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SubjectRe: [PATCH 1/1] kvm/book3s_64: Fixes crash caused by not cleaning vhost IOTLB
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On Wed, 2019-12-18 at 15:53 +1100, Alexey Kardashevskiy wrote:
> H_STUFF_TCE is always called with 0. Well, may be some AIX somewhere
> calls it with a value other than zero, and I probably saw some other
> value somewhere but in QEMU/KVM case it is 0 so you effectively disable
> in-kernel acceleration of H_STUFF_TCE which is
> undesirable.
>

Thanks for the feedback!

> For now we should disable in-kernel H_STUFF_TCE/... handlers in QEMU
> just like we do for VFIO for older host kernels:
>
> https://git.qemu.org/?p=qemu.git;a=blob;f=hw/ppc/spapr_iommu.c;h=3d3bcc86496a5277d62f7855fbb09c013c015f27;hb=HEAD#l208

I am still reading into this temporary solution, I could still not
understand how it works.

> I am not sure what a proper solution would be, something like an eventfd
> and KVM's kvmppc_h_stuff_tce() signaling vhost that the latter needs to
> invalidate iotlbs. Or we can just say that we do not allow KVM
> acceleration if there is vhost+iommu on the same liobn (== vPHB, pretty
> much). Thanks,

I am not used to eventfd, but i agree it's a valid solution to talk to
QEMU and then use it to send a message via /dev/vhost.
KVM -> QEMU -> vhost

But I can't get my mind out of another solution: doing it in
kernelspace. I am not sure how that would work, though.

If I could understand correctly, there is a vhost IOTLB per vhost_dev,
and H_STUFF_TCE is not called in 64-bit DMA case (for tce_value == 0
case, at least), which makes sense, given it doesn't need to invalidate
entries on IOTLB.

So, we would need to somehow replace `return H_TOO_HARD` in this patch
with code that could call vhost_process_iotlb_msg() with
VHOST_IOTLB_INVALIDATE.

For that, I would need to know what are the vhost_dev's of that
process, which I don't know if it's possible to do currently (or safe
at all).

I am thinking of linking all vhost_dev's with a list (list.h) that
could be searched, comparing `mm_struct *` of the calling task with all
vhost_dev's, and removing the entry of all IOTLB that hits.

Not sure if that's the best approach to find the related vhost_dev's.

What do you think?

Best regards,
Leonardo Bras
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