Re: [PATCH RFC 17/18] OvmfPkg/XenPvBlkDxe: Add BlockFront client.

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Wed, Jul 16, 2014 at 04:15:46PM +0100, Anthony PERARD wrote:
> This is the code that will do the actual communication between OVMF and
> a PV block backend, where the block device lives.
> 
> This implementation originally comes from Mini-OS, a part of the Xen
> Project.
> 
> Signed-off-by: Samuel Thibault <samuel.thibault@eu.citrix.com>
> Contributed-under: TianoCore Contribution Agreement 1.0
> Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
> ---
>  OvmfPkg/XenPvBlkDxe/BlockFront.c    | 583 ++++++++++++++++++++++++++++++++++++
>  OvmfPkg/XenPvBlkDxe/BlockFront.h    |  88 ++++++
>  OvmfPkg/XenPvBlkDxe/XenPvBlkDxe.c   |   8 +
>  OvmfPkg/XenPvBlkDxe/XenPvBlkDxe.inf |   2 +
>  4 files changed, 681 insertions(+)
>  create mode 100644 OvmfPkg/XenPvBlkDxe/BlockFront.c
>  create mode 100644 OvmfPkg/XenPvBlkDxe/BlockFront.h
> 
> diff --git a/OvmfPkg/XenPvBlkDxe/BlockFront.c b/OvmfPkg/XenPvBlkDxe/BlockFront.c
> new file mode 100644
> index 0000000..ff5641c
> --- /dev/null
> +++ b/OvmfPkg/XenPvBlkDxe/BlockFront.c
> @@ -0,0 +1,583 @@
> +/* Minimal block driver for Mini-OS.
> + * Copyright (c) 2007-2008 Samuel Thibault.
> + * Based on netfront.c.
> + */
> +
> +#include <Library/PrintLib.h>
> +#include <Library/DebugLib.h>
> +
> +#include "BlockFront.h"
> +
> +#include <IndustryStandard/Xen/io/protocols.h>
> +#include <IndustryStandard/Xen/io/xenbus.h>
> +
> +#include "inttypes.h"
> +
> +STATIC
> +XENSTORE_STATUS
> +XenbusReadInteger (
> +  IN  XENBUS_PROTOCOL *This,
> +  IN  CONST CHAR8 *Node,
> +  IN  BOOLEAN FromBackend,
> +  OUT UINT64 *ValuePtr
> +  )
> +{
> +  XENSTORE_STATUS Status;
> +  CHAR8 *p;
> +
> +  if (!FromBackend) {
> +    Status = This->XsRead (This, XST_NIL, Node, (VOID**)&p);
> +  } else {
> +    Status = This->XsBackendRead (This, XST_NIL, Node, (VOID**)&p);
> +  }
> +  if (Status != XENSTORE_STATUS_SUCCESS) {
> +    return Status;
> +  }
> +  // XXX Will assert if p overflow UINT64 ...
> +  *ValuePtr = AsciiStrDecimalToUint64 (p);
> +  FreePool (p);
> +  return Status;
> +}
> +
> +STATIC
> +VOID
> +XenPvBlockFree (
> +  IN XEN_BLOCK_FRONT_DEVICE *Dev
> +  )
> +{
> +  XENBUS_PROTOCOL *XenbusIo = Dev->XenbusIo;
> +
> +  if (Dev->RingRef != 0) {
> +    XenbusIo->GrantEndAccess (XenbusIo, Dev->RingRef);
> +  }
> +  if (Dev->Ring.sring != NULL) {
> +    FreePages (Dev->Ring.sring, 1);
> +  }
> +  if (Dev->EventChannel != 0) {
> +    XenbusIo->EventChannelClose (XenbusIo, Dev->EventChannel);
> +  }
> +  FreePool (Dev);
> +}
> +
> +XENSTORE_STATUS
> +XenPvBlkWaitForBackendState (
> +  IN  XEN_BLOCK_FRONT_DEVICE *Dev,
> +  IN  XenbusState            WaitedState,

ExpectedState

?

