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

[Corey Bryant]

On 05/24/2013 08:36 AM, Stefan Hajnoczi wrote:
> On Fri, May 24, 2013 at 08:13:27AM -0400, Stefan Berger wrote:
> > 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?
>
> How else: use asynchronous APIs like bdrv_aio_writev() or the coroutine
> versions (which eliminate the need for callbacks) like bdrv_co_writev().
>
> I'm preparing patches that allow the QEMU block layer to be used safely
> outside the QEMU global mutex.  Once this is possible it would be okay
> to use synchronous methods.

Ok thanks.  I'll use aio APIs next time around.  Just to be clear, does 
"eliminating the callback" mean I don't have to use a bottom-half if I 
use coroutine reads/writes?

--
Regards,
Corey Bryant