Re: [Qemu-devel] [PATCH 0/7] VNVRAM persistent storage

[Stefan Berger <stefanb@linux.vnet.ibm.com>]

On 05/24/2013 05:59 AM, Stefan Hajnoczi wrote:
> On Thu, May 23, 2013 at 01:44:40PM -0400, Corey Bryant wrote:
>> This patch series provides VNVRAM persistent storage support that
>> QEMU can use internally.  The initial target user will be a software
>> vTPM 1.2 backend that needs to store keys in VNVRAM and be able to
>> reboot/migrate and retain the keys.
>>
>> This support uses QEMU's block driver to provide persistent storage
>> by reading/writing VNVRAM data from/to a drive image.  The VNVRAM
>> drive image is provided with the -drive command line option just like
>> any other drive image and the vnvram_create() API will find it.
>>
>> The APIs allow for VNVRAM entries to be registered, one at a time,
>> each with a maximum blob size.  Entry blobs can then be read/written
>> from/to an entry on the drive.  Here's an example of usage:
>>
>> VNVRAM *vnvram;
>> int errcode
>> const VNVRAMEntryName entry_name;
>> const char *blob_w = "blob data";
>> char *blob_r;
>> uint32_t blob_r_size;
>>
>> vnvram = vnvram_create("drive-ide0-0-0", false, &errcode);
>> strcpy((char *)entry_name, "first-entry");
> VNVRAMEntryName is very prone to buffer overflow.  I hope real code
> doesn't use strcpy().  The cast is ugly, please don't hide the type.
>
>> vnvram_register_entry(vnvram, &entry_name, 1024);
>> vnvram_write_entry(vnvram, &entry_name, (char *)blob_w, strlen(blob_w)+1);
>> vnvram_read_entry(vnvram, &entry_name, &blob_r, &blob_r_size);
> These are synchronous functions.  If I/O is involved then this is a
> problem: QEMU will be blocked waiting for host I/O to complete and the
> big QEMU lock is held.  This can cause poor guest interactivity and poor
> scalability because vcpus cannot make progress, neither can the QEMU
> monitor respond.

The vTPM is going to run as a thread and will have to write state blobs 
into a bdrv. The above functions will typically be called from this 
thead. When I originally wrote the code, the vTPM thread could not write 
the blobs into bdrv directly, so I had to resort to sending a message to 
the main QEMU thread to write the data to the bdrv. How else could we do 
this?

    Stefan