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Re: [Qemu-devel] [PATCH RFC 0/3] vfio: allow to notify unmap for very bi

From: Peter Xu
Subject: Re: [Qemu-devel] [PATCH RFC 0/3] vfio: allow to notify unmap for very big region
Date: Fri, 20 Jan 2017 11:43:28 +0800
User-agent: Mutt/1.5.24 (2015-08-30)

On Thu, Jan 19, 2017 at 10:54:37AM -0700, Alex Williamson wrote:
> On Thu, 19 Jan 2017 17:25:29 +0800
> Peter Xu <address@hidden> wrote:
> > This requirement originates from the VT-d vfio series:
> > 
> >   https://lists.nongnu.org/archive/html/qemu-devel/2017-01/msg03495.html
> > 
> > The goal of this series is to allow IOMMU to notify unmap with very
> > big IOTLB range, for example, with base=0 and size=2^63-1 (to unmap
> > the whole address space).
> > 
> > The first patch is a good to have, for traces.
> > 
> > The second one is a cleanup of existing code, only.
> Sort of, it does add some overhead to the unmap path, but you remove
> that and more in the third patch.

Hmm, yes, I tried to get the ram pointer even for unmap. I should
remove the ending "only".

> > The third one moves the further RAM translation and check into map
> > operation logic, so that it'll free unmap operations.
> > 
> > The series is marked as RFC since I am not sure whether this is a
> > workable way. Anyway, please review to help confirm it.
> It seems reasonable to me,

Good to know that you didn't disagree on this. :) Then let me take
these patches along with that series in the next post (since that
series will depend on this one, so I'll keep them together, though
please maintainers decide on how you'll merge them when you want to),

> except for the example here of using 2^63-1,
> which I expect is to work around the vfio {un}map API bug as we
> discussed on irc.

Actually not only for that one, I don't know whether we have problem

(I mentioned this in IRC, in case you missed that, I paste it here as

static struct vfio_dma *vfio_find_dma(struct vfio_iommu *iommu,
                                      dma_addr_t start, size_t size)
        struct rb_node *node = iommu->dma_list.rb_node;

        while (node) {
                struct vfio_dma *dma = rb_entry(node, struct vfio_dma, node);

                if (start + size <= dma->iova)
                        node = node->rb_left;
                else if (start >= dma->iova + dma->size)
                          Could it overflow here? <--------------
                        node = node->rb_right;
                        return dma;

        return NULL;

I used 2^63-1 to try to avoid both cases.

> For everyone, the root of the problem is that the
> ioctls use:
>         __u64   iova;                           /* IO virtual address */
>         __u64   size;                           /* Size of mapping (bytes) */
> So we can only express a size of 2^64-1 and we have an off-by-one error
> trying to perform a map/unmap of an entire 64-bit space.  Note when
> designing an API, use start/end rather than base/size to avoid this.

Lesson learned.

> What I don't want to see is for this API bug to leak out into the rest
> of the QEMU code such that intel_iommu code, or iommu code in general
> subtly avoids it by artificially using a smaller range.  VT-d hardware
> has an actual physical address space of either 2^39 or 2^48 bits, so if
> you want to make the iommu address space match the device we're trying
> to emulate, that's perfectly fine.  AIUI, AMD-Vi does actually have a
> 64-bit address space on the IOMMU, so to handle that case I'd expect
> the simplest solution would be to track the and mapped iova high water
> mark per container in vfio and truncate unmaps to that high water end
> address.  Realistically we're probably not going to see iovas at the end
> of the 64-bit address space, but we can come up with some other
> workaround in the vfio code or update the kernel API if we do.  Thanks,

Agree that high watermark can be a good solution for VT-d. I'll use
that instead of 2^63-1. Though for AMD, if it supports 64bits, we may
still need to solve existing issues in the future, since in that case
the high watermark can be (hwaddr)-1 as well if guest specifies it.

(Though anyway I'm not sure when AMD vIOMMUs will be ready for device
 assignment, so I don't think that's a big issue at least for now)


-- peterx

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