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Re: [Xen-devel] [PATCH 2/12] VTPM mini-os: posix IO layer for blkfront i

To: Samuel Thibault <samuel.thibault@xxxxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH 2/12] VTPM mini-os: posix IO layer for blkfront in mini-os
From: Matthew Fioravante <matthew.fioravante@xxxxxxxxxx>
Date: Mon, 14 Mar 2011 17:32:31 -0400
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I think I totally misunderstood you. With the last case are you just referring to how I keep having to set the callback to NULL everytime? If so I totally agree with you. It should just set it back to NULL when its done.

I thought you were talking about how I was handling writes to partial blocks and that somehow the low level blkfront functions should be doing this transparently.

On 03/14/2011 03:29 PM, Samuel Thibault wrote:
Matthew Fioravante, le Mon 14 Mar 2011 15:07:30 -0400, a écrit :
On 03/11/2011 08:05 PM, Samuel Thibault wrote:
Matthew Fioravante, le Fri 11 Mar 2011 17:34:26 -0500, a écrit :
+      /*Make sure we have write permission */
+      if(dev->info.info&   VDISK_READONLY || dev->info.mode != O_RDWR) {
Good catch, actually testing a bitfield with != is a bad idea to begin
with anyway.
Err, it's not a bitfield, in mini-os it's a {0,1,2} enum.

Could you perhaps optimize when buf is actually aligned?  That would
save a copy.

This can be done but only if in the current iteration of the loop an
entire block is being read.

Since aiocb only operates on sectors it'll
read at minimum a whole sector into buf. If buf isnt big enough to hold
the data a secondary buffer with a copy operation will have to be done.

But I expect people using that interface to tend to allocate big aligned

+      /* Write operation */
+      else {
+        /* If we're writing a partial block, we need to read the current
contents first
+         * so we don't overwrite the extra bits with garbage */
+        if(blkoff != 0 || bytes<   blocksize) {
+           aiocb.aio_cb = NULL;
Maybe blkfront_aio_cb should do it itself?  It looks odd to have to do
it when reusing an aiocb structure.

It could, but then that changes the design of aiocb. Was it supposed to
be a very low level interface for just reading and writing blocks onto
the disk?
Well, I'd say the whole blkfront itself is a low-level interrface, and
your patch actually provides a higher one :)

This particular change in the design shouldn't break anything, since
aio_cb is actually filled by blkfront itself, so it makes sense that it
cleans it since it expects it to be cleaned.

Right now you have to set aio_nbytes and aio_offset to a multiple of
sector size. This could be changed to allow variable sizes.
Alternatively 2 new fields could be added to specify which portion
inside a block to operate on.

Can you send a partial block through the xen block frontend and backend

If not we would have to queue up a read and then a write
internally when the user requests a write. Its possible some users may
not want this forced behavior of 2 operations.
That's why I wouldn't recommend aio_nbytes/offset to be allowed to
be non-multiples of the sector size. That interface is meant to be
an efficient sector-transfer interface. Your posix layer can handle
flexibility for the user.


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