Re: [Xen-devel] [PATCH v2 2/2] block/xen-blkfront: Handle non-indirect grant with 64KB pages

[Roger Pau Monnà <roger.pau@xxxxxxxxxx>]

El 19/10/15 a les 16.19, Julien Grall ha escrit:
> The minimal size of request in the block framework is always PAGE_SIZE.
> It means that when 64KB guest is support, the request will at least be
> 64KB.
> 
> Although, if the backend doesn't support indirect descriptor (such as QDISK
> in QEMU), a ring request is only able to accommodate 11 segments of 4KB
> (i.e 44KB).
> 
> The current frontend is assuming that an I/O request will always fit in
> a ring request. This is not true any more when using 64KB page
> granularity and will therefore crash during boot.
> 
> On ARM64, the ABI is completely neutral to the page granularity used by
> the domU. The guest has the choice between different page granularity
> supported by the processors (for instance on ARM64: 4KB, 16KB, 64KB).
> This can't be enforced by the hypervisor and therefore it's possible to
> run guests using different page granularity.
> 
> So we can't mandate the block backend to support indirect descriptor
> when the frontend is using 64KB page granularity and have to fix it
> properly in the frontend.
> 
> The solution exposed below is based on modifying directly the frontend
> guest rather than asking the block framework to support smaller size
> (i.e < PAGE_SIZE). This is because the change is the block framework are
> not trivial as everything seems to relying on a struct *page (see [1]).
> Although, it may be possible that someone succeed to do it in the future
> and we would therefore be able to use it.
> 
> Given that a block request may not fit in a single ring request, a
> second request is introduced for the data that cannot fit in the first
> one. This means that the second ring request should never be used on
> Linux if the page size is smaller than 44KB.
> 
> To achieve the support of the extra ring request, the block queue size
> is divided by two. Therefore, the ring will always contain enough space
> to accommodate 2 ring requests. While this will reduce the overall
> performance, it will make the implementation more contained. The way
> forward to get better performance is to implement in the backend either
> indirect descriptor or multiple grants ring.
> 
> Note that the parameters blk_queue_max_* helpers haven't been updated.
> The block code will set the mimimum size supported and we may be able
> to support directly any change in the block framework that lower down
> the minimal size of a request.
> 
> [1] http://lists.xen.org/archives/html/xen-devel/2015-08/msg02200.html
> 
> Signed-off-by: Julien Grall <julien.grall@xxxxxxxxxx>

LGTM, only a couple of typos and a simplification:

Signed-off-by: Roger Pau Monnà <roger.pau@xxxxxxxxxx>

> ---
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> Cc: "Roger Pau MonnÃ" <roger.pau@xxxxxxxxxx>
> Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> Cc: David Vrabel <david.vrabel@xxxxxxxxxx>
> 
>     Changes in v2:
>         - Update the commit message
>         - Typoes
>         - Rename ring_req2/id2 to extra_ring_req/extra_id
> ---
>  drivers/block/xen-blkfront.c | 200 
> +++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 184 insertions(+), 16 deletions(-)
> 
> diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
> index 5982768..8ea2c97 100644
> --- a/drivers/block/xen-blkfront.c
> +++ b/drivers/block/xen-blkfront.c
> @@ -60,6 +60,20 @@
>  
>  #include <asm/xen/hypervisor.h>
>  
> +/*
> + * The minimal size of segment supportd by the block framework is PAGE_SIZE.
                                  ^supported
> + * When Linux is using a different page size than xen, it may not be possible
> + * to put all the data in a single segment.
> + * This can happen when the backend doesn't support indirect descriptor and
> + * therefore the maximum amount of data that a request can carry is
> + * BLKIF_MAX_SEGMENTS_PER_REQUEST * XEN_PAGE_SIZE = 44KB
> + *
> + * Note that we only support one extra request. So the Linux page size
> + * should be <= ( 2 * BLKIF_MAX_SEGMENTS_PER_REQUEST * XEN_PAGE_SIZE) =
> + * 88KB.
> + */
> +#define HAS_EXTRA_REQ (BLKIF_MAX_SEGMENTS_PER_REQUEST < XEN_PFN_PER_PAGE)
> +
>  enum blkif_state {
>       BLKIF_STATE_DISCONNECTED,
>       BLKIF_STATE_CONNECTED,
> @@ -79,6 +93,18 @@ struct blk_shadow {
>       struct grant **indirect_grants;
>       struct scatterlist *sg;
>       unsigned int num_sg;
> +     enum {
> +             REQ_WAITING,
> +             REQ_DONE,
> +             REQ_FAIL
> +     } status;
> +
> +     #define NO_ASSOCIATED_ID ~0UL
> +     /*
> +      * Id of the sibling if we ever need 2 requests when handling a
> +      * block I/O request
> +      */
> +     unsigned long associated_id;
>  };
>  
>  struct split_bio {
> @@ -467,6 +493,8 @@ static unsigned long blkif_ring_get_request(struct 
> blkfront_info *info,
>  
>       id = get_id_from_freelist(info);
>       info->shadow[id].request = req;
> +     info->shadow[id].status = REQ_WAITING;
> +     info->shadow[id].associated_id = NO_ASSOCIATED_ID;
>  
>       (*ring_req)->u.rw.id = id;
>  
> @@ -508,6 +536,9 @@ struct setup_rw_req {
>       bool need_copy;
>       unsigned int bvec_off;
>       char *bvec_data;
> +
> +     bool require_extra_req;
> +     struct blkif_request *extra_ring_req;
>  };
>  
>  static void blkif_setup_rw_req_grant(unsigned long gfn, unsigned int offset,
> @@ -521,8 +552,24 @@ static void blkif_setup_rw_req_grant(unsigned long gfn, 
> unsigned int offset,
>       unsigned int grant_idx = setup->grant_idx;
>       struct blkif_request *ring_req = setup->ring_req;
>       struct blkfront_info *info = setup->info;
> +     /*
> +      * We always use the shadow of the first request to store the list
> +      * of grant associated to the block I/O request. This made the
> +      * completion more easy to handle even if the block I/O request is
> +      * split.
> +      */
>       struct blk_shadow *shadow = &info->shadow[setup->id];
>  
> +     if (unlikely(setup->require_extra_req &&
> +                  grant_idx >= BLKIF_MAX_SEGMENTS_PER_REQUEST)) {
> +             /*
> +              * We are using the second request, setup grant_idx
> +              * to be the index of the segment array
> +              */
> +             grant_idx -= BLKIF_MAX_SEGMENTS_PER_REQUEST;
> +             ring_req = setup->extra_ring_req;
> +     }
> +
>       if ((ring_req->operation == BLKIF_OP_INDIRECT) &&
>           (grant_idx % GRANTS_PER_INDIRECT_FRAME == 0)) {
>               if (setup->segments)
> @@ -537,7 +584,11 @@ static void blkif_setup_rw_req_grant(unsigned long gfn, 
> unsigned int offset,
>  
>       gnt_list_entry = get_grant(&setup->gref_head, gfn, info);
>       ref = gnt_list_entry->gref;
> -     shadow->grants_used[grant_idx] = gnt_list_entry;
> +     /*
> +      * All the grants are stored in the shadow of the first
> +      * request. Therefore we have to use the global index
> +      */
> +     shadow->grants_used[setup->grant_idx] = gnt_list_entry;
>  
>       if (setup->need_copy) {
>               void *shared_data;
> @@ -579,11 +630,31 @@ static void blkif_setup_rw_req_grant(unsigned long gfn, 
> unsigned int offset,
>       (setup->grant_idx)++;
>  }
>  
> +static void blkif_setup_extra_req(struct blkif_request *first,
> +                               struct blkif_request *second)
> +{
> +     uint16_t nr_segments = first->u.rw.nr_segments;
> +
> +     /*
> +      * The second request is only present when the first request uses
> +      * all its segments. It's always the continuity of the first one.
> +      */
> +     first->u.rw.nr_segments = BLKIF_MAX_SEGMENTS_PER_REQUEST;
> +
> +     second->u.rw.nr_segments = nr_segments - BLKIF_MAX_SEGMENTS_PER_REQUEST;
> +     second->u.rw.sector_number = first->u.rw.sector_number +
> +             (BLKIF_MAX_SEGMENTS_PER_REQUEST * XEN_PAGE_SIZE) / 512;
> +
> +     second->u.rw.handle = first->u.rw.handle;
> +     second->operation = first->operation;
> +}
> +
>  static int blkif_queue_rw_req(struct request *req)
>  {
>       struct blkfront_info *info = req->rq_disk->private_data;
> -     struct blkif_request *ring_req;
> -     unsigned long id;
> +     struct blkif_request *ring_req, *extra_ring_req = NULL;
> +     unsigned long id, extra_id = NO_ASSOCIATED_ID;
