Re: [Xen-devel] [PATCH v2 3/4] x86/vtd: introduce a PVH implementation of iommu_inclusive_mapping

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

On Mon, Aug 28, 2017 at 06:14:45AM +0000, Tian, Kevin wrote:
> > From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> > Sent: Wednesday, August 23, 2017 4:19 PM
> > To: Roger Pau Monne <roger.pau@xxxxxxxxxx>
> > Cc: Tian, Kevin <kevin.tian@xxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
> > Subject: Re: [Xen-devel] [PATCH v2 3/4] x86/vtd: introduce a PVH
> > implementation of iommu_inclusive_mapping
> > 
> > >>> On 22.08.17 at 16:01, <roger.pau@xxxxxxxxxx> wrote:
> > > On Tue, Aug 22, 2017 at 06:31:27AM -0600, Jan Beulich wrote:
> > >> >>> On 11.08.17 at 18:43, <roger.pau@xxxxxxxxxx> wrote:
> > >> > On certain Intel systems, as far as I can tell almost all pre-Haswell 
> > >> > ones,
> > >> > trying to boot a PVH Dom0 will freeze the box completely, up to the
> > point that
> > >> > not even the watchdog works. The freeze happens exactly when
> > enabling the DMA
> > >> > remapping in the IOMMU, the last line seen is:
> > >> >
> > >> > (XEN) [VT-D]iommu_enable_translation: iommu->reg =
> > ffff82c00021b000
> > >> >
> > >> > In order to workaround this (which seems to be a lack of proper RMRR
> > entries,
> > >> > plus the IOMMU being unable to generate faults and freezing the
> > entire system)
> > >> > add a PVH specific implementation of iommu_inclusive_mapping, that
> > maps
> > >> > non-RAM, non-unusable regions into Dom0 p2m. Note that care is
> > taken to not map
> > >> > device MMIO regions that Xen is emulating, like the local APIC or the 
> > >> > IO
> > APIC.
> > >> >
> > >> > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> > >>
> > >> I don't mean to object to the patch, but it certainly would be helpful
> > >> to understand the behavior a little better, in particular also to
> > >> perhaps be able to derive what RMRRs are missing (which could
> > >> then be added via command line option instead of this all-or-norhing
> > >> approach). Kevin, could you perhaps help here?
> > >
> > > I tied that, but since the system freezes completely I have no idea
> > > what's missing. It's quite clear to me that it's related to the IOMMU
> > > and it's inability to properly generate a fault, but further than that
> > > I have no other clue.
> > 
> > Hence my request for Kevin to help (perhaps indirectly by pulling
> > in other Intel folks). Someone being able to check what the chipset
> > actually does or being able to observe what's going on in a logic
> > analyzer should be able to explain the observed behavior.
> > 
> 
> We don't have logic analyzer specifically to examine VTd, but yes
> we can help have a try whether it's reproducible in our side and
> then do some analysis.
> 
> what's the hardware configuration?

It's a Dell Precision T3600 with Xeon(R) CPU E5-1607 0 @ 3.00GHz, a
C600/X79 chipset and BIOS version A14.

Just trying to boot a Xen kernel with dom0=pvh will show the issue
(ie: you won't be able to reach the panic at the end of the Dom0
builder because the box will get stuck in the iommu_hwdom_init
call).

Roger.

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