linux-kernel - Re: [PATCH V2 0/7] vdpa live update

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Message-ID: <CACGkMEv_yJ6iwOexGGexVt=EEpr2SXHJSLFcH_7g1yh=L-eDgA@mail.gmail.com>
Date: Tue, 16 Jul 2024 13:30:17 +0800
From: Jason Wang <jasowang@...hat.com>
To: Steven Sistare <steven.sistare@...cle.com>
Cc: virtualization@...ts.linux-foundation.org, linux-kernel@...r.kernel.org, 
	"Michael S. Tsirkin" <mst@...hat.com>, Si-Wei Liu <si-wei.liu@...cle.com>, 
	Eugenio Perez Martin <eperezma@...hat.com>, Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, 
	Dragos Tatulea <dtatulea@...dia.com>
Subject: Re: [PATCH V2 0/7] vdpa live update

On Mon, Jul 15, 2024 at 10:29 PM Steven Sistare
<steven.sistare@...cle.com> wrote:
>
> On 7/14/2024 10:14 PM, Jason Wang wrote:
> > On Fri, Jul 12, 2024 at 9:19 PM Steve Sistare <steven.sistare@...cle.com> wrote:
> >>
> >> Live update is a technique wherein an application saves its state, exec's
> >> to an updated version of itself, and restores its state.  Clients of the
> >> application experience a brief suspension of service, on the order of
> >> 100's of milliseconds, but are otherwise unaffected.
> >>
> >> Define and implement interfaces that allow vdpa devices to be preserved
> >> across fork or exec, to support live update for applications such as QEMU.
> >> The device must be suspended during the update, but its DMA mappings are
> >> preserved, so the suspension is brief.
> >>
> >> The VHOST_NEW_OWNER ioctl transfers device ownership and pinned memory
> >> accounting from one process to another.
> >>
> >> The VHOST_BACKEND_F_NEW_OWNER backend capability indicates that
> >> VHOST_NEW_OWNER is supported.
> >>
> >> The VHOST_IOTLB_REMAP message type updates a DMA mapping with its userland
> >> address in the new process.
> >>
> >> The VHOST_BACKEND_F_IOTLB_REMAP backend capability indicates that
> >> VHOST_IOTLB_REMAP is supported and required.  Some devices do not
> >> require it, because the userland address of each DMA mapping is discarded
> >> after being translated to a physical address.
> >>
> >> Here is a pseudo-code sequence for performing live update, based on
> >> suspend + reset because resume is not yet widely available.  The vdpa device
> >> descriptor, fd, remains open across the exec.
> >>
> >>    ioctl(fd, VHOST_VDPA_SUSPEND)
> >>    ioctl(fd, VHOST_VDPA_SET_STATUS, 0)
> >
> > I don't understand why we need a reset after suspend, it looks to me
> > the previous suspend became meaningless.
>
> The suspend guarantees completion of in-progress DMA.  At least, that is
> my interpretation of why that is done for live migration in QEMU, which
> also does suspend + reset + re-create.  I am following the live migration
> model.

Yes, but any reason we need a reset after the suspension?

>
> >>    exec
> >>
> >>    ioctl(fd, VHOST_NEW_OWNER)
> >>
> >>    issue ioctls to re-create vrings
> >>
> >>    if VHOST_BACKEND_F_IOTLB_REMAP
> >
> > So the idea is for a device that is using a virtual address, it
> > doesn't need VHOST_BACKEND_F_IOTLB_REMAP at all?
>
> Actually the reverse: if the device translates virtual to physical when
> the mappings are created, and discards the virtual, then VHOST_IOTLB_REMAP
> is not needed.

Ok.

>
> >>        foreach dma mapping
> >>            write(fd, {VHOST_IOTLB_REMAP, new_addr})
> >>
> >>    ioctl(fd, VHOST_VDPA_SET_STATUS,
> >>              ACKNOWLEDGE | DRIVER | FEATURES_OK | DRIVER_OK)
> >
> >  From API level, this seems to be asymmetric as we have suspending but
> > not resuming?
>
> Again, I am just following the path taken by live migration.
> I will be happy to use resume when the devices and QEMU support it.
> The decision to use reset vs resume should not affect the definition
> and use of VHOST_NEW_OWNER and VHOST_IOTLB_REMAP.
>
> - Steve

Thanks