d to exclude RMRRs from reserved regions where
> we've already seen fit to allow the devices to participate in the IOMMU
> API, this would introduce a functional regression for assignment of
> these devices.  David, should we skip GFX and USB devices with RMRRs in
> intel_iommu_get_resv_regions() just as we do in
> device_is_rmrr_locked()?  Thanks,
> 
> Alex