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kvm/virtio: Set IFF_VNET_HDR when setting up tap fds

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IFF_VNET_HDR is a tun/tap flag that allows you to send and receive
large (i.e. GSO) packets and packets with partial checksums. Setting
the flag means that every packet is proceeded by the same header which
virtio uses to communicate GSO/csum metadata.

By enabling this flag on the tap fds we create, we greatly increase
the achievable throughput with virtio_net.

However, we need to be careful to only set the flag when a) QEMU has
support for this ABI and b) the value of the flag is queryable using
the TUNGETIFF ioctl.

It's nearly five months since kvm-74 - the first KVM release with this
feature - was released. Up until now, we've not added libvirt support
because there is no clean way to detect support for this in QEMU at
runtime. A brief attempt to add a "info capabilities" monitor command
to QEMU floundered. Perfect is the enemy of good enough. Probing the
KVM version will suffice for now.

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
This commit is contained in:
Mark McLoughlin 2009-01-27 11:12:05 +00:00
parent e15147c141
commit b4f62abbf1
5 changed files with 125 additions and 9 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

32
ChangeLog
View File

@ -1,3 +1,35 @@
Tue Jan 27 10:48:12 IST 2009 Mark McLoughlin <markmc@redhat.com>
kvm/virtio: Set IFF_VNET_HDR when setting up tap fds
IFF_VNET_HDR is a tun/tap flag that allows you to send and receive
large (i.e. GSO) packets and packets with partial checksums. Setting
the flag means that every packet is proceeded by the same header which
virtio uses to communicate GSO/csum metadata.
By enabling this flag on the tap fds we create, we greatly increase
the achievable throughput with virtio_net.
However, we need to be careful to only set the flag when a) QEMU has
support for this ABI and b) the value of the flag is queryable using
the TUNGETIFF ioctl.
It's nearly five months since kvm-74 - the first KVM release with this
feature - was released. Up until now, we've not added libvirt support
because there is no clean way to detect support for this in QEMU at
runtime. A brief attempt to add a "info capabilities" monitor command
to QEMU floundered. Perfect is the enemy of good enough. Probing the
KVM version will suffice for now.
* src/qemu_conf.[ch] (qemudExtractVersionInfo): detect the KVM version,
set QEMUD_CMD_FLAG_VNET_HDR if we have kvm-74 or newer
(qemudBuildCommandLine): if qemu support VNET_HDR and this is a
virtio interface, then use it
* src/bridge.[ch] (brProbeVnetHdr): only enable IFF_VNET_HDR if
the kernel bits are available
(brAddTap): add a @vnet_hdr arg, set IFF_VNET_HDR
Tue Jan 27 11:53:32 +0100 2009 Jim Meyering <meyering@redhat.com>
and Daniel P. Berrange <berrange@redhat.com>

67
src/bridge.c
View File

@ -47,6 +47,7 @@
#include "internal.h"
#include "memory.h"
#include "util.h"
#include "logging.h"
#define MAX_BRIDGE_ID 256
@ -402,11 +403,68 @@ static int brSetInterfaceMtu(brControl *ctl,
return ifSetMtu(ctl, ifname, mtu);
}
/**
* brProbeVnetHdr:
* @tapfd: a tun/tap file descriptor
*
* Check whether it is safe to enable the IFF_VNET_HDR flag on the
* tap interface.
*
* Setting IFF_VNET_HDR enables QEMU's virtio_net driver to allow
* guests to pass larger (GSO) packets, with partial checksums, to
* the host. This greatly increases the achievable throughput.
*
* It is only useful to enable this when we're setting up a virtio
* interface. And it is only *safe* to enable it when we know for
* sure that a) qemu has support for IFF_VNET_HDR and b) the running
* kernel implements the TUNGETIFF ioctl(), which qemu needs to query
* the supplied tapfd.
*
* Returns 0 in case of success or an errno code in case of failure.
*/
static int
brProbeVnetHdr(int tapfd)
{
#if defined(IFF_VNET_HDR) && defined(TUNGETFEATURES) && defined(TUNGETIFF)
unsigned int features;
struct ifreq dummy;
if (ioctl(tapfd, TUNGETFEATURES, &features) != 0) {
VIR_INFO0(_("Not enabling IFF_VNET_HDR; "
"TUNGETFEATURES ioctl() not implemented"));
return 0;
}
if (!(features & IFF_VNET_HDR)) {
VIR_INFO0(_("Not enabling IFF_VNET_HDR; "
"TUNGETFEATURES ioctl() reports no IFF_VNET_HDR"));
return 0;
}
/* The kernel will always return -1 at this point.
* If TUNGETIFF is not implemented then errno == EBADFD.
*/
if (ioctl(tapfd, TUNGETIFF, &dummy) != -1 || errno != EBADFD) {
VIR_INFO0(_("Not enabling IFF_VNET_HDR; "
"TUNGETIFF ioctl() not implemented"));
return 0;
}
VIR_INFO0(_("Enabling IFF_VNET_HDR"));
return 1;
#else
VIR_INFO0(_("Not enabling IFF_VNET_HDR; disabled at build time"));
return 0;
#endif
}
/**
* brAddTap:
* @ctl: bridge control pointer
* @bridge: the bridge name
* @ifname: the interface name (or name template)
* @vnet_hdr: whether to try enabling IFF_VNET_HDR
* @tapfd: file descriptor return value for the new tap device
*
* This function creates a new tap device on a bridge. @ifname can be either
@ -420,6 +478,7 @@ int
brAddTap(brControl *ctl,
const char *bridge,
char **ifname,
int vnet_hdr,
int *tapfd)
{
int id, subst, fd;
@ -435,6 +494,9 @@ brAddTap(brControl *ctl,
if ((fd = open("/dev/net/tun", O_RDWR)) < 0)
return errno;
if (vnet_hdr)
vnet_hdr = brProbeVnetHdr(fd);
do {
struct ifreq try;
int len;
@ -443,6 +505,11 @@ brAddTap(brControl *ctl,
try.ifr_flags = IFF_TAP|IFF_NO_PI;
#ifdef IFF_VNET_HDR
if (vnet_hdr)
try.ifr_flags |= IFF_VNET_HDR;
#endif
if (subst) {
len = snprintf(try.ifr_name, BR_IFNAME_MAXLEN, *ifname, id);
if (len >= BR_IFNAME_MAXLEN) {

