Re: [Qemu-devel] [PATCH qemu v10 10/14] spapr_pci: Enable vfio-pci hotplug

[Thomas Huth <thuth@redhat.com>]

On Mon,  6 Jul 2015 12:11:06 +1000
Alexey Kardashevskiy <aik@ozlabs.ru> wrote:

> sPAPR IOMMU is managing two copies of an TCE table:
> 1) a guest view of the table - this is what emulated devices use and
> this is where H_GET_TCE reads from;
> 2) a hardware TCE table - only present if there is at least one vfio-pci
> device on a PHB; it is updated via a memory listener on a PHB address
> space which forwards map/unmap requests to vfio-pci IOMMU host driver.
> 
> At the moment presence of vfio-pci devices on a bus affect the way
> the guest view table is allocated. If there is no vfio-pci on a PHB
> and the host kernel supports KVM acceleration of H_PUT_TCE, a table
> is allocated in KVM. However, if there is vfio-pci and we do yet not
> support KVM acceleration for these, the table has to be allocated
> by the userspace.
> 
> When vfio-pci device is hotplugged and there were no vfio-pci devices
> already, the guest view table could have been allocated by KVM which
> means that H_PUT_TCE is handled by the host kernel and since we
> do not support vfio-pci in KVM, the hardware table will not be updated.
> 
> This reallocates the guest view table in QEMU if the first vfio-pci
> device has just been plugged. spapr_tce_realloc_userspace() handles this.

I wonder whether it would help to improve the readability of the code
later if you put the description of the function into the code instead
of the commit message?

> This replays all the mappings to make sure that the tables are in sync.
> This will not have a visible effect though as for a new device
> the guest kernel will allocate-and-map new addresses and therefore
> existing mappings from emulated devices will not be used by vfio-pci
> devices.
> 
> This adds calls to spapr_phb_dma_capabilities_update() in PCI hotplug
> hooks.
> 
> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
> ---
...
> diff --git a/hw/ppc/spapr_pci.c b/hw/ppc/spapr_pci.c
> index 76c988f..d1fa157 100644
> --- a/hw/ppc/spapr_pci.c
> +++ b/hw/ppc/spapr_pci.c
> @@ -827,6 +827,43 @@ int spapr_phb_dma_reset(sPAPRPHBState *sphb)
>      return 0;
>  }
>  
> +static int spapr_phb_hotplug_dma_sync(sPAPRPHBState *sphb)
> +{
> +    int ret = 0, i;
> +    bool had_vfio = sphb->has_vfio;
> +    sPAPRTCETable *tcet;
> +
> +    spapr_phb_dma_capabilities_update(sphb);
> +
> +    if (!had_vfio && sphb->has_vfio) {

    if (had_vfio || !sphb->has_vfio) {
        return 0;
    }

... and then you can save one level of indentation for the following
for-loop.

> +        for (i = 0; i < SPAPR_PCI_DMA_MAX_WINDOWS; ++i) {
> +            tcet = spapr_tce_find_by_liobn(SPAPR_PCI_LIOBN(sphb->index, i));
> +            if (!tcet || !tcet->enabled) {
> +                continue;
> +            }
> +            if (tcet->fd >= 0) {
> +                /*
> +                 * We got first vfio-pci device on accelerated table.
> +                 * VFIO acceleration is not possible.
> +                 * Reallocate table in userspace and replay mappings.
> +                 */
> +                ret = spapr_tce_realloc_userspace(tcet, true);
> +                trace_spapr_pci_dma_realloc_update(tcet->liobn, ret);
> +            } else {
> +                /* There was no acceleration, so just replay mappings. */
> +                ret = spapr_tce_replay(tcet);
> +                trace_spapr_pci_dma_update(tcet->liobn, ret);
> +            }
> +            if (ret) {
> +                break;
> +            }
> +        }
> +        return ret;
> +    }
> +
> +    return 0;
> +}
> +
>  /* Macros to operate with address in OF binding to PCI */
>  #define b_x(x, p, l)    (((x) & ((1<<(l))-1)) << (p))
>  #define b_n(x)          b_x((x), 31, 1) /* 0 if relocatable */
...
> @@ -1130,6 +1174,9 @@ static void spapr_phb_remove_pci_device_cb(DeviceState *dev, void *opaque)
>       */
>      pci_device_reset(PCI_DEVICE(dev));
>      object_unparent(OBJECT(dev));
> +
> +    /* Actual VFIO device release happens from RCU so postpone DMA update */
> +    call_rcu1(&((sPAPRPHBState *)opaque)->rcu, spapr_phb_remove_sync_dma);

Too much brackets again for my taste ;-)

>  }
>  

 Thomas