[Skiboot] [RFC] docs: Specify V3 of the mbox protocol
Andrew Jeffery
andrew at aj.id.au
Mon Sep 11 17:01:45 AEST 2017
On Thu, 2017-09-07 at 17:14 +1000, Suraj Jitindar Singh wrote:
> Version 3 of the mbox protocol makes four protocol changes:
> - Add a requested block size argument to GET_MBOX_INFO
> - Add a no erase argument to MARK_DIRTY
> - Add a GET_FLASH_NAME command and support multiple flash devices
> - Add a MARK_LOCKED command
>
> Requested Block Size:
> The requested block size argument has been added to the GET_MBOX_INFO
> command to allow the host to request a specified block size which might
> be required to allow data manipulation at a finer granularity. The
> daemon should take this into account when choosing a block size for use
> which it will specify in the response as before. The daemon has final
> say and the host must use the block size the daemon chooses.
>
> No Erase:
> The no erase argument to the mark dirty command allows a host to specify
> that an area of flash should not be erased before being written to, as is
> the default behaviour. This can be used when a host has already erased a
> large area and then performs many small writes which would normally mean
> many erases due to the implicit erase before write, making this slow.
>
> Add GET_FLASH_NAME command:
> The ability to support multiple flash devices has been added so that the
> mbox protocol can be used to arbitrate access from the host to a number
> of flash devices which the daemon has access to. To facilitate this the
> GET_FLASH_INFO, CREATE_{READ/WRITE}_WINDOW, and MARK_LOCKED commands now
> take a flash ID, with the number of flash IDs allocated returned by the
> GET_MBOX_INFO commands. There is also a new command GET_FLASH_NAME used
> to convert a flash ID to a flash name.
>
> Add MARK_LOCKED command:
> The MARK_LOCKED command has been added to allow an area(s) of flash to be
> locked, that is that area must be treated as read only and the host is
> not allowed to dirty or erase any windows which map that area of flash.
> Additionally another error code LOCKED_ERROR was added to be returned
> when the host does try to dirty or erase a locked area.
>
> The host cannot lock a currently dirty or erased area of the current
> write window as it is not defined if the clean/dirty/erased value is
> what should actually be "locked".
>
> A locked area of flash remains so until the daemon receives an
> mboxctl --reset command and the locked areas are stored in a file on the
> BMC filesystem to ensure persistence across BMC reboots/daemon crashes.
>
> Multiple flash device support proposed and defined by:
> > William A. Kennington III <wak at google.com>
>
> > Signed-off-by: Suraj Jitindar Singh <sjitindarsingh at gmail.com>
> ---
> Documentation/mbox_protocol.md | 127 ++++++++++++++++++++++++++++++-----------
> 1 file changed, 93 insertions(+), 34 deletions(-)
>
> diff --git a/Documentation/mbox_protocol.md b/Documentation/mbox_protocol.md
> index bcd70a8..74863a7 100644
> --- a/Documentation/mbox_protocol.md
> +++ b/Documentation/mbox_protocol.md
> @@ -17,19 +17,21 @@ limitations under the License.
> This document describes a protocol for host to BMC communication via the
> mailbox registers present on the Aspeed 2400 and 2500 chips.
> This protocol is specifically designed to allow a host to request and manage
> -access to the flash with the specifics of how the host is required to control
> -this described below.
> +access to a flash device with the specifics of how the host is required to
> +control this described below.
>
> ## Version
>
> -Both version 1 and version 2 of the protocol are described below with version 2
> -specificities represented with V2 in brackets - (V2).
> +Version specific protocol functionalities are represented by the version number
> +in brackets next to the definition of the functionality. (e.g. (V2) for version
> +2 specific funtionality). All version specific functionality must also be
> +implemented by proceeding versions.
I don't know that this is something we want to lock ourselves into. We
never want to remove functionality? What if a command turns out to be
dangerous?
>
> ## Problem Overview
>
> "mbox" is the name we use to represent a protocol we have established between
> the host and the BMC via the Aspeed mailbox registers. This protocol is used
> -for the host to control the flash.
> +for the host to control access to the flash device(s).
>
> Prior to the mbox protocol, the host uses a backdoor into the BMC address space
> (the iLPC-to-AHB bridge) to directly manipulate the BMCs own flash controller.
> @@ -280,6 +282,14 @@ The host is not required to perform an erase before a write command and the
> BMC must ensure that a write performs as expected - that is if an erase is
> required before a write then the BMC must perform this itself.
>
> +The host may lock an area of flash using the MARK_LOCKED command. Any attempt
> +to mark dirty or erased this area of flash must fail with the LOCKED_ERROR
> +response code. The host may open a write window which contains a locked area
> +of flash however changes to a locked area of flash must never be written back
> +to the backing data source (i.e. that area of flash must be treated as read
> +only). An attempt to lock an area of flash which is not clean in the current
> +window must fail with PARAM_ERROR. (V3)
I think we need to consider how locked regions interact with the in-memory
caching of the flash data.
