Re: [Qemu-devel] [PATCH] virtio-pci: reduce modern_mem_bar size

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Sep 07, 2016 at 04:53:09PM +0300, Marcel Apfelbaum wrote:
> Currently each VQ Notification Virtio Capability is allocated
> on a different page. The idea is to enable split drivers within
> guests, however there are no known plans to do that.
> The allocation will result in a 8MB BAR, more than various
> guest firmwares pre-allocates for PCI Bridges hotplug process.
> 
> Reserve 4 bytes per VQ by default and add a new parameter
> "page-per-vq" to be used with split drivers.
> 
> Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
> ---
> Hi,
> 
> To be applied on top of:
>     [PATCH v5 1/2] pc: Add 2.8 machine (http://patchwork.ozlabs.org/patch/666812/)
> 
> Thanks,
> Marcel
> 
>  hw/virtio/virtio-pci.c | 18 +++++++++++-------
>  hw/virtio/virtio-pci.h |  6 ++++++
>  include/hw/compat.h    |  6 +++++-
>  3 files changed, 22 insertions(+), 8 deletions(-)
> 
> diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
> index 755f921..25e7ffa 100644
> --- a/hw/virtio/virtio-pci.c
> +++ b/hw/virtio/virtio-pci.c
> @@ -307,7 +307,7 @@ static int virtio_pci_ioeventfd_assign(DeviceState *d, EventNotifier *notifier,
>      MemoryRegion *modern_mr = &proxy->notify.mr;
>      MemoryRegion *modern_notify_mr = &proxy->notify_pio.mr;
>      MemoryRegion *legacy_mr = &proxy->bar;
> -    hwaddr modern_addr = QEMU_VIRTIO_PCI_QUEUE_MEM_MULT *
> +    hwaddr modern_addr = proxy->queue_mem_mult *
>                           virtio_get_queue_index(vq);
>      hwaddr legacy_addr = VIRTIO_PCI_QUEUE_NOTIFY;
>  
> @@ -1370,7 +1370,8 @@ static void virtio_pci_notify_write(void *opaque, hwaddr addr,
>                                      uint64_t val, unsigned size)
>  {
>      VirtIODevice *vdev = opaque;
> -    unsigned queue = addr / QEMU_VIRTIO_PCI_QUEUE_MEM_MULT;
> +    VirtIOPCIProxy *proxy = VIRTIO_PCI(DEVICE(vdev)->parent_bus->parent);
> +    unsigned queue = addr / proxy->queue_mem_mult;
>  
>      if (queue < VIRTIO_QUEUE_MAX) {
>          virtio_queue_notify(vdev, queue);
> @@ -1609,7 +1610,7 @@ static void virtio_pci_device_plugged(DeviceState *d, Error **errp)
>          struct virtio_pci_notify_cap notify = {
>              .cap.cap_len = sizeof notify,
>              .notify_off_multiplier =
> -                cpu_to_le32(QEMU_VIRTIO_PCI_QUEUE_MEM_MULT),
> +                cpu_to_le32(proxy->queue_mem_mult),
>          };
>          struct virtio_pci_cfg_cap cfg = {
>              .cap.cap_len = sizeof cfg,
> @@ -1717,6 +1718,9 @@ static void virtio_pci_realize(PCIDevice *pci_dev, Error **errp)
>      bool pcie_port = pci_bus_is_express(pci_dev->bus) &&
>                       !pci_bus_is_root(pci_dev->bus);
>  
> +    proxy->queue_mem_mult = (proxy->flags & VIRTIO_PCI_FLAG_PAGE_PER_VQ) ?
> +        QEMU_VIRTIO_PCI_QUEUE_MEM_MULT : 4;
> +
>      /*
>       * virtio pci bar layout used by default.
>       * subclasses can re-arrange things if needed.
> @@ -1744,8 +1748,7 @@ static void virtio_pci_realize(PCIDevice *pci_dev, Error **errp)
>      proxy->device.type = VIRTIO_PCI_CAP_DEVICE_CFG;
>  
>      proxy->notify.offset = 0x3000;
> -    proxy->notify.size =
> -        QEMU_VIRTIO_PCI_QUEUE_MEM_MULT * VIRTIO_QUEUE_MAX;
> +    proxy->notify.size = proxy->queue_mem_mult * VIRTIO_QUEUE_MAX;
>      proxy->notify.type = VIRTIO_PCI_CAP_NOTIFY_CFG;
>  
>      proxy->notify_pio.offset = 0x0;
> @@ -1754,8 +1757,7 @@ static void virtio_pci_realize(PCIDevice *pci_dev, Error **errp)
>  
>      /* subclasses can enforce modern, so do this unconditionally */
>      memory_region_init(&proxy->modern_bar, OBJECT(proxy), "virtio-pci",
> -                       2 * QEMU_VIRTIO_PCI_QUEUE_MEM_MULT *
> -                       VIRTIO_QUEUE_MAX);