> +  OUT XenbusState            *LastStatePtr OPTIONAL
> +  )
> +{
> +  XENBUS_PROTOCOL *XenbusIo = Dev->XenbusIo;
> +  XenbusState State;
> +  UINT64 Value;
> +  XENSTORE_STATUS Status;
> +
> +  while (TRUE) {
> +    Status = XenbusReadInteger (XenbusIo, "state", TRUE, &Value);
> +    if (Status != XENSTORE_STATUS_SUCCESS) {
> +      return Status;
> +    }
> +    if (Value > XenbusStateReconfigured) {
> +      return XENSTORE_STATUS_EIO;
> +    }
> +    State = Value;
> +    if (State >= WaitedState) {

Not State != WaitedState ?

> +      break;
> +    }
> +    DEBUG ((EFI_D_INFO,
> +            "XenPvBlk: waiting backend state %d, current: %d\n",
> +            WaitedState, State));
> +    XenbusIo->WaitForWatch (XenbusIo, Dev->StateWatchToken);
> +  }
> +
> +  if (LastStatePtr != NULL) {
> +    *LastStatePtr = State;
> +  }
> +
> +  return XENSTORE_STATUS_SUCCESS;
> +}
> +
> +EFI_STATUS
> +XenPvBlockFrontInitialization (
> +  IN  XENBUS_PROTOCOL         *XenbusIo,
> +  IN  CONST CHAR8             *NodeName,
> +  OUT XEN_BLOCK_FRONT_DEVICE  **DevPtr
> +  )
> +{
> +  XENSTORE_TRANSACTION xbt;
> +  CHAR8 *DeviceType;
> +  blkif_sring_t *SharedRing;
> +  XENSTORE_STATUS Status;
> +  XEN_BLOCK_FRONT_DEVICE *Dev;
> +  XenbusState State;
> +  UINT64 Value;
> +
> +  ASSERT (NodeName != NULL);
> +
> +  Dev = AllocateZeroPool (sizeof (XEN_BLOCK_FRONT_DEVICE));
> +  Dev->Signature = XEN_BLOCK_FRONT_SIGNATURE;
> +  Dev->NodeName = NodeName;
> +  Dev->XenbusIo = XenbusIo;
> +  Dev->DeviceId = XenbusIo->DeviceId;
> +
> +  XenbusIo->XsRead (XenbusIo, XST_NIL, "device-type", (VOID**)&DeviceType);
> +  if (AsciiStrCmp (DeviceType, "cdrom") == 0) {
> +    Dev->MediaInfo.CdRom = TRUE;
> +  } else {
> +    Dev->MediaInfo.CdRom = FALSE;
> +  }
> +  FreePool (DeviceType);
> +
> +  Status = XenbusReadInteger (XenbusIo, "backend-id", FALSE, &Value);
> +  if (Status != XENSTORE_STATUS_SUCCESS || Value > UINT16_MAX) {
> +    DEBUG ((EFI_D_ERROR, "XenPvBlk: Failed to get backend-id (%d)\n",
> +            Status));
> +    goto Error;
> +  }
> +  Dev->DomainId = Value;
> +  XenbusIo->EventChannelAllocate (XenbusIo, Dev->DomainId, &Dev->EventChannel);
> +
> +  SharedRing = (blkif_sring_t*) AllocatePages (1);
> +  SHARED_RING_INIT (SharedRing);
> +  FRONT_RING_INIT (&Dev->Ring, SharedRing, EFI_PAGE_SIZE);
> +  XenbusIo->GrantAccess (XenbusIo,
> +                         Dev->DomainId,
> +                         (INTN) SharedRing >> EFI_PAGE_SHIFT,
> +                         FALSE,
> +                         &Dev->RingRef);
> +
> +Again:
> +  Status = XenbusIo->XsTransactionStart (XenbusIo, &xbt);
> +  if (Status != XENSTORE_STATUS_SUCCESS) {
> +    DEBUG ((EFI_D_WARN, "XenPvBlk: Failed to start transaction, %d\n", Status));

.. and do you want to error out?