The BMC doesn't know that the memory backing the LPC window has been
written until it receives a MARK_DIRTY from the host. MARK_DIRTY is not
valid on a read window or a locked region, but that doesn't mean that
changes to a locked region can't persist in memory due to caching to
avoid flash access (which was a part of the motivation for memory-based
booting to begin with). This may naively fool host firmware into thinking it is
accessing unmolested data as the region is locked, whereas the region could
contain anything, just it will never be written to flash.
Should we require window requests intersecting a locked region always have at
least the locked region positively match the on-flash data? That way the
firmware that requested a region be locked could ensure it has whatever was on
the flash at the time it was locked by explicitly closing the window (read or
write) once finished with it, which requires a window be opened again for any
subsequent access. At that point (open) we can do the locked region intesection
calculation and check the data in the window as necessary.
Locked regions could thus be a performance hit if we always load the locked
regions from flash, but surely we prefer (some) integrity over performance.
Alternatively, you could cryptographically hash the per-block content of the
on-flash locked region during the MARK_LOCKED operation, then stash the hash
values away. On a CREATE_{READ,WRITE}_WINDOW operation you could hash any
intersecting, in-cache locked blocks and compare to the stashed hash values to
incur only one set of flash accesses (initial hash calculations) rather than n.
> +
> ### BMC Events
>
> The BMC can raise events with the host asynchronously to communicate to the
> @@ -316,6 +326,8 @@ MARK_WRITE_DIRTY 0x07
> WRITE_FLUSH 0x08
> BMC_EVENT_ACK 0x09
> MARK_WRITE_ERASED 0x0a (V2)
> +GET_FLASH_NAME 0x0b (V3)
> +MARK_LOCKED 0x0c (V3)
> ```
>
> ### Responses
> @@ -329,6 +341,7 @@ TIMEOUT 5
> BUSY 6 (V2)
> WINDOW_ERROR 7 (V2)
> SEQ_ERROR 8 (V2)
> +LOCKED_ERROR 9 (V3)
> ```
>
> ### Sequence Numbers
> @@ -368,6 +381,10 @@ BUSY - Daemon in suspended state (currently unable to access flash)
> WINDOW_ERROR - Command not valid for active window or no active window
> - Try opening an appropriate window and retrying the command
>
> +SEQ_ERROR - Invalid sequence number supplied with command
> +
> +LOCKED_ERROR - Tried to mark dirty or erased locked area of flash
> +
> ### Information
> - All multibyte messages are LSB first (little endian)
> - All responses must have a valid return code in byte 13
> @@ -394,9 +411,7 @@ Sizes and addresses specified in blocks must be converted to bytes by
> multiplying by the block size.
> ```
> Command:
> - RESET_STATE
> - Implemented in Versions:
> - V1, V2
Addressing my concern above about locking ourselves in, I prefer we
allow for a set of statements like:
Added in: V1
Removed in: V4
I guess we continue to annotate the command argument registers as we have been
already.
> + RESET_STATE (V1)
> Arguments:
> -
> Response:
> @@ -408,9 +423,7 @@ Command:
> pre mailbox protocol. Final behavior is still TBD.
>
> Command:
> - GET_MBOX_INFO
> - Implemented in Versions:
> - V1, V2
> + GET_MBOX_INFO (V1)
> Arguments:
> V1:
> Args 0: API version
> @@ -418,6 +431,10 @@ Command:
> V2:
> Args 0: API version
>
> + V3:
> + Args 0: API version
> + Args 1: Requested block size (shift)
> +
> Response:
> V1:
> Args 0: API version
> @@ -430,6 +447,14 @@ Command:
> Args 3-4: reserved
> Args 5: Block size as power of two (encoded as a shift)
> Args 6-7: Suggested Timeout (seconds)
> +
> + V3:
> + Args 0: API version
> + Args 1-2: reserved
> + Args 3-4: reserved
> + Args 5: Block size as power of two (encoded as a shift)
> + Args 6-7: Suggested Timeout (seconds)
> + Args 8: Num Allocated Flash IDs
> Notes:
> The suggested timeout is a hint to the host as to how long
> it should wait after issuing a command to the BMC before it
> @@ -439,25 +464,32 @@ Command:
> the BMC does not wish to provide a hint in which case the host
> must choose some reasonable value.
>
> + The host may require
I'd use 'desire' instead of 'require', because the host must obey the
BMC's response regarding the blocksize.
> a specific block size and thus can request
> + this by giving a hint to the daemon (may be zero). The daemon
> + may use this to select the block size which it will use however
> + is free to ignore this.
Maybe 'it' instead of 'this' at the end of the sentence?