Maybe add
	/* PCI BAR regions must be powers of 2 */

> +                       pow2ceil(proxy->notify.offset + proxy->notify.size));
>  
>      memory_region_init_alias(&proxy->modern_cfg,
>                               OBJECT(proxy),
> @@ -1833,6 +1835,8 @@ static Property virtio_pci_properties[] = {
>                      VIRTIO_PCI_FLAG_MODERN_PIO_NOTIFY_BIT, false),
>      DEFINE_PROP_BIT("x-disable-pcie", VirtIOPCIProxy, flags,
>                      VIRTIO_PCI_FLAG_DISABLE_PCIE_BIT, false),
> +    DEFINE_PROP_BIT("page-per-vq", VirtIOPCIProxy, flags,
> +                    VIRTIO_PCI_FLAG_PAGE_PER_VQ_BIT, false),
>      DEFINE_PROP_END_OF_LIST(),
>  };
>  
> diff --git a/hw/virtio/virtio-pci.h b/hw/virtio/virtio-pci.h
> index 25fbf8a..0fd0d37 100644
> --- a/hw/virtio/virtio-pci.h
> +++ b/hw/virtio/virtio-pci.h
> @@ -64,6 +64,7 @@ enum {
>      VIRTIO_PCI_FLAG_MIGRATE_EXTRA_BIT,
>      VIRTIO_PCI_FLAG_MODERN_PIO_NOTIFY_BIT,
>      VIRTIO_PCI_FLAG_DISABLE_PCIE_BIT,
> +    VIRTIO_PCI_FLAG_PAGE_PER_VQ_BIT,
>  };
>  
>  /* Need to activate work-arounds for buggy guests at vmstate load. */
> @@ -84,6 +85,10 @@ enum {
>  #define VIRTIO_PCI_FLAG_MODERN_PIO_NOTIFY \
>      (1 << VIRTIO_PCI_FLAG_MODERN_PIO_NOTIFY_BIT)
>  
> +/* page per vq flag to be used by split drivers within guests */
> +#define VIRTIO_PCI_FLAG_PAGE_PER_VQ \
> +    (1 << VIRTIO_PCI_FLAG_PAGE_PER_VQ_BIT)
> +
>  typedef struct {
>      MSIMessage msg;
>      int virq;
> @@ -138,6 +143,7 @@ struct VirtIOPCIProxy {
>      uint32_t msix_bar;
>      uint32_t modern_io_bar;
>      uint32_t modern_mem_bar;
> +    int queue_mem_mult;
>      int config_cap;
>      uint32_t flags;
>      bool disable_modern;

I would rather use wrapper functions to calculate it,
to avoid duplicating data.
E.g.
static inline int virtio_pci_queue_mem_mult(struct VirtIOPCIProxy *)
{
	return (proxy->flags & VIRTIO_PCI_FLAG_PAGE_PER_VQ) ?
		QEMU_VIRTIO_PCI_QUEUE_MEM_MULT : 4;
}


Otherwise, looks fine.

> diff --git a/include/hw/compat.h b/include/hw/compat.h
> index 08dd4fb..a1d6694 100644
> --- a/include/hw/compat.h
> +++ b/include/hw/compat.h
> @@ -2,7 +2,11 @@
>  #define HW_COMPAT_H
>  
>  #define HW_COMPAT_2_7 \
> -        /* empty */
> +    {\
> +        .driver   = "virtio-pci",\
> +        .property = "page-per-vq",\
> +        .value    = "on",\
> +    },
>  
>  #define HW_COMPAT_2_6 \
>      {\
> -- 
> 2.5.5