> +  }
> +
> +  Status = XenbusIo->XsPrintf (XenbusIo, xbt, NodeName, "ring-ref", "%d",
> +                               Dev->RingRef);
> +  if (Status != XENSTORE_STATUS_SUCCESS) {
> +    DEBUG ((EFI_D_ERROR, "XenPvBlk: Failed to write ring-ref.\n"));
> +    goto AbortTransaction;
> +  }
> +  Status = XenbusIo->XsPrintf (XenbusIo, xbt, NodeName,
> +                               "event-channel", "%d", Dev->EventChannel);
> +  if (Status != XENSTORE_STATUS_SUCCESS) {
> +    DEBUG ((EFI_D_ERROR, "XenPvBlk: Failed to write event-channel.\n"));
> +    goto AbortTransaction;
> +  }
> +  Status = XenbusIo->XsPrintf (XenbusIo, xbt, NodeName,
> +                               "protocol", "%a", XEN_IO_PROTO_ABI_NATIVE);
> +  if (Status != XENSTORE_STATUS_SUCCESS) {
> +    DEBUG ((EFI_D_ERROR, "XenPvBlk: Failed to write protocol.\n"));
> +    goto AbortTransaction;
> +  }
> +
> +  Status = XenbusIo->SetState (XenbusIo, xbt, XenbusStateConnected);
> +  if (Status != XENSTORE_STATUS_SUCCESS) {
> +    DEBUG ((EFI_D_ERROR, "XenPvBlk: Failed switch state.\n"));
> +    goto AbortTransaction;
> +  }
> +
> +  Status = XenbusIo->XsTransactionEnd (XenbusIo, xbt, FALSE);
> +  if (Status == XENSTORE_STATUS_EAGAIN) {
> +    goto Again;
> +  }
> +
> +  XenbusIo->RegisterWatchBackend (XenbusIo, "state", &Dev->StateWatchToken);
> +
> +  // Waiting backend
> +  Status = XenPvBlkWaitForBackendState (Dev, XenbusStateConnected, &State);
> +  if (Status != XENSTORE_STATUS_SUCCESS || State != XenbusStateConnected) {
> +    DEBUG ((EFI_D_ERROR,
> +            "XenPvBlk: backend for %a/%d not available, rc=%d state=%d\n",
> +            XenbusIo->Type, XenbusIo->DeviceId, Status, State));
> +    goto Error2;
> +  }
> +
> +  Status = XenbusReadInteger (XenbusIo, "info", TRUE, &Value);
> +  if (Status != XENSTORE_STATUS_SUCCESS || Value > UINT32_MAX) {
> +    goto Error2;
> +  }
> +  Dev->MediaInfo.VDiskInfo = Value;
> +  if (Dev->MediaInfo.VDiskInfo & VDISK_READONLY) {
> +    Dev->MediaInfo.ReadWrite = FALSE;
> +  } else {
> +    Dev->MediaInfo.ReadWrite = TRUE;
> +  }
> +
> +  Status = XenbusReadInteger (XenbusIo, "sectors", TRUE, &Dev->MediaInfo.Sectors);
> +  if (Status != XENSTORE_STATUS_SUCCESS) {
> +    goto Error2;
> +  }
> +
> +  Status = XenbusReadInteger (XenbusIo, "sector-size", TRUE, &Value);
> +  if (Status != XENSTORE_STATUS_SUCCESS || Value > UINT32_MAX) {
> +    goto Error2;
> +  }
> +  Dev->MediaInfo.SectorSize = Value;

Why not &Dev->MediaInfo.SectorSize in XenbusReadInteger?

Is it because of types? If so, please add a comment stating that.

