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Re: [Qemu-devel] Design of the blobstore [API of the NVRAM]

From: Stefan Berger
Subject: Re: [Qemu-devel] Design of the blobstore [API of the NVRAM]
Date: Fri, 16 Sep 2011 07:36:31 -0400
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On 09/16/2011 06:35 AM, Stefan Hajnoczi wrote:
On Thu, Sep 15, 2011 at 08:34:55AM -0400, Stefan Berger wrote:
On 09/15/2011 07:17 AM, Stefan Hajnoczi wrote:

Everything is kind of changing now. But here's what I have right now:

     tb->s.tpm_ltpms->nvram = nvram_setup(tpm_ltpms->drive_id,&errcode);
     if (!tb->s.tpm_ltpms->nvram) {
         fprintf(stderr, "Could not find nvram.\n");
         return errcode;


     rc = nvram_start(tpm_ltpms->nvram, fail_on_encrypted_drive);

Above first sets up the NVRAM using the drive's id. That is the
-tpmdev ...,nvram=my-bs, parameter. This establishes the NVRAM.
Subsequently the blobs to be written into the NVRAM are registered.
The nvram_start then reconciles the registered NVRAM blobs with
those found on disk and if everything fits together the result is
'rc = 0' and the NVRAM is ready to go. Other devices can than do the
same also with the same NVRAM or another NVRAM. (NVRAM now after
renaming from blobstore).

Reading from NVRAM in case of the TPM is a rare event. It happens in
the context of QEMU's main thread:

     if (nvram_read_data(tpm_ltpms->nvram,
                         0, NULL, NULL) ||
                         0, NULL, NULL))
         tpm_ltpms->had_fatal_error = true;

Above reads the data of 2 blobs synchronously. This happens during startup.

Writes are depending on what the user does with the TPM. He can
trigger lots of updates to persistent state if he performs certain
operations, i.e., persisting keys inside the TPM.

     rc = nvram_write_data(tpm_ltpms->nvram,
                           what, tsb->buffer, tsb->size,
                           VNVRAM_ASYNC_F | VNVRAM_WAIT_COMPLETION_F,
                           NULL, NULL);

Above writes a TPM blob into the NVRAM. This is triggered by the TPM
thread and notifies the QEMU main thread to write the blob into
NVRAM. I do this synchronously at the moment not using the last two
parameters for callback after completion but the two flags. The
first is to notify the main thread the 2nd flag is to wait for the
completion of the request (using a condition internally).

Here are the protos:

VNVRAM *nvram_setup(const char *drive_id, int *errcode);

int nvram_start(VNVRAM *, bool fail_on_encrypted_drive);

int nvram_register_blob(VNVRAM *bs, enum NVRAMEntryType type,
                         unsigned int maxsize);

unsigned int nvram_get_totalsize(VNVRAM *bs);
unsigned int nvram_get_totalsize_kb(VNVRAM *bs);

typedef void NVRAMRWFinishCB(void *opaque, int errcode, bool is_write,
                              unsigned char **data, unsigned int len);

int nvram_write_data(VNVRAM *bs, enum NVRAMEntryType type,
                      const unsigned char *data, unsigned int len,
                      int flags, NVRAMRWFinishCB cb, void *opaque);

As said, things are changing right now, so this is to give an impression...
Thanks, these details are interesting.  I interpreted the blobstore as a
key-value store but these example show it as a stream.  No IDs or
IMO the only stuff we should store there are blobs retrievable via keys (names) -- no metadata.

offsets are given, the reads are just performed in order and move
through the NVRAM.  If it stays this simple then bdrv_*() is indeed a
There are no offsets because there's some intelligence in the blobstore/NVRAM that lays out the data onto the disk. That's why there is a directory. This in turn allows the sharing of the NVRAM by possibly multiple drivers where the driver-writer doesn't need to lay out the blobs him-/herself.
natural way to do this - although my migration point remains since this
feature adds a new requirement for shared storage when it would be
pretty easy to put this stuff in the vm data stream (IIUC the TPM NVRAM
is relatively small?).
It's just another image. You have to treat it like the VM's 'main' image. Block migration works fine on it just that it may be difficult for a user to handle migration flags if one image is on shared storage and the other isn't.


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