Re: [PATCH 3/4] Intel pci: Limit dmar_init_reserved_ranges

[Mike Habeck]

On 03/31/2011 06:25 PM, Mike Travis wrote:
> I'll probably need help from our Hardware PCI Engineer to help explain
> this further, though here's a pointer to an earlier email thread:
>
> http://marc.info/?l=linux-kernel&m=129259816925973&w=2
>
> I'll also dig out the specs you're asking for.
>
> Thanks,
> Mike
>
> Chris Wright wrote:
> > * Mike Travis (travis@xxxxxxx) wrote:
> > > Chris - did you have any comment on this patch?
> >
> > It doesn't actually look right to me. It means that particular range
> > is no longer reserved. But perhaps I've misunderstood something.
> >
> > > Mike Travis wrote:
> > > > dmar_init_reserved_ranges() reserves the card's MMIO ranges to
> > > > prevent handing out a DMA map that would overlap with the MMIO range.
> > > > The problem while the Nvidia GPU has 64bit BARs, it's capable of
> > > > receiving > 40bit PIOs, but can't generate > 40bit DMAs.
> >
> > I don't undertand what you mean here.

What Mike is getting at is there is no reason to reserve the MMIO
range if it's greater than the dma_mask, given the MMIO range is
outside of what the IOVA code will ever hand back to the IOMMU
code.  In this case the nVidia card has a 64bit BAR and is assigned
the MMIO range [0xf8200000000 - 0xf820fffffff].  But the Nvidia
card can only generate a 40bit DMA (thus has a 40bit dma_mask). If
the IOVA code honors the limit_pfn (i.e., dma_mask) passed in it
will never hand back a >40bit address back to the IOMMU code. Thus
there is no reason to reserve the cards MMIO range if it is greater
than the dma_mask. (And that is what the patch is doing).

More below,,,

> > > > So when the iommu code reserves these MMIO ranges a > 40bit
> > > > entry ends up getting in the rbtree. On a UV test system with
> > > > the Nvidia cards, the BARs are:
> > > >
> > > > 0001:36:00.0 VGA compatible controller: nVidia Corporation
> > > > GT200GL Region 0: Memory at 92000000 (32-bit, non-prefetchable)
> > > > [size=16M]
> > > > Region 1: Memory at f8200000000 (64-bit, prefetchable) [size=256M]
> > > > Region 3: Memory at 90000000 (64-bit, non-prefetchable) [size=32M]
> > > >
> > > > So this 44bit MMIO address 0xf8200000000 ends up in the rbtree. As DMA
> > > > maps get added and deleted from the rbtree we can end up getting a cached
> > > > entry to this 0xf8200000000 entry... this is what results in the code
> > > > handing out the invalid DMA map of 0xf81fffff000:
> > > >
> > > > [ 0xf8200000000-1 >> PAGE_SIZE << PAGE_SIZE ]
> > > >
> > > > The IOVA code needs to better honor the "limit_pfn" when allocating
> > > > these maps.
> >
> > This means we could get the MMIO address range (it's no longer reserved).

Not true, the MMIO address is greater than the dma_mask (i.e., the
limit_pfn passed into alloc_iova()) thus the IOVA code will never
hand back that address range given it's greater than the dma_mask).

> > It seems to me the DMA transaction would then become a peer to peer
> > transaction if ACS is not enabled, which could show up as random register
> > write in that GPUs 256M BAR (i.e. broken).
> >
> > The iova allocation should not hand out an address bigger than the
> > dma_mask. What is the device's dma_mask?

Agree.  But there is a bug.  The IOVA doesn't validate the limit_pfn
if it uses the cached entry.  One could argue that it should validate
the limit_pfn, but then again a entry outside the limit_pfn should
have never got into the rbtree...  (it got in due to the IOMMU's
dmar_init_reserved_ranges() adding it).

-mike

> > thanks,
> > -chris

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