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Diffstat (limited to 'tools/virtio/ringtest/ring.c')
-rw-r--r-- | tools/virtio/ringtest/ring.c | 270 |
1 files changed, 270 insertions, 0 deletions
diff --git a/tools/virtio/ringtest/ring.c b/tools/virtio/ringtest/ring.c new file mode 100644 index 0000000000..58e7d33bdd --- /dev/null +++ b/tools/virtio/ringtest/ring.c @@ -0,0 +1,270 @@ +// SPDX-License-Identifier: GPL-2.0-only +/* + * Copyright (C) 2016 Red Hat, Inc. + * Author: Michael S. Tsirkin <mst@redhat.com> + * + * Simple descriptor-based ring. virtio 0.9 compatible event index is used for + * signalling, unconditionally. + */ +#define _GNU_SOURCE +#include "main.h" +#include <stdlib.h> +#include <stdio.h> +#include <string.h> + +/* Next - Where next entry will be written. + * Prev - "Next" value when event triggered previously. + * Event - Peer requested event after writing this entry. + */ +static inline bool need_event(unsigned short event, + unsigned short next, + unsigned short prev) +{ + return (unsigned short)(next - event - 1) < (unsigned short)(next - prev); +} + +/* Design: + * Guest adds descriptors with unique index values and DESC_HW in flags. + * Host overwrites used descriptors with correct len, index, and DESC_HW clear. + * Flags are always set last. + */ +#define DESC_HW 0x1 + +struct desc { + unsigned short flags; + unsigned short index; + unsigned len; + unsigned long long addr; +}; + +/* how much padding is needed to avoid false cache sharing */ +#define HOST_GUEST_PADDING 0x80 + +/* Mostly read */ +struct event { + unsigned short kick_index; + unsigned char reserved0[HOST_GUEST_PADDING - 2]; + unsigned short call_index; + unsigned char reserved1[HOST_GUEST_PADDING - 2]; +}; + +struct data { + void *buf; /* descriptor is writeable, we can't get buf from there */ + void *data; +} *data; + +struct desc *ring; +struct event *event; + +struct guest { + unsigned avail_idx; + unsigned last_used_idx; + unsigned num_free; + unsigned kicked_avail_idx; + unsigned char reserved[HOST_GUEST_PADDING - 12]; +} guest; + +struct host { + /* we do not need to track last avail index + * unless we have more than one in flight. + */ + unsigned used_idx; + unsigned called_used_idx; + unsigned char reserved[HOST_GUEST_PADDING - 4]; +} host; + +/* implemented by ring */ +void alloc_ring(void) +{ + int ret; + int i; + + ret = posix_memalign((void **)&ring, 0x1000, ring_size * sizeof *ring); + if (ret) { + perror("Unable to allocate ring buffer.\n"); + exit(3); + } + event = calloc(1, sizeof(*event)); + if (!event) { + perror("Unable to allocate event buffer.\n"); + exit(3); + } + guest.avail_idx = 0; + guest.kicked_avail_idx = -1; + guest.last_used_idx = 0; + host.used_idx = 0; + host.called_used_idx = -1; + for (i = 0; i < ring_size; ++i) { + struct desc desc = { + .index = i, + }; + ring[i] = desc; + } + guest.num_free = ring_size; + data = calloc(ring_size, sizeof(*data)); + if (!data) { + perror("Unable to allocate data buffer.\n"); + exit(3); + } +} + +/* guest side */ +int add_inbuf(unsigned len, void *buf, void *datap) +{ + unsigned head, index; + + if (!guest.num_free) + return -1; + + guest.num_free--; + head = (ring_size - 1) & (guest.avail_idx++); + + /* Start with a write. On MESI architectures this helps + * avoid a shared state with consumer that is polling this descriptor. + */ + ring[head].addr = (unsigned long)(void*)buf; + ring[head].len = len; + /* read below might bypass write above. That is OK because it's just an + * optimization. If this happens, we will get the cache line in a + * shared state which is unfortunate, but probably not worth it to + * add an explicit full barrier to avoid this. + */ + barrier(); + index = ring[head].index; + data[index].buf = buf; + data[index].data = datap; + /* Barrier A (for pairing) */ + smp_release(); + ring[head].flags = DESC_HW; + + return 0; +} + +void *get_buf(unsigned *lenp, void **bufp) +{ + unsigned head = (ring_size - 1) & guest.last_used_idx; + unsigned index; + void *datap; + + if (ring[head].flags & DESC_HW) + return NULL; + /* Barrier B (for pairing) */ + smp_acquire(); + *lenp = ring[head].len; + index = ring[head].index & (ring_size - 1); + datap = data[index].data; + *bufp = data[index].buf; + data[index].buf = NULL; + data[index].data = NULL; + guest.num_free++; + guest.last_used_idx++; + return datap; +} + +bool used_empty() +{ + unsigned head = (ring_size - 1) & guest.last_used_idx; + + return (ring[head].flags & DESC_HW); +} + +void disable_call() +{ + /* Doing nothing to disable calls might cause + * extra interrupts, but reduces the number of cache misses. + */ +} + +bool enable_call() +{ + event->call_index = guest.last_used_idx; + /* Flush call index write */ + /* Barrier D (for pairing) */ + smp_mb(); + return used_empty(); +} + +void kick_available(void) +{ + bool need; + + /* Flush in previous flags write */ + /* Barrier C (for pairing) */ + smp_mb(); + need = need_event(event->kick_index, + guest.avail_idx, + guest.kicked_avail_idx); + + guest.kicked_avail_idx = guest.avail_idx; + if (need) + kick(); +} + +/* host side */ +void disable_kick() +{ + /* Doing nothing to disable kicks might cause + * extra interrupts, but reduces the number of cache misses. + */ +} + +bool enable_kick() +{ + event->kick_index = host.used_idx; + /* Barrier C (for pairing) */ + smp_mb(); + return avail_empty(); +} + +bool avail_empty() +{ + unsigned head = (ring_size - 1) & host.used_idx; + + return !(ring[head].flags & DESC_HW); +} + +bool use_buf(unsigned *lenp, void **bufp) +{ + unsigned head = (ring_size - 1) & host.used_idx; + + if (!(ring[head].flags & DESC_HW)) + return false; + + /* make sure length read below is not speculated */ + /* Barrier A (for pairing) */ + smp_acquire(); + + /* simple in-order completion: we don't need + * to touch index at all. This also means we + * can just modify the descriptor in-place. + */ + ring[head].len--; + /* Make sure len is valid before flags. + * Note: alternative is to write len and flags in one access - + * possible on 64 bit architectures but wmb is free on Intel anyway + * so I have no way to test whether it's a gain. + */ + /* Barrier B (for pairing) */ + smp_release(); + ring[head].flags = 0; + host.used_idx++; + return true; +} + +void call_used(void) +{ + bool need; + + /* Flush in previous flags write */ + /* Barrier D (for pairing) */ + smp_mb(); + + need = need_event(event->call_index, + host.used_idx, + host.called_used_idx); + + host.called_used_idx = host.used_idx; + + if (need) + call(); +} |