xv6-65oo2/kernel/virtio_disk.c

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//
// driver for qemu's virtio disk device.
// uses qemu's mmio interface to virtio.
// qemu presents a "legacy" virtio interface.
//
// qemu ... -drive file=fs.img,if=none,format=raw,id=x0 -device virtio-blk-device,drive=x0,bus=virtio-mmio-bus.0
//
#include "types.h"
#include "riscv.h"
#include "defs.h"
#include "param.h"
#include "memlayout.h"
#include "spinlock.h"
#include "sleeplock.h"
#include "fs.h"
#include "buf.h"
#include "virtio.h"
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// the address of virtio mmio register r.
#define R(r) ((volatile uint32 *)(VIRTIO0 + (r)))
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static struct disk {
// the virtio driver and device mostly communicate through a set of
// structures in RAM. pages[] allocates that memory. pages[] is a
// global (instead of calls to kalloc()) because it must consist of
// two contiguous pages of page-aligned physical memory.
char pages[2*PGSIZE];
// pages[] is divided into three regions (descriptors, avail, and
// used), as explained in Section 2.6 of the virtio specification
// for the legacy interface.
// https://docs.oasis-open.org/virtio/virtio/v1.1/virtio-v1.1.pdf
// the first region of pages[] is a set (not a ring) of DMA
// descriptors, with which the driver tells the device where to read
// and write individual disk operations. there are NUM descriptors.
// most commands consist of a "chain" (a linked list) of a couple of
// these descriptors.
// points into pages[].
struct virtq_desc *desc;
// next is a ring in which the driver writes descriptor numbers
// that the driver would like the device to process. it only
// includes the head descriptor of each chain. the ring has
// NUM elements.
// points into pages[].
struct virtq_avail *avail;
// finally a ring in which the device writes descriptor numbers that
// the device has finished processing (just the head of each chain).
// there are NUM used ring entries.
// points into pages[].
struct virtq_used *used;
// our own book-keeping.
char free[NUM]; // is a descriptor free?
uint16 used_idx; // we've looked this far in used[2..NUM].
// track info about in-flight operations,
// for use when completion interrupt arrives.
// indexed by first descriptor index of chain.
struct {
struct buf *b;
char status;
} info[NUM];
// disk command headers.
// one-for-one with descriptors, for convenience.
struct virtio_blk_req ops[NUM];
struct spinlock vdisk_lock;
} __attribute__ ((aligned (PGSIZE))) disk;
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void
virtio_disk_init(void)
{
uint32 status = 0;
initlock(&disk.vdisk_lock, "virtio_disk");
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if(*R(VIRTIO_MMIO_MAGIC_VALUE) != 0x74726976 ||
*R(VIRTIO_MMIO_VERSION) != 1 ||
*R(VIRTIO_MMIO_DEVICE_ID) != 2 ||
*R(VIRTIO_MMIO_VENDOR_ID) != 0x554d4551){
panic("could not find virtio disk");
}
status |= VIRTIO_CONFIG_S_ACKNOWLEDGE;
*R(VIRTIO_MMIO_STATUS) = status;
status |= VIRTIO_CONFIG_S_DRIVER;
*R(VIRTIO_MMIO_STATUS) = status;
// negotiate features
uint64 features = *R(VIRTIO_MMIO_DEVICE_FEATURES);
features &= ~(1 << VIRTIO_BLK_F_RO);
features &= ~(1 << VIRTIO_BLK_F_SCSI);
features &= ~(1 << VIRTIO_BLK_F_CONFIG_WCE);
features &= ~(1 << VIRTIO_BLK_F_MQ);
features &= ~(1 << VIRTIO_F_ANY_LAYOUT);
features &= ~(1 << VIRTIO_RING_F_EVENT_IDX);
features &= ~(1 << VIRTIO_RING_F_INDIRECT_DESC);
*R(VIRTIO_MMIO_DRIVER_FEATURES) = features;
// tell device that feature negotiation is complete.
status |= VIRTIO_CONFIG_S_FEATURES_OK;
*R(VIRTIO_MMIO_STATUS) = status;
// tell device we're completely ready.
status |= VIRTIO_CONFIG_S_DRIVER_OK;
*R(VIRTIO_MMIO_STATUS) = status;
*R(VIRTIO_MMIO_GUEST_PAGE_SIZE) = PGSIZE;
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// initialize queue 0.
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*R(VIRTIO_MMIO_QUEUE_SEL) = 0;
uint32 max = *R(VIRTIO_MMIO_QUEUE_NUM_MAX);
if(max == 0)
panic("virtio disk has no queue 0");
if(max < NUM)
panic("virtio disk max queue too short");
*R(VIRTIO_MMIO_QUEUE_NUM) = NUM;
memset(disk.pages, 0, sizeof(disk.pages));
*R(VIRTIO_MMIO_QUEUE_PFN) = ((uint64)disk.pages) >> PGSHIFT;
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// desc = pages -- num * virtq_desc
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// avail = pages + 0x40 -- 2 * uint16, then num * uint16
// used = pages + 4096 -- 2 * uint16, then num * vRingUsedElem
disk.desc = (struct virtq_desc *) disk.pages;
disk.avail = (struct virtq_avail *)(disk.pages + NUM*sizeof(struct virtq_desc));
disk.used = (struct virtq_used *) (disk.pages + PGSIZE);
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// all NUM descriptors start out unused.
