The descriptor table (a.k.a. "queue") consists of descriptors (=
VRING_DESC).
An individual descriptor describes a contiguous buffer, to be transferred
uni-directionally between host and guest.
Several descriptors in the descriptor table can be linked into a
descriptor chain, specifying a bi-directional scatter-gather transfer
between host and guest. Such a descriptor chain is also known as "virtio
request".
(The descriptor table can host sereval descriptor chains (in-flight virtio
requests) in parallel, but the OVMF driver supports at most one chain, at
any point in time.)
The first descriptor in any descriptor chain is called "head descriptor".
In order to submit a number of parallel requests (= a set of independent
descriptor chains) from the guest to the host, the guest must put *only*
the head descriptor of each separate chain onto the Available Ring.
VirtioLib currently places each descriptor in the same chain onto the
Available Ring separately. Accordingly, the host interprets each of these
dependent descriptors as a head descriptor of a separate descriptor chain
(= virtio request), and serves them in parallel.
Anatomy of a single virtio-blk READ request (a descriptor chain with three
descriptors):
virtio-blk request header, prepared by guest:
VirtioAppendDesc PhysAddr=3FBC6050 Size=16 Flags=1 Head=1232 Next=1232
payload to be filled in by host:
VirtioAppendDesc PhysAddr=3B934C00 Size=32768 Flags=3 Head=1232 Next=1233
host status, to be filled in by host:
VirtioAppendDesc PhysAddr=3FBC604F Size=1 Flags=2 Head=1232 Next=1234
Processing on the host side -- each descriptor is interpreted as a
standalone request (virtqueue_pop), and submitted/collected separately
to/from AIO:
virtio_queue_notify vdev VDEV vq VQ#0
virtqueue_pop vq VQ#0 elem EL#0 in_num 2 out_num 1
bdrv_aio_readv bs BDRV sector_num 585792 nb_sectors 64 opaque REQ#0
virtqueue_pop vq VQ#0 elem EL#1 in_num 2 out_num 1
bdrv_aio_readv bs BDRV sector_num 585792 nb_sectors 64 opaque REQ#1
virtqueue_pop vq VQ#0 elem EL#2 in_num 2 out_num 1
bdrv_aio_readv bs BDRV sector_num 585792 nb_sectors 64 opaque REQ#2
virtio_blk_rw_complete req REQ#0 ret 0
virtio_blk_req_complete req REQ#0 status 0
virtio_blk_rw_complete req REQ#1 ret 0
virtio_blk_req_complete req REQ#1 status 0
virtio_blk_rw_complete req REQ#2 ret 0
virtio_blk_req_complete req REQ#2 status 0
This can (and sometimes does) lead to data corruption; for example the
CRC32 of the data received from the host for the same (never rewritten)
LBA varies dependent on the race.
SynchronousRequest Lba=585792 BufSiz=32768 ReqIsWrite=0 Crc32=BF68A44D
Fix this by:
- building the only one descriptor chain supported by VirtioLib always at
the beginning of the descriptor table,
- make sure only the head descriptor of this chain is put on the Available
Ring,
- request the virtio spec's language to be cleaned up
<http://lists.linuxfoundation.org/pipermail/virtualization/2013-April/024032.html>.
VirtioAppendDesc PhysAddr=3FBC6040 Size=16 Flags=1 Head=0 Next=0
VirtioAppendDesc PhysAddr=3B934C00 Size=32768 Flags=3 Head=0 Next=1
VirtioAppendDesc PhysAddr=3FBC603F Size=1 Flags=2 Head=0 Next=2
virtio_queue_notify vdev VDEV vq VQ#0
virtqueue_pop vq VQ#0 elem EL#0 in_num 2 out_num 1
bdrv_aio_readv bs BDRV sector_num 585792 nb_sectors 64 opaque REQ#0
virtio_blk_rw_complete req REQ#0 ret 0
virtio_blk_req_complete req REQ#0 status 0
SynchronousRequest Lba=585792 BufSiz=32768 ReqIsWrite=0 Crc32=1EEB2B07
(The Crc32 was double-checked with edk2's and Linux's guest IDE driver.)
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <[email protected]>
---
OvmfPkg/Include/Library/VirtioLib.h | 27 +++++++---------
OvmfPkg/Library/VirtioLib/VirtioLib.c | 52 +++++++++++++++++++-------------
2 files changed, 43 insertions(+), 36 deletions(-)
diff --git a/OvmfPkg/Include/Library/VirtioLib.h
b/OvmfPkg/Include/Library/VirtioLib.h
index 448dd2b..3bebd9f 100644
--- a/OvmfPkg/Include/Library/VirtioLib.h
+++ b/OvmfPkg/Include/Library/VirtioLib.h
@@ -140,8 +140,8 @@ VirtioRingUninit (
// request.
//
typedef struct {
- UINT16 HeadIdx;
- UINT16 NextAvailIdx;
+ UINT16 HeadDescIdx;
+ UINT16 NextDescIdx;
} DESC_INDICES;
@@ -169,9 +169,8 @@ VirtioPrepare (
Append a contiguous buffer for transmission / reception via the virtio ring.
- This function implements the following sections from virtio-0.9.5:
+ This function implements the following section from virtio-0.9.5:
- 2.4.1.1 Placing Buffers into the Descriptor Table
- - 2.4.1.2 Updating the Available Ring
Free space is taken as granted, since the individual drivers support only
synchronous requests and host side status is processed in lock-step with
@@ -199,13 +198,9 @@ VirtioPrepare (
In *Indices:
- @param [in] HeadIdx The index identifying the head buffer (first
- buffer appended) belonging to this same
- request.
-
- @param [in out] NextAvailIdx On input, the index identifying the next
- descriptor available to carry the buffer. On
- output, incremented by one, modulo 2^16.