> The value in the response is the block
> + size which must be used for all further requests until a new
> + size is negotiated by another call to GET_MBOX_INFO. (V3)
> +
> Command:
> - GET_FLASH_INFO
> - Implemented in Versions:
> - V1, V2
> + GET_FLASH_INFO (V1)
> Arguments:
> + V1, V2:
> -
> +
> + V3:
> + Args 0: Flash ID
> Response:
> V1:
> Args 0-3: Flash size (bytes)
> Args 4-7: Erase granule (bytes)
>
> - V2:
> + V2, V3:
> Args 0-1: Flash size (blocks)
> Args 2-3: Erase granule (blocks)
>
> Command:
> - CREATE_{READ/WRITE}_WINDOW
> - Implemented in Versions:
> - V1, V2
> + CREATE_{READ/WRITE}_WINDOW (V1)
> Arguments:
> V1:
> Args 0-1: Requested flash offset (blocks)
> @@ -466,11 +498,15 @@ Command:
> Args 0-1: Requested flash offset (blocks)
> Args 2-3: Requested flash size to access (blocks)
>
> + V3:
> + Args 0-1: Requested flash offset (blocks)
> + Args 2-3: Requested flash size to access (blocks)
> + Args 4: Flash ID
> Response:
> V1:
> Args 0-1: LPC bus address of window (blocks)
>
> - V2:
> + V2, V3:
> Args 0-1: LPC bus address of window (blocks)
> Args 2-3: Window size (blocks)
> Args 4-5: Flash offset mapped by window (blocks)
> @@ -504,9 +540,7 @@ Command:
> window.
>
> Command:
> - CLOSE_WINDOW
> - Implemented in Versions:
> - V1, V2
> + CLOSE_WINDOW (V1)
> Arguments:
> V1:
> -
> @@ -533,9 +567,7 @@ Command:
> evicted from the cache.
>
> Command:
> - MARK_WRITE_DIRTY
> - Implemented in Versions:
> - V1, V2
> + MARK_WRITE_DIRTY (V1)
> Arguments:
> V1:
> Args 0-1: Flash offset to mark from base of flash (blocks)
> @@ -544,6 +576,7 @@ Command:
> V2:
> Args 0-1: Window offset to mark (blocks)
> Args 2-3: Number to mark dirty at offset (blocks)
> + Args 4 : Don't Erase Before Write (V3)
>
> Response:
> -
> @@ -558,10 +591,15 @@ Command:
> block. If the offset + number exceeds the size of the active
> window then the command must not succeed.
>
> + The host can give a hint to the daemon that is doesn't have to
> + erase a flash area before writing to it by setting ARG[4]. This
> + means that the daemon will blindly perform a write to that area
> + and will not try to erase it before hand. This can be used if
> + the host knows that a large area has already been erased for
> + example but then wants to perform many small writes.
> +
> Command
> - WRITE_FLUSH
> - Implemented in Versions:
> - V1, V2
> + WRITE_FLUSH (V1)
> Arguments:
> V1:
> Args 0-1: Flash offset to mark from base of flash (blocks)
> @@ -585,9 +623,7 @@ Command
>
>
> Command:
> - BMC_EVENT_ACK
> - Implemented in Versions:
> - V1, V2
> + BMC_EVENT_ACK (V1)
> Arguments:
> Args 0: Bits in the BMC status byte (mailbox data
> register 15) to ack
> @@ -598,9 +634,7 @@ Command:
> supplied in mailbox register 15.
>
> Command:
> - MARK_WRITE_ERASED
> - Implemented in Versions:
> - V2
> + MARK_WRITE_ERASED (V2)
> Arguments:
> V2:
> Args 0-1: Window offset to erase (blocks)
> @@ -617,6 +651,31 @@ Command:
> number is the number of blocks of the active window to erase
> starting at offset. If the offset + number exceeds the size of
> the active window then the command must not succeed.
> +
> +Command:
> + GET_FLASH_NAME (V3)
> + Arguments:
> + Args 0: Flash ID
> + Response:
> + Args 0-10: Flash Name / UID
Hopefully 11 bytes is enough. Well, 10 I guess - should we specify that the
value at index 10 be '\0'? I feel like not requiring that could lead to
undesirable implementations.
I guess we don't want to go down the path of continued responses or magic
windows to accommodate larger names?
> + Notes:
> + Describes a flash with some kind of identifier useful to the
> + host system. This is typically a null-padded string.
> +
> +Command:
> + MARK_LOCKED (V3)
> + Arguments:
> + Args 0-1: Flash offset to lock (blocks)
> + Args 2-3: Number to lock at offset (blocks)
I guess it's hard to get away from using block-sized granuality. Do we
currently have any partitions that we might lock that are less than a block
size? Or are all partitions expected to be block-size aligned?
Cheers,
Andrew
> + Args 4: Flash ID
> + Response:
> + -
> + Notes:
> + Lock an area of flash so that the host can't mark it dirty or
> + erased. If the requested area is within the current window and
> + that area is currently marked dirty or erased then this command
> + must fail.
> +
> ```
>
> ### BMC Events in Detail:
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