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for(int i = 0; i < NUM; i++)
disk.free[i] = 1;
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// plic.c and trap.c arrange for interrupts from VIRTIO0_IRQ.
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}
// find a free descriptor, mark it non-free, return its index.
static int
alloc_desc()
{
for(int i = 0; i < NUM; i++){
if(disk.free[i]){
disk.free[i] = 0;
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return i;
}
}
return -1;
}
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// mark a descriptor as free.
static void
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free_desc(int i)
{
if(i >= NUM)
panic("free_desc 1");
if(disk.free[i])
panic("free_desc 2");
disk.desc[i].addr = 0;
disk.desc[i].len = 0;
disk.desc[i].flags = 0;
disk.desc[i].next = 0;
disk.free[i] = 1;
wakeup(&disk.free[0]);
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}
// free a chain of descriptors.
static void
free_chain(int i)
{
while(1){
int flag = disk.desc[i].flags;
int nxt = disk.desc[i].next;
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free_desc(i);
if(flag & VRING_DESC_F_NEXT)
i = nxt;
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else
break;
}
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}
// allocate three descriptors (they need not be contiguous).
// disk transfers always use three descriptors.
static int
alloc3_desc(int *idx)
{
for(int i = 0; i < 3; i++){
idx[i] = alloc_desc();
if(idx[i] < 0){
for(int j = 0; j < i; j++)
free_desc(idx[j]);
return -1;
}
}
return 0;
}
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void
virtio_disk_rw(struct buf *b, int write)
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{
uint64 sector = b->blockno * (BSIZE / 512);
acquire(&disk.vdisk_lock);
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// the spec's Section 5.2 says that legacy block operations use
// three descriptors: one for type/reserved/sector, one for the
// data, one for a 1-byte status result.
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// allocate the three descriptors.
int idx[3];
while(1){
if(alloc3_desc(idx) == 0) {
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break;
}
sleep(&disk.free[0], &disk.vdisk_lock);
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}
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// format the three descriptors.
// qemu's virtio-blk.c reads them.
struct virtio_blk_req *buf0 = &disk.ops[idx[0]];
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if(write)
buf0->type = VIRTIO_BLK_T_OUT; // write the disk
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else
buf0->type = VIRTIO_BLK_T_IN; // read the disk
buf0->reserved = 0;
buf0->sector = sector;
disk.desc[idx[0]].addr = (uint64) buf0;
disk.desc[idx[0]].len = sizeof(struct virtio_blk_req);
disk.desc[idx[0]].flags = VRING_DESC_F_NEXT;
disk.desc[idx[0]].next = idx[1];
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disk.desc[idx[1]].addr = (uint64) b->data;
disk.desc[idx[1]].len = BSIZE;
if(write)
disk.desc[idx[1]].flags = 0; // device reads b->data
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else
disk.desc[idx[1]].flags = VRING_DESC_F_WRITE; // device writes b->data
disk.desc[idx[1]].flags |= VRING_DESC_F_NEXT;
disk.desc[idx[1]].next = idx[2];
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disk.info[idx[0]].status = 0xff; // device writes 0 on success
disk.desc[idx[2]].addr = (uint64) &disk.info[idx[0]].status;
disk.desc[idx[2]].len = 1;
disk.desc[idx[2]].flags = VRING_DESC_F_WRITE; // device writes the status
disk.desc[idx[2]].next = 0;
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// record struct buf for virtio_disk_intr().
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b->disk = 1;
disk.info[idx[0]].b = b;
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// tell the device the first index in our chain of descriptors.
disk.avail->ring[disk.avail->idx % NUM] = idx[0];
__sync_synchronize();
// tell the device another avail ring entry is available.
disk.avail->idx += 1; // not % NUM ...
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__sync_synchronize();
*R(VIRTIO_MMIO_QUEUE_NOTIFY) = 0; // value is queue number
// Wait for virtio_disk_intr() to say request has finished.
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while(b->disk == 1) {
sleep(b, &disk.vdisk_lock);
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}
disk.info[idx[0]].b = 0;
free_chain(idx[0]);
release(&disk.vdisk_lock);
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}
void
virtio_disk_intr()
{
acquire(&disk.vdisk_lock);
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// the device won't raise another interrupt until we tell it
// we've seen this interrupt, which the following line does.
// this may race with the device writing new entries to
// the "used" ring, in which case we may process the new
// completion entries in this interrupt, and have nothing to do
// in the next interrupt, which is harmless.
*R(VIRTIO_MMIO_INTERRUPT_ACK) = *R(VIRTIO_MMIO_INTERRUPT_STATUS) & 0x3;
__sync_synchronize();
// the device increments disk.used->idx when it
// adds an entry to the used ring.
while(disk.used_idx != disk.used->idx){
__sync_synchronize();
int id = disk.used->ring[disk.used_idx % NUM].id;
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if(disk.info[id].status != 0)
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panic("virtio_disk_intr status");
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struct buf *b = disk.info[id].b;
b->disk = 0; // disk is done with buf
wakeup(b);
disk.used_idx += 1;
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}
release(&disk.vdisk_lock);
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}