> +
> +  // Default value
> +  Value = 0;
> +  Status = XenbusReadInteger (XenbusIo, "feature-barrier", TRUE, &Value);
> +  if (Value == 1) {
> +    Dev->MediaInfo.FeatureBarrier = TRUE;
> +  } else {
> +    Dev->MediaInfo.FeatureBarrier = FALSE;
> +  }
> +
> +  // Default value
> +  Value = 0;
> +  XenbusReadInteger (XenbusIo, "feature-flush-cache", TRUE, &Value);
> +  if (Value == 1) {
> +    Dev->MediaInfo.FeatureFlushCache = TRUE;
> +  } else {
> +    Dev->MediaInfo.FeatureFlushCache = FALSE;
> +  }
> +
> +  DEBUG ((EFI_D_INFO, "XenPvBlk: New disk with %d sectors of %d bytes\n",
> +          Dev->MediaInfo.Sectors, Dev->MediaInfo.SectorSize));
> +
> +  *DevPtr = Dev;
> +  return EFI_SUCCESS;
> +
> +Error2:
> +  XenbusIo->UnregisterWatch (XenbusIo, Dev->StateWatchToken);
> +  XenbusIo->XsRemove (XenbusIo, XST_NIL, "ring-ref");
> +  XenbusIo->XsRemove (XenbusIo, XST_NIL, "event-channel");

XenbusIo->XsRemove (XenbusIo, XST_NIL, "protocol");

?
> +  goto Error;
> +AbortTransaction:
> +  XenbusIo->XsTransactionEnd (XenbusIo, xbt, TRUE);
> +Error:
> +  XenPvBlockFree (Dev);
> +  return EFI_DEVICE_ERROR;
> +}
> +
> +VOID
> +XenPvBlockFrontShutdown (
> +  IN XEN_BLOCK_FRONT_DEVICE *Dev
> +  )
> +{
> +  XENBUS_PROTOCOL *XenbusIo = Dev->XenbusIo;
> +  XENSTORE_STATUS Status;
> +  UINT64 Value;
> +
> +  XenPvBlockSync (Dev);
> +
> +  Status = XenbusIo->SetState (XenbusIo, XST_NIL, XenbusStateClosing);
> +  if (Status != XENSTORE_STATUS_SUCCESS) {
> +    DEBUG ((EFI_D_ERROR,
> +            "XenPvBlk: error while changing state to Closing: %d\n",
> +            Status));
> +    goto Close;
> +  }
> +
> +  Status = XenPvBlkWaitForBackendState (Dev, XenbusStateClosing, NULL);
> +  if (Status != XENSTORE_STATUS_SUCCESS) {
> +    DEBUG ((EFI_D_ERROR,
> +            "XenPvBlk: error while waiting for closing backend state: %d\n",
> +            Status));
> +    goto Close;
> +  }
> +
> +  Status = XenbusIo->SetState (XenbusIo, XST_NIL, XenbusStateClosed);
> +  if (Status != XENSTORE_STATUS_SUCCESS) {
> +    DEBUG ((EFI_D_ERROR,
> +            "XenPvBlk: error while changing state to Closed: %d\n",
> +            Status));
> +    goto Close;
> +  }
> +
> +  Status = XenPvBlkWaitForBackendState (Dev, XenbusStateClosed, NULL);
> +  if (Status != XENSTORE_STATUS_SUCCESS) {
> +    DEBUG ((EFI_D_ERROR,
> +            "XenPvBlk: error while waiting for closed backend state: %d\n",
> +            Status));
> +    goto Close;
> +  }
> +
> +  Status = XenbusIo->SetState (XenbusIo, XST_NIL, XenbusStateInitialising);
> +  if (Status != XENSTORE_STATUS_SUCCESS) {
> +    DEBUG ((EFI_D_ERROR,
> +            "XenPvBlk: error while changing state to initialising: %d\n",
> +            Status));
> +    goto Close;
> +  }
> +
> +  while (TRUE) {
> +    Status = XenbusReadInteger (XenbusIo, "state", TRUE, &Value);
> +    if (Status != XENSTORE_STATUS_SUCCESS) {
> +      DEBUG ((EFI_D_ERROR,
> +              "XenPvBlk: error while waiting for new backend state: %d\n",
> +              Status));
> +      goto Close;
> +    }
> +    if (Value < XenbusStateInitWait || Value >= XenbusStateClosed) {
> +      break;
> +    }
> +    DEBUG ((EFI_D_INFO,
> +            "XenPvBlk: waiting backend state %d, current: %d\n",
> +            XenbusStateInitWait, Value));
> +    XenbusIo->WaitForWatch (XenbusIo, Dev->StateWatchToken);
> +  }
> +
> +Close:
> +  XenbusIo->UnregisterWatch (XenbusIo, Dev->StateWatchToken);
> +  XenbusIo->XsRemove (XenbusIo, XST_NIL, "ring-ref");
> +  XenbusIo->XsRemove (XenbusIo, XST_NIL, "event-channel");

"protocol" 
> +
> +  XenPvBlockFree (Dev);
> +}
> +
> +STATIC
> +VOID
> +XenPvBlockWaitSlot (
> +  IN XEN_BLOCK_FRONT_DEVICE *Dev
> +  )
> +{
> +  /* Wait for a slot */
> +  if (RING_FULL (&Dev->Ring)) {
> +    while (TRUE) {
> +      XenPvBlockAsyncIoPoll (Dev);
> +      if (!RING_FULL(&Dev->Ring)) {
> +        break;
> +      }
> +      /* Really no slot, go to sleep. */
> +      // XXX wait for an event on Dev->EventChannel
> +      DEBUG ((EFI_D_INFO, "%a %d, waiting\n", __func__, __LINE__));
> +    }
> +  }
> +}
> +
> +/* Issue an aio */
> +VOID
> +XenPvBlockAsyncIo (
> +  IN OUT XEN_BLOCK_FRONT_IO *IoData,
> +  IN     BOOLEAN            IsWrite
> +  )
> +{
> +  XEN_BLOCK_FRONT_DEVICE *Dev = IoData->Dev;
> +  XENBUS_PROTOCOL *XenbusIo = Dev->XenbusIo;
> +  blkif_request_t *Request;
> +  RING_IDX RingIndex;
> +  BOOLEAN Notify;
> +  INT32 NumSegments, Index;
> +  UINTN Start, End;
> +
> +  // Can't io at non-sector-aligned location
> +  ASSERT(!(IoData->Offset & (Dev->MediaInfo.SectorSize - 1)));
> +  // Can't io non-sector-sized amounts
> +  ASSERT(!(IoData->Size & (Dev->MediaInfo.SectorSize - 1)));
> +  // Can't io non-sector-aligned buffer
> +  ASSERT(!((UINTN) IoData->Buffer & (Dev->MediaInfo.SectorSize - 1)));
> +
> +  Start = (UINTN) IoData->Buffer & ~EFI_PAGE_MASK;
> +  End = ((UINTN) IoData->Buffer + IoData->Size + EFI_PAGE_SIZE - 1) & ~EFI_PAGE_MASK;
> +  IoData->NumRef = NumSegments = (End - Start) / EFI_PAGE_SIZE;
> +
> +  // XXX is this comment still true ?

I think it is outdated, as Linux would do max DMA up to the segment
size that is advertised. Ditto for the SCSI stack.

> +  /* qemu's IDE max multsect is 16 (8KB) and SCSI max DMA was set to 32KB,
> +   * so max 44KB can't happen */
> +  ASSERT (NumSegments <= BLKIF_MAX_SEGMENTS_PER_REQUEST);

But this check is OK as we can't truly fit more on the ring.
> +
> +  XenPvBlockWaitSlot (Dev);

What happens if there are multiple threads calling this
function? Don't we need a mutex to only allow
one thread to grab an RingIndex?

> +  RingIndex = Dev->Ring.req_prod_pvt;
> +  Request = RING_GET_REQUEST (&Dev->Ring, RingIndex);
> +
> +  Request->operation = IsWrite ? BLKIF_OP_WRITE : BLKIF_OP_READ;
> +  Request->nr_segments = NumSegments;
> +  Request->handle = Dev->DeviceId;
> +  Request->id = (UINTN) IoData;
> +  Request->sector_number = IoData->Offset / 512;
> +
> +  for (Index = 0; Index < NumSegments; Index++) {
> +    Request->seg[Index].first_sect = 0;

Uh? 
> +    Request->seg[Index].last_sect = EFI_PAGE_SIZE / 512 - 1;

Uh? I thought you wanted to use the real sector values?
> +  }
> +  Request->seg[0].first_sect = ((UINTN) IoData->Buffer & EFI_PAGE_MASK) / 512;
> +  Request->seg[NumSegments - 1].last_sect =
> +      (((UINTN) IoData->Buffer + IoData->Size - 1) & EFI_PAGE_MASK) / 512;

What about in between 0 and NumSegments? You should also
fill out those values with valid entries.

