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Re: [Qemu-devel] [GSoC] Help needed in implementing live migration
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [Qemu-devel] [GSoC] Help needed in implementing live migration |
Date: |
Fri, 28 Jun 2019 12:18:37 +0100 |
User-agent: |
Mutt/1.12.0 (2019-05-25) |
* Stefan Hajnoczi (address@hidden) wrote:
> On Thu, Jun 27, 2019 at 05:02:33AM +0530, Sukrit Bhatnagar wrote:
> > On Tue, 25 Jun 2019 at 00:11, Dr. David Alan Gilbert
> > <address@hidden> wrote:
> > >
> > > * Sukrit Bhatnagar (address@hidden) wrote:
> > > > Hi David,
> > > >
> > > > I am Sukrit, GSoC participant working on PVRDMA live migration.
> > > > We had a short chat about vmxnet3 migration about a week ago
> > > > on the IRC channel.
> > > >
> > > > I am facing an issue while doing migration of the pvrdma device.
> > > > While loading the device state, we need to perform a few dma
> > > > mappings on the destination. But on the destination, the migration
> > > > fails due a BounceBuffer being locked (in_use). This global
> > > > BounceBuffer is used in address_space_map/unmap functions
> > > > which the rdma_pci_dma_map/unmap calls.
> > > > Essentially, we need a way to remap guest physical address on
> > > > the destination after migration.
> > > >
> > > > I had posted an RFC a while ago on the list:
> > > > https://lists.gnu.org/archive/html/qemu-devel/2019-06/msg04924.html
> > > > https://lists.gnu.org/archive/html/qemu-devel/2019-06/msg04923.html
> > > >
> > > > My mentors (Marcel and Yuval) told me to ask you for help
> > > > regarding this. It would be really great if you can guide me in
> > > > finding a workaround for this.
> > >
> > > Hi,
> > > I'll have a look; I need to get some other things finished first.
> >
> > Adding cc: qemu-devel, sorry for the private email.
>
> I haven't looked deeply but it's surprising that you're hitting
> BounceBuffer. My understanding is that's an old mechanism for
> supporting exotic things like DMAing to/from device MMIO registers.
>
> Modern machines and guest software usually don't do this. I wonder why
> you're hitting this case.
>
> If you look at the BounceBuffer code there's an API to register a
> callback (cpu_register_map_client()). That's how the case of multiple
> BounceBuffers is supposed to be handled.
>
> Can you double-check your code and figure out how it got here? I don't
> think it should be taking this path.
Looking at address_space_map I see it uses bounce buffers if
!memory_access_is_direct, and that seems to be a check to see if
the memory region is actually a normal block of RAM.
Dave
> Stefan
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK