Virtio guests send VIRTIO_MMIO_QUEUE_NOTIFY notification when they need to notify the backend of an update to the status of the virtqueue. The backend or another entity, polls the MMIO address for updates to know when the notification is sent. It works well if the backend does this polling by itself. But as we move towards generic backend implementations, we end up implementing this in a separate user-space program. Generally, the Virtio backends are implemented to work with the Eventfd based mechanism. In order to make such backends work with Xen, another software layer needs to do the polling and send an event via eventfd to the backend once the notification from guest is received. This results in an extra context switch. This is not a new problem in Linux though. It is present with other hypervisors like KVM, etc. as well. The generic solution implemented in the kernel for them is to provide an IOCTL call to pass the address to poll and eventfd, which lets the kernel take care of polling and raise an event on the eventfd, instead of handling this in user space (which involves an extra context switch). This patch adds similar support for xen. Inspired by existing implementations for KVM, etc.. This also copies ioreq.h header file (only struct ioreq and related macros) from Xen's source tree (Top commit 5d84f07fe6bf ("xen/pci: drop remaining uses of bool_t")).
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| @@ -110,6 +110,22 @@ struct privcmd_irqfd { · 109 unmodified lines | |||
| 110 | __u8 pad[2]; | 110 | __u8 pad[2]; |
| 111 | }; | 111 | }; |
| 112 | 112 | ||
| 113 | /* For privcmd_ioeventfd::flags */ | ||
| 114 | #define PRIVCMD_IOEVENTFD_FLAG_DEASSIGN (1 << 0) | ||
| 115 | |||
| 116 | struct privcmd_ioeventfd { | ||
| 117 | __u64 ioreq; | ||
| 118 | __u64 ports; | ||
| 119 | __u64 addr; | ||
| 120 | __u32 addr_len; | ||
| 121 | __u32 event_fd; | ||
| 122 | __u32 vcpus; | ||
| 123 | __u32 vq; | ||
| 124 | __u32 flags; | ||
| 125 | domid_t dom; | ||
| 126 | __u8 pad[2]; | ||
| 127 | }; | ||
| 128 | |||
| 113 | /* | 129 | /* |
| 114 | * @cmd: IOCTL_PRIVCMD_HYPERCALL | 130 | * @cmd: IOCTL_PRIVCMD_HYPERCALL |
| 115 | * @arg: &privcmd_hypercall_t | 131 | * @arg: &privcmd_hypercall_t |
| @@ -139,5 +155,7 @@ struct privcmd_irqfd { · 23 unmodified lines | |||
| 139 | _IOC(_IOC_NONE, 'P', 7, sizeof(struct privcmd_mmap_resource)) | 155 | _IOC(_IOC_NONE, 'P', 7, sizeof(struct privcmd_mmap_resource)) |
| 140 | #define IOCTL_PRIVCMD_IRQFD \ | 156 | #define IOCTL_PRIVCMD_IRQFD \ |
| 141 | _IOW('P', 8, struct privcmd_irqfd) | 157 | _IOW('P', 8, struct privcmd_irqfd) |
| 158 | #define IOCTL_PRIVCMD_IOEVENTFD \ | ||
| 159 | _IOW('P', 9, struct privcmd_ioeventfd) | ||
| 142 | 160 | ||
| 143 | #endif /* __LINUX_PUBLIC_PRIVCMD_H__ */ | 161 | #endif /* __LINUX_PUBLIC_PRIVCMD_H__ */ |