RE: [PATCH V2 mlx5-next 12/14] vfio/mlx5: Implement vfio_pci driver for mlx5 devices

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



> From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Sent: Monday, November 8, 2021 8:36 PM
> 
> On Mon, Nov 08, 2021 at 08:53:20AM +0000, Tian, Kevin wrote:
> > > From: Jason Gunthorpe <jgg@xxxxxxxxxx>
> > > Sent: Tuesday, October 26, 2021 11:19 PM
> > >
> > > On Tue, Oct 26, 2021 at 08:42:12AM -0600, Alex Williamson wrote:
> > >
> > > > > This is also why I don't like it being so transparent as it is
> > > > > something userspace needs to care about - especially if the HW
> cannot
> > > > > support such a thing, if we intend to allow that.
> > > >
> > > > Userspace does need to care, but userspace's concern over this should
> > > > not be able to compromise the platform and therefore making VF
> > > > assignment more susceptible to fatal error conditions to comply with a
> > > > migration uAPI is troublesome for me.
> > >
> > > It is an interesting scenario.
> > >
> > > I think it points that we are not implementing this fully properly.
> > >
> > > The !RUNNING state should be like your reset efforts.
> > >
> > > All access to the MMIO memories from userspace should be revoked
> > > during !RUNNING
> >
> > This assumes that vCPUs must be stopped before !RUNNING is entered
> > in virtualization case. and it is true today.
> >
> > But it may not hold when talking about guest SVA and I/O page fault [1].
> > The problem is that the pending requests may trigger I/O page faults
> > on guest page tables. W/o running vCPUs to handle those faults, the
> > quiesce command cannot complete draining the pending requests
> > if the device doesn't support preempt-on-fault (at least it's the case for
> > some Intel and Huawei devices, possibly true for most initial SVA
> > implementations).
> 
> It cannot be ordered any other way.
> 
> vCPUs must be stopped first, then the PCI devices must be stopped
> after, otherwise the vCPU can touch a stopped a device while handling
> a fault which is unreasonable.
> 
> However, migrating a pending IOMMU fault does seem unreasonable as well.
> 
> The NDA state can potentially solve this:
> 
>   RUNNING | VCPU RUNNING - Normal
>   NDMA | RUNNING | VCPU RUNNING - Halt and flush DMA, and thus all
> faults
>   NDMA | RUNNING - Halt all MMIO access

should be two steps?

NDMA | RUNNING - vCPU stops access to the device
NDMA - halt all MMIO access by revoking mapping

>   0 - Halted everything

yes, adding a new state sounds better than reordering the vcpu/device
stop sequence.

> 
> Though this may be more disruptive to the vCPUs as they could spin on
> DMA/interrupts that will not come.

it's inevitable regardless how we define the migration states. the
actual impact depends on how long 'Halt and flush DMA' will take.

Thanks
Kevin




[Index of Archives]     [KVM ARM]     [KVM ia64]     [KVM ppc]     [Virtualization Tools]     [Spice Development]     [Libvirt]     [Libvirt Users]     [Linux USB Devel]     [Linux Audio Users]     [Yosemite Questions]     [Linux Kernel]     [Linux SCSI]     [XFree86]

  Powered by Linux