From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:47021) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1V74wW-0008Ft-19 for qemu-devel@nongnu.org; Wed, 07 Aug 2013 10:42:34 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1V74wQ-0006b9-16 for qemu-devel@nongnu.org; Wed, 07 Aug 2013 10:42:27 -0400 Received: from mx1.redhat.com ([209.132.183.28]:33975) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1V74wP-0006aw-Oh for qemu-devel@nongnu.org; Wed, 07 Aug 2013 10:42:21 -0400 Received: from int-mx12.intmail.prod.int.phx2.redhat.com (int-mx12.intmail.prod.int.phx2.redhat.com [10.5.11.25]) by mx1.redhat.com (8.14.4/8.14.4) with ESMTP id r77EgKAS016155 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=OK) for ; Wed, 7 Aug 2013 10:42:20 -0400 Date: Wed, 7 Aug 2013 10:42:17 -0400 From: Jeff Cody Message-ID: <20130807144217.GC22280@localhost.localdomain> References: <595d71cf1cd25ac9deff521bceb82b8c3da888d0.1375325971.git.jcody@redhat.com> <20130801134441.GD2387@stefanha-thinkpad.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130801134441.GD2387@stefanha-thinkpad.redhat.com> Subject: Re: [Qemu-devel] [PATCH v2 2/9] block: vhdx - add header update capability. List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Stefan Hajnoczi Cc: kwolf@redhat.com, famz@redhat.com, qemu-devel@nongnu.org On Thu, Aug 01, 2013 at 03:44:41PM +0200, Stefan Hajnoczi wrote: > On Wed, Jul 31, 2013 at 11:23:47PM -0400, Jeff Cody wrote: > > @@ -212,6 +242,24 @@ bool vhdx_checksum_is_valid(uint8_t *buf, size_t size, int crc_offset) > > > > > > /* > > + * This generates a UUID that is compliant with the MS GUIDs used > > + * in the VHDX spec (and elsewhere). > > + * > > + * We can do this with uuid_generate if uuid.h is present, > > + * however not all systems have uuid and the generation is > > + * pretty straightforward for the DCE + random usage case > > The talk of uuid.h not being available confuses me. The code has no > #ifdef and we do not define uuid_generate() ourselves. > > Is this an outdated comment? > > > +/* Update the VHDX headers > > + * > > + * This follows the VHDX spec procedures for header updates. > > + * > > + * - non-current header is updated with largest sequence number > > + */ > > +static int vhdx_update_header(BlockDriverState *bs, BDRVVHDXState *s, bool rw) > > "rw" should be named "generate_file_write_guid". If you call > vhdx_update_header() again later you do not need to update FileWriteGuid > again according to the spec. There's probably no harm in doing so but > the spec explicitly says "If this is the first header update within the > session, use a new value for the FileWriteGuid". > > This means that future calls to vhdx_update_header() in the same session > should set "generate_file_write_guid" to false. Therefore "rw" is not > the right name. > Reading through your comments again made me realize there is a misunderstanding (a good sign in itself that 'rw' should be renamed). The 'rw' is not to generate the FileWriteGuid, but rather the DataWriteGuid. FileWriteGuid is copied from our session_guid, so it is only generated once and used through the entire open session. The s->session_guid field is generated upon vhdx_open(), for usage if the file is opened BDRV_O_RDWR. The DataWriteGuid is generated before any user-visible writes (anything observable via a virtual disk read). And that DataWriteGuid is what the 'rw' field is used for, and the function vhdx_user_visible_write() handles calling vhdx_update_headers() with that field set to true. I'll rename 'rw' to 'generate_data_write_guid'. > > +{ > > + int ret = 0; > > + int hdr_idx = 0; > > + uint64_t header_offset = VHDX_HEADER1_OFFSET; > > + > > + VHDXHeader *active_header; > > + VHDXHeader *inactive_header; > > + VHDXHeader header_le; > > + uint8_t *buffer; > > + > > + /* operate on the non-current header */ > > + if (s->curr_header == 0) { > > + hdr_idx = 1; > > + header_offset = VHDX_HEADER2_OFFSET; > > + } > > + > > + active_header = s->headers[s->curr_header]; > > + inactive_header = s->headers[hdr_idx]; > > + > > + inactive_header->sequence_number = active_header->sequence_number + 1; > > + > > + /* a new file guid must be generate before any file write, including > > s/generate/generated/ > > > + * headers */ > > + memcpy(&inactive_header->file_write_guid, &s->session_guid, > > + sizeof(MSGUID)); > > + > > + /* a new data guid only needs to be generate before any guest-visible > > + * writes, so update it if the image is opened r/w. */ > > + if (rw) { > > + vhdx_guid_generate(&inactive_header->data_write_guid); > > + } > > + > > + /* the header checksum is not over just the packed size of VHDXHeader, > > + * but rather over the entire 'reserved' range for the header, which is > > + * 4KB (VHDX_HEADER_SIZE). */ > > + > > + buffer = qemu_blockalign(bs, VHDX_HEADER_SIZE); > > + /* we can't assume the extra reserved bytes are 0 */ > > + ret = bdrv_pread(bs->file, header_offset, buffer, VHDX_HEADER_SIZE); > > + if (ret < 0) { > > + goto exit; > > + } > > + /* overwrite the actual VHDXHeader portion */ > > + memcpy(buffer, inactive_header, sizeof(VHDXHeader)); > > + inactive_header->checksum = vhdx_update_checksum(buffer, > > + VHDX_HEADER_SIZE, 4); > > + vhdx_header_le_export(inactive_header, &header_le); > > + bdrv_pwrite_sync(bs->file, header_offset, &header_le, sizeof(VHDXHeader)); > > This function can fail and it's a good indicator that future I/Os will > also be in trouble. Please propagate the error return. > > > @@ -801,8 +955,7 @@ static int vhdx_open(BlockDriverState *bs, QDict *options, int flags) > > } > > > > if (flags & BDRV_O_RDWR) { > > - ret = -ENOTSUP; > > - goto fail; > > + vhdx_update_headers(bs, s, false); > > Error handling is missing. At this point it looks like we cannot write > to the file. Propagating the error seems reasonable.