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Re: [Qemu-devel] [PATCH RFC 0/3] vfio: allow to notify unmap for very bi
From: |
Alex Williamson |
Subject: |
Re: [Qemu-devel] [PATCH RFC 0/3] vfio: allow to notify unmap for very big region |
Date: |
Fri, 20 Jan 2017 10:14:01 -0700 |
On Fri, 20 Jan 2017 20:27:18 +0800
Peter Xu <address@hidden> wrote:
> On Fri, Jan 20, 2017 at 11:43:28AM +0800, Peter Xu wrote:
>
> [...]
>
> > > 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.
>
> Okay when I replied I didn't notice this "watermark" may need more
> than several (even tens of) LOCs. :(
>
> Considering that I see no further usage of this watermark, I'm
> thinking whether it's okay I directly use (1ULL << VTD_MGAW) here as
> the watermark - it's simple, efficient and secure imho.
Avoiding the issue based on the virtual iommu hardware properties is a
fine solution, my intention was only to discourage introduction of
artificial limitations in the surrounding code to avoid this vfio
issue. Thanks,
Alex