Re: [PATCH v11 0/10] iommu/vt-d: Fix intel vt-d faults in kdump kernel

[David Woodhouse <dwmw2@infradead.org>]

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On Mon, 2015-06-08 at 18:13 +0200, Joerg Roedel wrote:
> On Mon, Jun 08, 2015 at 04:50:24PM +0100, David Woodhouse wrote:
> > On Mon, 2015-06-08 at 17:29 +0200, Joerg Roedel wrote:
> > > Hmm, I also limited this functionality to kdump kernels. Do we still
> > > need to preserve these extended data structures even when there is no
> > > upstream support for them yet?
> > 
> > We *do* have upstream support. The 4.1 kernel will use the extended
> > root/context tables and will set the DMA_RTADDR_RTT bit in the Root
> > Table Address register, even though it doesn't yet actually *use* any
> > of the shiny new bits in the extended context tables.
> > 
> > So the code which copies the context tables needs to take that into
> > account.
> 
> Right, I missed that until now. So what the code with my changes does
> is, it sets the DMA_RTADDR_RTT bit as it would do on a normal boot. But
> unlike the root entry table address, this bit can not be changed while
> translation is active.
> 
> So I think we need to read out that bit when we find translation enabled
> and if it is different from what we would set it to, we bail out of any
> copying, disable translation and proceed as in a normal boot.

Given that this is only for kdump and not the general case of kexec,
that's probably tolerable. Of course we do still need to make it *not*
broken for the case where DMA_RTADDR_RTT is set, as it is at the
moment.

And I suspect if we're doing that, it might be simple enough to make it
convert to/from the extended page tables. I don't think we want to
preserve PASID tables; only the "second level" (i.e. traditional)
translation. So we could happily pull the page table pointer out of
either kind of context entry, and install it into either kind. I think
there's a simple mapping of translation types too. I need to sort out
the translation types when adding the real PASID support (imminently!)
anyway.

-- 
dwmw2

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