diff options
Diffstat (limited to 'drivers/block')
| -rw-r--r-- | drivers/block/lguest_blk.c | 169 |
1 files changed, 157 insertions, 12 deletions
diff --git a/drivers/block/lguest_blk.c b/drivers/block/lguest_blk.c index 5b79d07..93e3c40 100644 --- a/drivers/block/lguest_blk.c +++ b/drivers/block/lguest_blk.c @@ -1,6 +1,12 @@ -/* A simple block driver for lguest. +/*D:400 + * The Guest block driver * - * Copyright 2006 Rusty Russell <rusty@rustcorp.com.au> IBM Corporation + * This is a simple block driver, which appears as /dev/lgba, lgbb, lgbc etc. + * The mechanism is simple: we place the information about the request in the + * device page, then use SEND_DMA (containing the data for a write, or an empty + * "ping" DMA for a read). + :*/ +/* Copyright 2006 Rusty Russell <rusty@rustcorp.com.au> IBM Corporation * * This program is free software; you can redistribute it and/or modify * it under the terms of the GNU General Public License as published by @@ -25,27 +31,50 @@ static char next_block_index = 'a'; +/*D:420 Here is the structure which holds all the information we need about + * each Guest block device. + * + * I'm sure at this stage, you're wondering "hey, where was the adventure I was + * promised?" and thinking "Rusty sucks, I shall say nasty things about him on + * my blog". I think Real adventures have boring bits, too, and you're in the + * middle of one. But it gets better. Just not quite yet. */ struct blockdev { + /* The block queue infrastructure wants a spinlock: it is held while it + * calls our block request function. We grab it in our interrupt + * handler so the responses don't mess with new requests. */ spinlock_t lock; - /* The disk structure for the kernel. */ + /* The disk structure registered with kernel. */ struct gendisk *disk; - /* The major number for this disk. */ + /* The major device number for this disk, and the interrupt. We only + * really keep them here for completeness; we'd need them if we + * supported device unplugging. */ int major; int irq; + /* The physical address of this device's memory page */ unsigned long phys_addr; - /* The mapped block page. */ + /* The mapped memory page for convenient acces. */ struct lguest_block_page *lb_page; - /* We only have a single request outstanding at a time. */ + /* We only have a single request outstanding at a time: this is it. */ struct lguest_dma dma; struct request *req; }; -/* Jens gave me this nice helper to end all chunks of a request. */ +/*D:495 We originally used end_request() throughout the driver, but it turns + * out that end_request() is deprecated, and doesn't actually end the request + * (which seems like a good reason to deprecate it!). It simply ends the first + * bio. So if we had 3 bios in a "struct request" we would do all 3, + * end_request(), do 2, end_request(), do 1 and end_request(): twice as much + * work as we needed to do. + * + * This reinforced to me that I do not understand the block layer. + * + * Nonetheless, Jens Axboe gave me this nice helper to end all chunks of a + * request. This improved disk speed by 130%. */ static void end_entire_request(struct request *req, int uptodate) { if (end_that_request_first(req, uptodate, req->hard_nr_sectors)) @@ -55,30 +84,62 @@ static void end_entire_request(struct request *req, int uptodate) end_that_request_last(req, uptodate); } +/* I'm told there are only two stories in the world worth telling: love and + * hate. So there used to be a love scene here like this: + * + * Launcher: We could make beautiful I/O together, you and I. + * Guest: My, that's a big disk! + * + * Unfortunately, it was just too raunchy for our otherwise-gentle tale. */ + +/*D:490 This is the interrupt handler, called when a block read or write has + * been completed for us. */ static irqreturn_t lgb_irq(int irq, void *_bd) { + /* We handed our "struct blockdev" as the argument to request_irq(), so + * it is passed through to us here. This tells us which device we're + * dealing with in case we have more than one. */ struct blockdev *bd = _bd; unsigned long flags; + /* We weren't doing anything? Strange, but could happen if we shared + * interrupts (we don't!). */ if (!bd->req) { pr_debug("No work!\n"); return IRQ_NONE; } + /* Not done yet? That's equally strange. */ if (!bd->lb_page->result) { pr_debug("No result!