From 93de2a6a4b91b72607136dd656edf03fb399d27f Mon Sep 17 00:00:00 2001 From: Sean Christopherson Date: Wed, 23 Sep 2026 09:37:21 -0700 Subject: KVM: SEV: Do cache maintenance on the source VM during intra-host migration Manually perform cache maintenance on the source VM during intra-host migration to ensure no stale data is left in CPU caches after the VM is destroyed. Because the source VM is "converted" to a non-SEV VM, KVM's memory reclaim flows won't trigger cache maintenance, e.g. when all guest memory is reclaimed in response to detaching from the mmu_notifier. Note, relying on the destination VM to do cache maintenance isn't an option as KVM doesn't require identical guest memory configurations, i.e. the source VM may have access to memory that the destination VM does not. Enforcing equivalent memory configurations is infeasible, as it would require a *deep* comparison of memslots, e.g. to verify that not only are the memslot identical, but what the memslots point at is also identical. Fixes: b56639318bb2 ("KVM: SEV: Add support for SEV intra host migration") Cc: stable@vger.kernel.org Reported-by: Stefan Teodorescu Signed-off-by: Sean Christopherson Message-ID: <20260923163721.1584779-3-seanjc@google.com> Signed-off-by: Paolo Bonzini --- Documentation/networking/driver.rst | 143 ++++++++++++++++++++++++++++++++++++ 1 file changed, 143 insertions(+) create mode 100644 Documentation/networking/driver.rst (limited to 'Documentation/networking/driver.rst') diff --git a/Documentation/networking/driver.rst b/Documentation/networking/driver.rst new file mode 100644 index 000000000..920c0ec98 --- /dev/null +++ b/Documentation/networking/driver.rst @@ -0,0 +1,143 @@ +.. SPDX-License-Identifier: GPL-2.0 + +===================== +Softnet Driver Issues +===================== + +Probing guidelines +================== + +Address validation +------------------ + +Any hardware layer address you obtain for your device should +be verified. For example, for ethernet check it with +linux/etherdevice.h:is_valid_ether_addr() + +Close/stop guidelines +===================== + +Quiescence +---------- + +After the ndo_stop routine has been called, the hardware must +not receive or transmit any data. All in flight packets must +be aborted. If necessary, poll or wait for completion of +any reset commands. + +Auto-close +---------- + +The ndo_stop routine will be called by unregister_netdevice +if device is still UP. + +Transmit path guidelines +======================== + +Stop queues in advance +---------------------- + +The ndo_start_xmit method must not return NETDEV_TX_BUSY under +any normal circumstances. It is considered a hard error unless +there is no way your device can tell ahead of time when its +transmit function will become busy. + +Instead it must maintain the queue properly. For example, +for a driver implementing scatter-gather this means: + +.. code-block:: c + + static u32 drv_tx_avail(struct drv_ring *dr) + { + u32 used = READ_ONCE(dr->prod) - READ_ONCE(dr->cons); + + return dr->tx_ring_size - (used & dr->tx_ring_mask); + } + + static netdev_tx_t drv_hard_start_xmit(struct sk_buff *skb, + struct net_device *dev) + { + struct drv *dp = netdev_priv(dev); + struct netdev_queue *txq; + struct drv_ring *dr; + int idx; + + idx = skb_get_queue_mapping(skb); + dr = dp->tx_rings[idx]; + txq = netdev_get_tx_queue(dev, idx); + + //... + /* This should be a very rare race - log it. */ + if (drv_tx_avail(dr) <= skb_shinfo(skb)->nr_frags + 1) { + netif_tx_stop_queue(txq); + netdev_warn(dev, "Tx Ring full when queue awake!\n"); + return NETDEV_TX_BUSY; + } + + //... queue packet to card ... + + netdev_tx_sent_queue(txq, skb->len); + + //... update tx producer index using WRITE_ONCE() ... + + if (!netif_txq_maybe_stop(txq, drv_tx_avail(dr), + MAX_SKB_FRAGS + 1, 2 * MAX_SKB_FRAGS)) + dr->stats.stopped++; + + //... + return NETDEV_TX_OK; + } + +And then at the end of your TX reclamation event handling: + +.. code-block:: c + + //... update tx consumer index using WRITE_ONCE() ... + + netif_txq_completed_wake(txq, cmpl_pkts, cmpl_bytes, + drv_tx_avail(dr), 2 * MAX_SKB_FRAGS); + +Lockless queue stop / wake helper macros +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +.. kernel-doc:: include/net/netdev_queues.h + :doc: Lockless queue stopping / waking helpers. + +The standard macros like netif_txq_maybe_stop(), netif_txq_try_stop() etc. +are well tested, prefer them over local synchronization schemes. + +No exclusive ownership +---------------------- + +An ndo_start_xmit method must not modify the shared parts of a +cloned SKB. + +Timely completions +------------------ + +Do not forget that once you return NETDEV_TX_OK from your +ndo_start_xmit method, it is your driver's responsibility to free +up the SKB and in some finite amount of time. + +For example, this means that it is not allowed for your TX +mitigation scheme to let TX packets "hang out" in the TX +ring unreclaimed forever if no new TX packets are sent. +This error can deadlock sockets waiting for send buffer room +to be freed up. + +If you return NETDEV_TX_BUSY from the ndo_start_xmit method, you +must not keep any reference to that SKB and you must not attempt +to free it up. + +Error message reporting +======================= + +A number of driver configuration interfaces pass a Netlink extended ACK +(``extack``) object to the driver (either directly as an argument or +as a member of a parameter struct). The drivers should try to report +most errors via the ``extack`` object. System level exceptions, +indicating that system or device is misbehaving or is in bad state, +should continue to be reported to system logs. + +Messages should be passed **either** via ``extack`` **or** to system logs. +Drivers should not try to report the same information to both. -- cgit v1.3.1