* Re: [Qemu-devel] [GSoC] Help needed in implementing live migration
[not found] ` <20190624184133.GW2726@work-vm>
@ 2019-06-26 23:32 ` Sukrit Bhatnagar
2019-06-28 9:38 ` Stefan Hajnoczi
0 siblings, 1 reply; 3+ messages in thread
From: Sukrit Bhatnagar @ 2019-06-26 23:32 UTC (permalink / raw)
To: Dr. David Alan Gilbert, qemu-devel
On Tue, 25 Jun 2019 at 00:11, Dr. David Alan Gilbert
<dgilbert@redhat.com> wrote:
>
> * Sukrit Bhatnagar (skrtbhtngr@gmail.com) 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.
> Dave
> > Thanks,
> > Sukrit
> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [Qemu-devel] [GSoC] Help needed in implementing live migration
2019-06-26 23:32 ` [Qemu-devel] [GSoC] Help needed in implementing live migration Sukrit Bhatnagar
@ 2019-06-28 9:38 ` Stefan Hajnoczi
2019-06-28 11:18 ` Dr. David Alan Gilbert
0 siblings, 1 reply; 3+ messages in thread
From: Stefan Hajnoczi @ 2019-06-28 9:38 UTC (permalink / raw)
To: Sukrit Bhatnagar; +Cc: Dr. David Alan Gilbert, qemu-devel
[-- Attachment #1: Type: text/plain, Size: 2057 bytes --]
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
> <dgilbert@redhat.com> wrote:
> >
> > * Sukrit Bhatnagar (skrtbhtngr@gmail.com) 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.
Stefan
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^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [Qemu-devel] [GSoC] Help needed in implementing live migration
2019-06-28 9:38 ` Stefan Hajnoczi
@ 2019-06-28 11:18 ` Dr. David Alan Gilbert
0 siblings, 0 replies; 3+ messages in thread
From: Dr. David Alan Gilbert @ 2019-06-28 11:18 UTC (permalink / raw)
To: Stefan Hajnoczi; +Cc: Sukrit Bhatnagar, qemu-devel
* Stefan Hajnoczi (stefanha@gmail.com) 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
> > <dgilbert@redhat.com> wrote:
> > >
> > > * Sukrit Bhatnagar (skrtbhtngr@gmail.com) 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 / dgilbert@redhat.com / Manchester, UK
^ permalink raw reply [flat|nested] 3+ messages in thread
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2019-06-26 23:32 ` [Qemu-devel] [GSoC] Help needed in implementing live migration Sukrit Bhatnagar
2019-06-28 9:38 ` Stefan Hajnoczi
2019-06-28 11:18 ` Dr. David Alan Gilbert
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