Re: [Qemu-devel] [PATCH 3/4] ide: add support for cancelable read requests

[Stefan Hajnoczi <stefanha@gmail.com>]

On Mon, Oct 12, 2015 at 02:27:24PM +0200, Peter Lieven wrote:
> this patch adds a new aio readv compatible function which copies
> all data through a bounce buffer. The benefit is that these requests
> can be flagged as canceled to avoid guest memory corruption when
> a canceled request is completed by the backend at a later stage.
> 
> If an IDE protocol wants to use this function it has to pipe
> all read requests through ide_readv_cancelable and it may then
> enable requests_cancelable in the IDEState.
> 
> If this state is enable we can avoid the blocking blk_drain_all
> in case of a BMDMA reset.
> 
> Currently only read operations are cancelable thus we can only
> use this logic for read-only devices.

Naming is confusing here.  Requests are already "cancelable" using
bdv_aio_cancel().

Please use a different name, for example "orphan" requests.  These are
requests that QEMU still knows about but the guest believes are
complete.  Or maybe "IDEBufferedRequest" since data is transferred
through a bounce buffer.

> Signed-off-by: Peter Lieven <pl@kamp.de>
> ---
>  hw/ide/core.c     | 54 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  hw/ide/internal.h | 16 ++++++++++++++++
>  hw/ide/pci.c      | 42 ++++++++++++++++++++++++++++--------------
>  3 files changed, 98 insertions(+), 14 deletions(-)
> 
> diff --git a/hw/ide/core.c b/hw/ide/core.c
> index 317406d..24547ce 100644
> --- a/hw/ide/core.c
> +++ b/hw/ide/core.c
> @@ -561,6 +561,59 @@ static bool ide_sect_range_ok(IDEState *s,
>      return true;
>  }
>  
> +static void ide_readv_cancelable_cb(void *opaque, int ret)
> +{
> +    IDECancelableRequest *req = opaque;
> +    if (!req->canceled) {
> +        if (!ret) {
> +            qemu_iovec_from_buf(req->org_qiov, 0, req->buf, req->org_qiov->size);
> +        }
> +        req->org_cb(req->org_opaque, ret);
> +    }
> +    QLIST_REMOVE(req, list);
> +    qemu_vfree(req->buf);
> +    qemu_iovec_destroy(&req->qiov);
> +    g_free(req);
> +}
> +
> +#define MAX_CANCELABLE_REQS 16
> +
> +BlockAIOCB *ide_readv_cancelable(IDEState *s, int64_t sector_num,
> +                                 QEMUIOVector *iov, int nb_sectors,
> +                                 BlockCompletionFunc *cb, void *opaque)
> +{
> +    BlockAIOCB *aioreq;
> +    IDECancelableRequest *req;
> +    int c = 0;
> +
> +    QLIST_FOREACH(req, &s->cancelable_requests, list) {
> +        c++;
> +    }
> +    if (c > MAX_CANCELABLE_REQS) {
> +        return NULL;
> +    }

A BH is probably needed here to schedule an cb(-EIO) call since this
function isn't supposed to return NULL if it's a direct replacement for
blk_aio_readv().

> +
> +    req = g_new0(IDECancelableRequest, 1);
> +    qemu_iovec_init(&req->qiov, 1);

It saves a g_new() call if you add a struct iovec field to
IDECancelableRequest and use qemu_iovec_init_external() instead of
qemu_iovec_init().

The qemu_iovec_destroy() calls must be dropped when an external struct
iovec is used.

The qemu_iovec_init_external() call must be moved after the
qemu_blockalign() and struct iovec setup below.

> +    req->buf = qemu_blockalign(blk_bs(s->blk), iov->size);
> +    qemu_iovec_add(&req->qiov, req->buf, iov->size);
> +    req->org_qiov = iov;
> +    req->org_cb = cb;
> +    req->org_opaque = opaque;
> +
> +    aioreq = blk_aio_readv(s->blk, sector_num, &req->qiov, nb_sectors,
> +                           ide_readv_cancelable_cb, req);
> +    if (aioreq == NULL) {
> +        qemu_vfree(req->buf);
> +        qemu_iovec_destroy(&req->qiov);
> +        g_free(req);
> +    } else {
> +        QLIST_INSERT_HEAD(&s->cancelable_requests, req, list);
> +    }
> +
> +    return aioreq;
> +}
> +
>  static void ide_sector_read(IDEState *s);
>  
>  static void ide_sector_read_cb(void *opaque, int ret)
> @@ -805,6 +858,7 @@ void ide_start_dma(IDEState *s, BlockCompletionFunc *cb)
>      s->bus->retry_unit = s->unit;
>      s->bus->retry_sector_num = ide_get_sector(s);
>      s->bus->retry_nsector = s->nsector;
> +    s->bus->s = s;

How is 's' different from 'unit' and 'retry_unit'?

The logic for switching between units is already a little tricky since
the guest can write to the hardware registers while requests are
in-flight.

Please don't duplicate "active unit" state, that increases the risk of
inconsistencies.

Can you use idebus_active_if() to get an equivalent IDEState pointer
without storing s?