> +  for (Index = 0; Index < NumSegments; Index++) {
> +    UINTN Data = Start + Index * EFI_PAGE_SIZE;
> +    if (!IsWrite) {
> +      /* Trigger CoW if needed */

Not sure I understand that reference?

> +      *(CHAR8 *)(Data + (Request->seg[Index].first_sect << 9)) = 0;
> +      MemoryFence ();
> +    }
> +    XenbusIo->GrantAccess (XenbusIo, Dev->DomainId,
> +                           Data >> EFI_PAGE_SHIFT, IsWrite,
> +                           &Request->seg[Index].gref);
> +    IoData->GrantRef[Index] = Request->seg[Index].gref;
> +  }
> +
> +  Dev->Ring.req_prod_pvt = RingIndex + 1;
> +
> +  MemoryFence ();
> +  RING_PUSH_REQUESTS_AND_CHECK_NOTIFY (&Dev->Ring, Notify);
> +
> +  if (Notify) {
> +    XenbusIo->EventChannelNotify (XenbusIo, Dev->EventChannel);
> +  }
> +}
> +
> +STATIC
> +VOID
> +XenPvBlockAsyncIoCallback (
> +  IN OUT XEN_BLOCK_FRONT_IO *IoData,
> +  IN     EFI_STATUS         Status
> +  )
> +{
> +  IoData->Data = (VOID *) 1;
> +  IoData->Callback = NULL;
> +}
> +
> +VOID
> +XenPvBlockIo (
> +  IN OUT XEN_BLOCK_FRONT_IO *IoData,
> +  IN     BOOLEAN            IsWrite
> +  )
> +{
> +  ASSERT (IoData->Callback == NULL);
> +  // XXX Signal/Event instead ?

Maybe for later versions. I think this is OK for right now, thought
it will mean we can only do one I/O .. and have then to wait
until it is completed.

That limititation should be mentioned in the commit description.

> +  IoData->Callback = XenPvBlockAsyncIoCallback;
> +  XenPvBlockAsyncIo (IoData, IsWrite);
> +  IoData->Data = NULL;
> +
> +  while (TRUE) {
> +    XenPvBlockAsyncIoPoll (IoData->Dev);
> +    if (IoData->Data) {
> +      break;
> +    }
> +  }
> +}
> +
> +STATIC
> +VOID
> +XenPvBlockPushOperation (
> +  IN XEN_BLOCK_FRONT_DEVICE *Dev,
> +  IN UINT8                  Operation,
> +  IN UINT64                 Id
> +  )
> +{
> +  INT32 Index;
> +  blkif_request_t *Request;
> +  BOOLEAN Notify;
> +
> +  XenPvBlockWaitSlot (Dev);

How do we make sure that there aren't multiple threads
calling this?