1
src/bridge.h
View File

@ -63,6 +63,7 @@ int brDeleteInterface (brControl *ctl,
int brAddTap (brControl *ctl,
const char *bridge,
char **ifname,
int vnet_hdr,
int *tapfd);
int brSetInterfaceUp (brControl *ctl,

33
src/qemu_conf.c
View File

@ -353,7 +353,7 @@ int qemudExtractVersionInfo(const char *qemu,
int newstdout = -1;
int ret = -1, status;
unsigned int major, minor, micro;
unsigned int version;
unsigned int version, kvm_version;
unsigned int flags = 0;
if (retflags)
@ -371,10 +371,13 @@ int qemudExtractVersionInfo(const char *qemu,
if (len < 0)
goto cleanup2;
if (sscanf(help, "QEMU PC emulator version %u.%u.%u",
&major, &minor, &micro) != 3) {
if (sscanf(help, "QEMU PC emulator version %u.%u.%u (kvm-%u)",
&major, &minor, &micro, &kvm_version) != 4)
kvm_version = 0;
if (!kvm_version && sscanf(help, "QEMU PC emulator version %u.%u.%u",
&major, &minor, &micro) != 3)
goto cleanup2;
}
version = (major * 1000 * 1000) + (minor * 1000) + micro;
@ -394,6 +397,8 @@ int qemudExtractVersionInfo(const char *qemu,
flags |= QEMUD_CMD_FLAG_DRIVE_BOOT;
if (version >= 9000)
flags |= QEMUD_CMD_FLAG_VNC_COLON;
if (kvm_version >= 74)
flags |= QEMUD_CMD_FLAG_VNET_HDR;
if (retversion)
*retversion = version;
@ -404,6 +409,8 @@ int qemudExtractVersionInfo(const char *qemu,
qemudDebug("Version %d %d %d Cooked version: %d, with flags ? %d",
major, minor, micro, version, flags);
if (kvm_version)
qemudDebug("KVM version %d detected", kvm_version);
cleanup2:
VIR_FREE(help);
@ -467,7 +474,8 @@ qemudNetworkIfaceConnect(virConnectPtr conn,
int **tapfds,
int *ntapfds,
virDomainNetDefPtr net,
int vlan)
int vlan,
int vnet_hdr)
{
char *brname;
char tapfdstr[4+3+32+7];
@ -517,7 +525,7 @@ qemudNetworkIfaceConnect(virConnectPtr conn,
}
if ((err = brAddTap(driver->brctl, brname,
&net->ifname, &tapfd))) {
&net->ifname, vnet_hdr, &tapfd))) {
if (errno == ENOTSUP) {
/* In this particular case, give a better diagnostic. */
qemudReportError(conn, NULL, NULL, VIR_ERR_INTERNAL_ERROR,
@ -1029,9 +1037,16 @@ int qemudBuildCommandLine(virConnectPtr conn,
case VIR_DOMAIN_NET_TYPE_NETWORK:
case VIR_DOMAIN_NET_TYPE_BRIDGE:
{
char *tap = qemudNetworkIfaceConnect(conn, driver,
tapfds, ntapfds,
net, vlan);
char *tap;
int vnet_hdr = 0;
if (qemuCmdFlags & QEMUD_CMD_FLAG_VNET_HDR &&
net->model && STREQ(net->model, "virtio"))
vnet_hdr = 1;
tap = qemudNetworkIfaceConnect(conn, driver,
tapfds, ntapfds,
net, vlan, vnet_hdr);
if (tap == NULL)
goto error;
ADD_ARG(tap);

1
src/qemu_conf.h
View File

@ -48,6 +48,7 @@ enum qemud_cmd_flags {
QEMUD_CMD_FLAG_NAME = (1 << 5),
QEMUD_CMD_FLAG_UUID = (1 << 6),
QEMUD_CMD_FLAG_DOMID = (1 << 7), /* Xenner only */
QEMUD_CMD_FLAG_VNET_HDR = (1 << 8),
};
/* Main driver state */