\n"); return IRQ_NONE; } + /* We have to grab the lock before ending the request. */ spin_lock_irqsave(&bd->lock, flags); + /* "result" is 1 for success, 2 for failure: end_entire_request() wants + * to know whether this succeeded or not. */ end_entire_request(bd->req, bd->lb_page->result == 1); + /* Clear out request, it's done. */ bd->req = NULL; + /* Reset incoming DMA for next time. */ bd->dma.used_len = 0; + /* Ready for more reads or writes */ blk_start_queue(bd->disk->queue); spin_unlock_irqrestore(&bd->lock, flags); + + /* The interrupt was for us, we dealt with it. */ return IRQ_HANDLED; } +/*D:480 The block layer's "struct request" contains a number of "struct bio"s, + * each of which contains "struct bio_vec"s, each of which contains a page, an + * offset and a length. + * + * Fortunately there are iterators to help us walk through the "struct + * request". Even more fortunately, there were plenty of places to steal the + * code from. We pack the "struct request" into our "struct lguest_dma" and + * return the total length. */ static unsigned int req_to_dma(struct request *req, struct lguest_dma *dma) { unsigned int i = 0, idx, len = 0; @@ -87,8 +148,13 @@ static unsigned int req_to_dma(struct request *req, struct lguest_dma *dma) rq_for_each_bio(bio, req) { struct bio_vec *bvec; bio_for_each_segment(bvec, bio, idx) { + /* We told the block layer not to give us too many. */ BUG_ON(i == LGUEST_MAX_DMA_SECTIONS); + /* If we had a zero-length segment, it would look like + * the end of the data referred to by the "struct + * lguest_dma", so make sure that doesn't happen. */ BUG_ON(!bvec->bv_len); + /* Convert page & offset to a physical address */ dma->addr[i] = page_to_phys(bvec->bv_page) + bvec->bv_offset; dma->len[i] = bvec->bv_len; @@ -96,26 +162,39 @@ static unsigned int req_to_dma(struct request *req, struct lguest_dma *dma) i++; } } + /* If the array isn't full, we mark the end with a 0 length */ if (i < LGUEST_MAX_DMA_SECTIONS) dma->len[i] = 0; return len; } +/* This creates an empty DMA, useful for prodding the Host without sending data + * (ie. when we want to do a read) */ static void empty_dma(struct lguest_dma *dma) { dma->len[0] = 0; } +/*D:470 Setting up a request is fairly easy: */ static void setup_req(struct blockdev *bd, int type, struct request *req, struct lguest_dma *dma) { + /* The type is 1 (write) or 0 (read). */ bd->lb_page->type = type; + /* The sector on disk where the read or write starts. */ bd->lb_page->sector = req->sector; + /* The result is initialized to 0 (unfinished). */ bd->lb_page->result = 0; + /* The current request (so we can end it in the interrupt handler). */ bd->req = req; + /* The number of bytes: returned as a side-effect of req_to_dma(), + * which packs the block layer's "struct request" into our "struct + * lguest_dma" */ bd->lb_page->bytes = req_to_dma(req, dma); } +/*D:450 Write is pretty straightforward: we pack the request into a "struct + * lguest_dma", then use SEND_DMA to send the request. */ static void do_write(struct blockdev *bd, struct request *req) { struct lguest_dma send; @@ -126,6 +205,9 @@ static void do_write(struct blockdev *bd, struct request *req) lguest_send_dma(bd->phys_addr, &send); } +/* Read is similar to write, except we pack the request into our receive + * "struct lguest_dma" and send through an empty DMA just to tell the Host that + * there's a request pending. */ static void do_read(struct blockdev *bd, struct request *req) { struct lguest_dma ping; @@ -137,21 +219,30 @@ static void do_read(struct blockdev *bd, struct request *req) lguest_send_dma(bd->phys_addr, &ping); } +/*D:440 This where requests come in: we get handed the request queue and are + * expected to pull a "struct request" off it until we've finished them or + * we're waiting for a reply: */ static void do_lgb_request(struct request_queue *q) { struct blockdev *bd; struct request *req; again: + /* This sometimes returns NULL even on the very first time around. I + * wonder if it's something to do with letting elves handle the request + * queue... */ req = elv_next_request(q); if (!req) return; + /* We attached the struct blockdev to the disk: get it back */ bd = req->rq_disk->private_data; - /* Sometimes we get repeated requests after blk_stop_queue. */ + /* Sometimes we get repeated requests after blk_stop_queue(), but we + * can only handle one at a time. */ if (bd->req) return; + /* We only do reads and writes: no tricky business! */ if (!blk_fs_request(req)) { pr_debug("Got non-command 0x%08x\n", req->cmd_type); req->errors++; @@ -164,20 +255,31 @@ again: else do_read(bd, req); - /* Wait for interrupt to tell us it's done. */ + /* We've put out the request, so stop any more coming in until we get + * an interrupt, which takes us to lgb_irq() to re-enable the queue. */ blk_stop_queue(q); } +/*D:430 This is the "struct block_device_operations" we attach to the disk at + * the end of lguestblk_probe(). It doesn't seem to want much. */ static struct block_device_operations lguestblk_fops = { .owner = THIS_MODULE, }; +/*D:425 Setting up a disk device seems to involve a lot of code. I'm not sure + * quite why. I do know that the IDE code sent two or