> +     bool require_extra_req = false;
>       int i;
>       struct setup_rw_req setup = {
>               .grant_idx = 0,
> @@ -628,19 +699,19 @@ static int blkif_queue_rw_req(struct request *req)
>       /* Fill out a communications ring structure. */
>       id = blkif_ring_get_request(info, req, &ring_req);
>  
> -     BUG_ON(info->max_indirect_segments == 0 &&
> -            GREFS(req->nr_phys_segments) > BLKIF_MAX_SEGMENTS_PER_REQUEST);
> -     BUG_ON(info->max_indirect_segments &&
> -            GREFS(req->nr_phys_segments) > info->max_indirect_segments);
> -
>       num_sg = blk_rq_map_sg(req->q, req, info->shadow[id].sg);
>       num_grant = 0;
>       /* Calculate the number of grant used */
>       for_each_sg(info->shadow[id].sg, sg, num_sg, i)
>              num_grant += gnttab_count_grant(sg->offset, sg->length);
>  
> +     require_extra_req = info->max_indirect_segments == 0 &&
> +             num_grant > BLKIF_MAX_SEGMENTS_PER_REQUEST;
> +     BUG_ON(!HAS_EXTRA_REQ && require_extra_req);
> +
>       info->shadow[id].num_sg = num_sg;
> -     if (num_grant > BLKIF_MAX_SEGMENTS_PER_REQUEST) {
> +     if (num_grant > BLKIF_MAX_SEGMENTS_PER_REQUEST &&
> +         likely(!require_extra_req)) {
>               /*
>                * The indirect operation can only be a BLKIF_OP_READ or
>                * BLKIF_OP_WRITE
> @@ -680,10 +751,30 @@ static int blkif_queue_rw_req(struct request *req)
>                       }
>               }
>               ring_req->u.rw.nr_segments = num_grant;
> +             if (unlikely(require_extra_req)) {
> +                     extra_id = blkif_ring_get_request(info, req,
> +                                                       &extra_ring_req);
> +                     /*
> +                      * Only the first request contains the scatter-gather
> +                      * list.
> +                      */
> +                     info->shadow[extra_id].num_sg = 0;
> +
> +                     blkif_setup_extra_req(ring_req, extra_ring_req);
> +
> +                     /* Link the 2 requests together */
> +                     info->shadow[extra_id].associated_id = id;
> +                     info->shadow[id].associated_id = extra_id;
> +             }
>       }
>  
>       setup.ring_req = ring_req;
>       setup.id = id;
> +
> +     setup.require_extra_req = require_extra_req;
> +     if (unlikely(require_extra_req))
> +             setup.extra_ring_req = extra_ring_req;
> +
>       for_each_sg(info->shadow[id].sg, sg, num_sg, i) {
>               BUG_ON(sg->offset + sg->length > PAGE_SIZE);
>  
> @@ -706,6 +797,8 @@ static int blkif_queue_rw_req(struct request *req)
>  
>       /* Keep a private copy so we can reissue requests when recovering. */
>       info->shadow[id].req = *ring_req;
> +     if (unlikely(require_extra_req))
> +             info->shadow[extra_id].req = *extra_ring_req;
>  
>       if (new_persistent_gnts)
>               gnttab_free_grant_references(setup.gref_head);
> @@ -797,7 +890,16 @@ static int xlvbd_init_blk_queue(struct gendisk *gd, u16 
> sector_size,
>       memset(&info->tag_set, 0, sizeof(info->tag_set));
>       info->tag_set.ops = &blkfront_mq_ops;
>       info->tag_set.nr_hw_queues = 1;
> -     info->tag_set.queue_depth =  BLK_RING_SIZE(info);
> +     if (HAS_EXTRA_REQ && info->max_indirect_segments == 0) {
> +             /*
> +              * When indirect descriptior is not supported, the I/O request
                                 ^descriptors are
> +              * will be split between multiple request in the ring.
> +              * To avoid problems when sending the request, divide by
> +              * 2 the depth of the queue.
> +              */
> +             info->tag_set.queue_depth =  BLK_RING_SIZE(info) / 2;
> +     } else
> +             info->tag_set.queue_depth = BLK_RING_SIZE(info);
>       info->tag_set.numa_node = NUMA_NO_NODE;
>       info->tag_set.flags = BLK_MQ_F_SHOULD_MERGE | BLK_MQ_F_SG_MERGE;
>       info->tag_set.cmd_size = 0;
> @@ -1217,19 +1319,67 @@ static void blkif_copy_from_grant(unsigned long gfn, 
> unsigned int offset,
>       kunmap_atomic(shared_data);
>  }
>  
> -static void blkif_completion(struct blk_shadow *s, struct blkfront_info 
> *info,
> +static bool blkif_completion(unsigned long *id, struct blkfront_info *info,
>                            struct blkif_response *bret)
>  {
>       int i = 0;
>       struct scatterlist *sg;
>       int num_sg, num_grant;
> +     struct blk_shadow *s = &info->shadow[*id];
>       struct copy_from_grant data = {
> -             .s = s,
>               .grant_idx = 0,
>       };
>  
>       num_grant = s->req.operation == BLKIF_OP_INDIRECT ?
>               s->req.u.indirect.nr_segments : s->req.u.rw.nr_segments;
> +
> +     /* The I/O request may be split in two */
> +     if (unlikely(s->associated_id != NO_ASSOCIATED_ID)) {
> +             struct blk_shadow *s2 = &info->shadow[s->associated_id];
> +
> +             /* Keep the status of the current response in shadow */
> +             s->status = (bret->status == BLKIF_RSP_OKAY) ?
> +                     REQ_DONE : REQ_FAIL;
> +
> +             /* Wait the second response if not yet here */
> +             if (s2->status == REQ_WAITING)
> +                     return 0;
> +
> +             /*
> +              * The status of the current response will be used in
> +              * order to know if the request has failed.
> +              * Update the current response status only if has not
> +              * failed.
> +              */
> +             if (bret->status == BLKIF_RSP_OKAY && s2->status == REQ_FAIL)