> +  Index = Dev->Ring.req_prod_pvt;
> +  Request = RING_GET_REQUEST(&Dev->Ring, Index);
> +  Request->operation = Operation;
> +  Request->nr_segments = 0;
> +  Request->handle = Dev->DeviceId;
> +  Request->id = Id;
> +  /* Not needed anyway, but the backend will check it */
> +  Request->sector_number = 0;
> +  Dev->Ring.req_prod_pvt = Index + 1;
> +  MemoryFence ();
> +  RING_PUSH_REQUESTS_AND_CHECK_NOTIFY (&Dev->Ring, Notify);
> +  if (Notify) {
> +    XENBUS_PROTOCOL *XenbusIo = Dev->XenbusIo;
> +    XenbusIo->EventChannelNotify (XenbusIo, Dev->EventChannel);
> +  }
> +}
> +
> +VOID
> +XenPvBlockSync (
> +  IN XEN_BLOCK_FRONT_DEVICE *Dev
> +  )
> +{
> +  if (Dev->MediaInfo.ReadWrite) {
> +    if (Dev->MediaInfo.FeatureBarrier) {
> +      XenPvBlockPushOperation (Dev, BLKIF_OP_WRITE_BARRIER, 0);
> +    }
> +
> +    if (Dev->MediaInfo.FeatureFlushCache) {
> +      XenPvBlockPushOperation (Dev, BLKIF_OP_FLUSH_DISKCACHE, 0);
> +    }
> +  }
> +
> +  /* Note: This won't finish if another thread enqueues requests.  */
> +  while (TRUE) {
> +    XenPvBlockAsyncIoPoll (Dev);
> +    if (RING_FREE_REQUESTS (&Dev->Ring) == RING_SIZE (&Dev->Ring)) {
> +      break;
> +    }
> +  }
> +}
> +
> +VOID
> +XenPvBlockAsyncIoPoll (
> +  IN XEN_BLOCK_FRONT_DEVICE *Dev
> +  )
> +{
> +  RING_IDX ProducerIndex, ConsumerIndex;
> +  blkif_response_t *Response;
> +  INT32 More;
> +
> +  do {
> +    ProducerIndex = Dev->Ring.sring->rsp_prod;
> +    /* Ensure we see queued responses up to 'ProducerIndex'. */
> +    MemoryFence ();
> +    ConsumerIndex = Dev->Ring.rsp_cons;
> +
> +    while (ConsumerIndex != ProducerIndex) {
> +      XEN_BLOCK_FRONT_IO *IoData;
> +      INT16 Status;
> +
> +      Response = RING_GET_RESPONSE (&Dev->Ring, ConsumerIndex);
> +
> +      IoData = (VOID *) (UINTN) Response->id;
> +      Status = Response->status;
> +
> +      switch (Response->operation) {
> +      case BLKIF_OP_READ:
> +      case BLKIF_OP_WRITE:
> +        {
> +          INT32 Index;
> +
> +          if (Status != BLKIF_RSP_OKAY) {
> +            DEBUG ((EFI_D_ERROR,
> +                    "XenPvBlk: "
> +                    "%a error %d on %a at offset %Ld, num bytes %Ld\n",
> +                    Response->operation == BLKIF_OP_READ ? "read" : "write",
> +                    Status, IoData->Dev->NodeName,
> +                    IoData->Offset,
> +                    IoData->Size));
> +          }
> +
> +          for (Index = 0; Index < IoData->NumRef; Index++) {
> +            Dev->XenbusIo->GrantEndAccess (Dev->XenbusIo, IoData->GrantRef[Index]);
> +          }
> +
> +          break;
> +        }
> +
> +      case BLKIF_OP_WRITE_BARRIER:
> +        if (Status != BLKIF_RSP_OKAY) {
> +          DEBUG ((EFI_D_ERROR, "XenPvBlk: write barrier error %d\n", Status));
> +        }
> +        break;
> +      case BLKIF_OP_FLUSH_DISKCACHE:
> +        if (Status != BLKIF_RSP_OKAY) {
> +          DEBUG ((EFI_D_ERROR, "XenPvBlk: flush error %d\n", Status));
> +        }
> +        break;
> +
> +      default:
> +        DEBUG ((EFI_D_ERROR,
> +                "XenPvBlk: unrecognized block operation %d response (status %d)\n",
> +                Response->operation, Status));
> +        break;
> +      }
> +
> +      Dev->Ring.rsp_cons = ++ConsumerIndex;
> +      /* Nota: callback frees IoData itself */
> +      if (IoData && IoData->Callback) {
> +        IoData->Callback (IoData, Status ? EFI_DEVICE_ERROR : EFI_SUCCESS);
> +      }
> +      if (Dev->Ring.rsp_cons != ConsumerIndex) {
> +        /* We reentered, we must not continue here */

Uh, how does that happen?

> +        break;
> +      }
> +    }
> +
> +    RING_FINAL_CHECK_FOR_RESPONSES (&Dev->Ring, More);
> +  } while (More != 0);
> +}