three of the maintainers + * insane, perhaps this is the fringe of the same disease? + * + * As in the console code, the probe function gets handed the generic + * lguest_device from lguest_bus.c: */ static int lguestblk_probe(struct lguest_device *lgdev) { struct blockdev *bd; int err; int irqflags = IRQF_SHARED; + /* First we allocate our own "struct blockdev" and initialize the easy + * fields. */ bd = kmalloc(sizeof(*bd), GFP_KERNEL); if (!bd) return -ENOMEM; @@ -187,59 +289,100 @@ static int lguestblk_probe(struct lguest_device *lgdev) bd->req = NULL; bd->dma.used_len = 0; bd->dma.len[0] = 0; + /* The descriptor in the lguest_devices array provided by the Host + * gives the Guest the physical page number of the device's page. */ bd->phys_addr = (lguest_devices[lgdev->index].pfn << PAGE_SHIFT); + /* We use lguest_map() to get a pointer to the device page */ bd->lb_page = lguest_map(bd->phys_addr, 1); if (!bd->lb_page) { err = -ENOMEM; goto out_free_bd; } + /* We need a major device number: 0 means "assign one dynamically". */ bd->major = register_blkdev(0, "lguestblk"); if (bd->major < 0) { err = bd->major; goto out_unmap; } + /* This allocates a "struct gendisk" where we pack all the information + * about the disk which the rest of Linux sees. We ask for one minor + * number; I do wonder if we should be asking for more. */ bd->disk = alloc_disk(1); if (!bd->disk) { err = -ENOMEM; goto out_unregister_blkdev; } + /* Every disk needs a queue for requests to come in: we set up the + * queue with a callback function (the core of our driver) and the lock + * to use. */ bd->disk->queue = blk_init_queue(do_lgb_request, &bd->lock); if (!bd->disk->queue) { err = -ENOMEM; goto out_put_disk; } - /* We can only handle a certain number of sg entries */ + /* We can only handle a certain number of pointers in our SEND_DMA + * call, so we set that with blk_queue_max_hw_segments(). This is not + * to be confused with blk_queue_max_phys_segments() of course! I + * know, who could possibly confuse the two? + * + * Well, it's simple to tell them apart: this one seems to work and the + * other one didn't. */ blk_queue_max_hw_segments(bd->disk->queue, LGUEST_MAX_DMA_SECTIONS); - /* Buffers must not cross page boundaries */ + + /* Due to technical limitations of our Host (and simple coding) we + * can't have a single buffer which crosses a page boundary. Tell it + * here. This means that our maximum request size is 16 + * (LGUEST_MAX_DMA_SECTIONS) pages. */ blk_queue_segment_boundary(bd->disk->queue, PAGE_SIZE-1); + /* We name our disk: this becomes the device name when udev does its + * magic thing and creates the device node, such as /dev/lgba. + * next_block_index is a global which starts at 'a'. Unfortunately + * this simple increment logic means that the 27th disk will be called + * "/dev/lgb{". In that case, I recommend having at least 29 disks, so + * your /dev directory will be balanced. */ sprintf(bd->disk->disk_name, "lgb%c", next_block_index++); + + /* We look to the device descriptor again to see if this device's + * interrupts are expected to be random. If they are, we tell the irq + * subsystem. At the moment this bit is always set. */ if (lguest_devices[lgdev->index].features & LGUEST_DEVICE_F_RANDOMNESS) irqflags |= IRQF_SAMPLE_RANDOM; + + /* Now we have the name and irqflags, we can request the interrupt; we + * give it the "struct blockdev" we have set up to pass to lgb_irq() + * when there is an interrupt. */ err = request_irq(bd->irq, lgb_irq, irqflags, bd->disk->disk_name, bd); if (err) goto out_cleanup_queue; + /* We bind our one-entry DMA pool to the key for this block device so + * the Host can reply to our requests. The key is equal to the + * physical address of the device's page, which is conveniently + * unique. */ err = lguest_bind_dma(bd->phys_addr, &bd->dma, 1, bd->irq); if (err) goto out_free_irq; + /* We finish our disk initialization and add the disk to the system. */ bd->disk->major = bd->major; bd->disk->first_minor = 0; bd->disk->private_data = bd; bd->disk->fops = &lguestblk_fops; - /* This is initialized to the disk size by the other end. */ + /* This is initialized to the disk size by the Launcher. */ set_capacity(bd->disk, bd->lb_page->num_sectors); add_disk(bd->disk); printk(KERN_INFO "%s: device %i at major %d\n", bd->disk->disk_name, lgdev->index, bd->major); + /* We don't need to keep the "struct blockdev" around, but if we ever + * implemented device removal, we'd need this. */ lgdev->private = bd; return 0; @@ -258,6 +401,8 @@ out_free_bd: return err; } +/*D:410 The boilerplate code for registering the lguest block driver is just + * like the console: */ static struct lguest_driver lguestblk_drv = { .name = "lguestblk", .owner = THIS_MODULE, |