+ @param [in out] NextDescIdx On input, the index identifying the next
+ descriptor to carry the buffer. On output,
+ incremented by one, modulo 2^16.
**/
VOID
@@ -221,8 +216,8 @@ VirtioAppendDesc (
/**
- Notify the host about appended descriptors and wait until it processes the
- last one (ie. all of them).
+ Notify the host about the descriptor chain just built, and wait until the
+ host processes it.
@param[in] PciIo The target virtio PCI device to notify.
@@ -230,8 +225,10 @@ VirtioAppendDesc (
@param[in out] Ring The virtio ring with descriptors to submit.
- @param[in] Indices The function waits until the host processes
- descriptors up to Indices->NextAvailIdx.
+ In *Indices:
+
+ @param[in] HeadDescIdx Identifies the head descriptor of the descriptor
+ chain.
@return Error code from VirtioWrite() if it fails.
diff --git a/OvmfPkg/Library/VirtioLib/VirtioLib.c
b/OvmfPkg/Library/VirtioLib/VirtioLib.c
index 8398c9d..db619ee 100644
--- a/OvmfPkg/Library/VirtioLib/VirtioLib.c
+++ b/OvmfPkg/Library/VirtioLib/VirtioLib.c
@@ -306,8 +306,11 @@ VirtioPrepare (
//
// Prepare for virtio-0.9.5, 2.4.1 Supplying Buffers to the Device.
//
- Indices->HeadIdx = *Ring->Avail.Idx;
- Indices->NextAvailIdx = Indices->HeadIdx;
+ // Since we support only one in-flight descriptor chain, we can always build
+ // that chain starting at entry #0 of the descriptor table.
+ //
+ Indices->HeadDescIdx = 0;
+ Indices->NextDescIdx = Indices->HeadDescIdx;
}
@@ -315,9 +318,8 @@ VirtioPrepare (
Append a contiguous buffer for transmission / reception via the virtio ring.
- This function implements the following sections from virtio-0.9.5:
+ This function implements the following section from virtio-0.9.5:
- 2.4.1.1 Placing Buffers into the Descriptor Table
- - 2.4.1.2 Updating the Available Ring
Free space is taken as granted, since the individual drivers support only
synchronous requests and host side status is processed in lock-step with
@@ -345,13 +347,9 @@ VirtioPrepare (
In *Indices:
- @param [in] HeadIdx The index identifying the head buffer (first
- buffer appended) belonging to this same
- request.
-
- @param [in out] NextAvailIdx On input, the index identifying the next
- descriptor available to carry the buffer. On
- output, incremented by one, modulo 2^16.
+ @param [in out] NextDescIdx On input, the index identifying the next
+ descriptor to carry the buffer. On output,
+ incremented by one, modulo 2^16.
**/
VOID
@@ -366,20 +364,18 @@ VirtioAppendDesc (
{
volatile VRING_DESC *Desc;
- Desc = &Ring->Desc[Indices->NextAvailIdx % Ring->QueueSize];
+ Desc = &Ring->Desc[Indices->NextDescIdx++ % Ring->QueueSize];
Desc->Addr = BufferPhysAddr;
Desc->Len = BufferSize;
Desc->Flags = Flags;
- Ring->Avail.Ring[Indices->NextAvailIdx++ % Ring->QueueSize] =
- Indices->HeadIdx % Ring->QueueSize;
- Desc->Next = Indices->NextAvailIdx % Ring->QueueSize;
+ Desc->Next = Indices->NextDescIdx % Ring->QueueSize;
}
/**
- Notify the host about appended descriptors and wait until it processes the
- last one (ie. all of them).
+ Notify the host about the descriptor chain just built, and wait until the
+ host processes it.
@param[in] PciIo The target virtio PCI device to notify.
@@ -387,8 +383,10 @@ VirtioAppendDesc (
@param[in out] Ring The virtio ring with descriptors to submit.
- @param[in] Indices The function waits until the host processes
- descriptors up to Indices->NextAvailIdx.
+ In *Indices:
+
+ @param[in] HeadDescIdx Identifies the head descriptor of the descriptor
+ chain.
@return Error code from VirtioWrite() if it fails.
@@ -405,14 +403,26 @@ VirtioFlush (
IN DESC_INDICES *Indices
)
{
+ UINT16 NextAvailIdx;
EFI_STATUS Status;
UINTN PollPeriodUsecs;
//
+ // virtio-0.9.5, 2.4.1.2 Updating the Available Ring
+ //
+ // It is not exactly clear from the wording of the virtio-0.9.5
+ // specification, but each entry in the Available Ring references only the
+ // head descriptor of any given descriptor chain.
+ //
+ NextAvailIdx = *Ring->Avail.Idx;
+ Ring->Avail.Ring[NextAvailIdx++ % Ring->QueueSize] =
+ Indices->HeadDescIdx % Ring->QueueSize;
+
+ //
// virtio-0.9.5, 2.4.1.3 Updating the Index Field
//
MemoryFence();
- *Ring->Avail.Idx = Indices->NextAvailIdx;
+ *Ring->Avail.Idx = NextAvailIdx;
//
// virtio-0.9.5, 2.4.1.4 Notifying the Device -- gratuitous notifications are
@@ -439,7 +449,7 @@ VirtioFlush (
//
PollPeriodUsecs = 1;
MemoryFence();
- while (*Ring->Used.Idx != Indices->NextAvailIdx) {
+ while (*Ring->Used.Idx != NextAvailIdx) {
gBS->Stall (PollPeriodUsecs); // calls AcpiTimerLib::MicroSecondDelay
if (PollPeriodUsecs < 1024) {
--
1.7.1
------------------------------------------------------------------------------
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