>      if (s->bus->dma->ops->start_dma) {
>          s->bus->dma->ops->start_dma(s->bus->dma, s, cb);
>      }
> diff --git a/hw/ide/internal.h b/hw/ide/internal.h
> index 05e93ff..ad188c2 100644
> --- a/hw/ide/internal.h
> +++ b/hw/ide/internal.h
> @@ -343,6 +343,16 @@ enum ide_dma_cmd {
>  #define ide_cmd_is_read(s) \
>  	((s)->dma_cmd == IDE_DMA_READ)
>  
> +typedef struct IDECancelableRequest {
> +    QLIST_ENTRY(IDECancelableRequest) list;
> +    QEMUIOVector qiov;
> +    uint8_t *buf;
> +    QEMUIOVector *org_qiov;
> +    BlockCompletionFunc *org_cb;
> +    void *org_opaque;

Please don't shorten names, original_* is clearer than org_*.

> +    bool canceled;
> +} IDECancelableRequest;
> +
>  /* NOTE: IDEState represents in fact one drive */
>  struct IDEState {
>      IDEBus *bus;
> @@ -396,6 +406,8 @@ struct IDEState {
>      BlockAIOCB *pio_aiocb;
>      struct iovec iov;
>      QEMUIOVector qiov;
> +    QLIST_HEAD(, IDECancelableRequest) cancelable_requests;
> +    bool requests_cancelable;
>      /* ATA DMA state */
>      int32_t io_buffer_offset;
>      int32_t io_buffer_size;
> @@ -468,6 +480,7 @@ struct IDEBus {
>      uint8_t retry_unit;
>      int64_t retry_sector_num;
>      uint32_t retry_nsector;
> +    IDEState *s;
>  };
>  
>  #define TYPE_IDE_DEVICE "ide-device"
> @@ -572,6 +585,9 @@ void ide_set_inactive(IDEState *s, bool more);
>  BlockAIOCB *ide_issue_trim(BlockBackend *blk,
>          int64_t sector_num, QEMUIOVector *qiov, int nb_sectors,
>          BlockCompletionFunc *cb, void *opaque);
> +BlockAIOCB *ide_readv_cancelable(IDEState *s, int64_t sector_num,
> +                                 QEMUIOVector *iov, int nb_sectors,
> +                                 BlockCompletionFunc *cb, void *opaque);
>  
>  /* hw/ide/atapi.c */
>  void ide_atapi_cmd(IDEState *s);
> diff --git a/hw/ide/pci.c b/hw/ide/pci.c
> index d31ff88..5587183 100644
> --- a/hw/ide/pci.c
> +++ b/hw/ide/pci.c
> @@ -240,21 +240,35 @@ void bmdma_cmd_writeb(BMDMAState *bm, uint32_t val)
>      /* Ignore writes to SSBM if it keeps the old value */
>      if ((val & BM_CMD_START) != (bm->cmd & BM_CMD_START)) {
>          if (!(val & BM_CMD_START)) {
> -            /*
> -             * We can't cancel Scatter Gather DMA in the middle of the
> -             * operation or a partial (not full) DMA transfer would reach
> -             * the storage so we wait for completion instead (we beahve
> -             * like if the DMA was completed by the time the guest trying
> -             * to cancel dma with bmdma_cmd_writeb with BM_CMD_START not
> -             * set).
> -             *
> -             * In the future we'll be able to safely cancel the I/O if the
> -             * whole DMA operation will be submitted to disk with a single
> -             * aio operation with preadv/pwritev.
> -             */
>              if (bm->bus->dma->aiocb) {
> -                blk_drain_all();
> -                assert(bm->bus->dma->aiocb == NULL);
> +                if (bm->bus->s && bm->bus->s->requests_cancelable) {
> +                    /*
> +                     * If the used IDE protocol supports request cancelation we
> +                     * can flag requests as canceled here and disable DMA.
> +                     * The IDE protocol used MUST use ide_readv_cancelable for all
> +                     * read operations and then subsequently can enable this code
> +                     * path. Currently this is only supported for read-only
> +                     * devices.
> +                    */
> +                    IDECancelableRequest *req;
> +                    QLIST_FOREACH(req, &bm->bus->s->cancelable_requests, list) {
> +                        if (!req->canceled) {
> +                            req->org_cb(req->org_opaque, -ECANCELED);
> +                        }
> +                        req->canceled = true;
> +                    }
> +                } else {
> +                    /*
> +                     * We can't cancel Scatter Gather DMA in the middle of the
> +                     * operation or a partial (not full) DMA transfer would reach
> +                     * the storage so we wait for completion instead (we beahve
> +                     * like if the DMA was completed by the time the guest trying
> +                     * to cancel dma with bmdma_cmd_writeb with BM_CMD_START not
> +                     * set).
> +                     */
> +                    blk_drain_all();
> +                    assert(bm->bus->dma->aiocb == NULL);

This assertion applies is both branches of the if statement, it could be
moved after the if statement.

> +                }
>              }
>              bm->status &= ~BM_STATUS_DMAING;
>          } else {
> -- 
> 1.9.1
>