This could be simplified by only checking if s2->status == REQ_FAIL.

> +                     bret->status = BLKIF_RSP_ERROR;
> +
> +             /*
> +              * All the grants is stored in the first shadow in order
> +              * to make the completion code simpler.
> +              */
> +             num_grant += s2->req.u.rw.nr_segments;
> +
> +             /*
> +              * The two responses may not come in order. Only the
> +              * first request will store the scatter-gather list
> +              */
> +             if (s2->num_sg != 0) {
> +                     /* Update "id" with the ID of the first response */
> +                     *id = s->associated_id;
> +                     s = s2;
> +             }
> +
> +             /*
> +              * We don't need anymore the second request, so recycling
> +              * it now.
> +              */
> +             if (add_id_to_freelist(info, s->associated_id))
> +                     WARN(1, "%s: can't recycle the second part (id = %ld) 
> of the request\n",
> +                          info->gd->disk_name, s->associated_id);
> +     }
> +
> +     data.s = s;
>       num_sg = s->num_sg;
>  
>       if (bret->operation == BLKIF_OP_READ && info->feature_persistent) {
> @@ -1299,6 +1449,8 @@ static void blkif_completion(struct blk_shadow *s, 
> struct blkfront_info *info,
>                       }
>               }
>       }
> +
> +     return 1;
>  }
>  
>  static irqreturn_t blkif_interrupt(int irq, void *dev_id)
> @@ -1339,8 +1491,14 @@ static irqreturn_t blkif_interrupt(int irq, void 
> *dev_id)
>               }
>               req  = info->shadow[id].request;
>  
> -             if (bret->operation != BLKIF_OP_DISCARD)
> -                     blkif_completion(&info->shadow[id], info, bret);
> +             if (bret->operation != BLKIF_OP_DISCARD) {
> +                     /*
> +                      * We may need to wait for an extra response if the
> +                      * I/O request is split in 2
> +                      */
> +                     if (!blkif_completion(&id, info, bret))
> +                             continue;
> +             }
>  
>               if (add_id_to_freelist(info, id)) {
>                       WARN(1, "%s: response to %s (id %ld) couldn't be 
> recycled!\n",
> @@ -1863,8 +2021,18 @@ static int blkfront_setup_indirect(struct 
> blkfront_info *info)
>       unsigned int psegs, grants;
>       int err, i;
>  
> -     if (info->max_indirect_segments == 0)
> -             grants = BLKIF_MAX_SEGMENTS_PER_REQUEST;
> +     if (info->max_indirect_segments == 0) {
> +             if (!HAS_EXTRA_REQ)
> +                     grants = BLKIF_MAX_SEGMENTS_PER_REQUEST;
> +             else {
> +                     /*
> +                      * When an extra req is required, the maximum
> +                      * grants supported is related to the size of the
> +                      * Linux block segment
> +                      */
> +                     grants = GRANTS_PER_PSEG;
> +             }
> +     }
>       else
>               grants = info->max_indirect_segments;
>       psegs = grants / GRANTS_PER_PSEG;
> 


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