From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([140.186.70.92]:42555) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1RJial-0003ep-VK for qemu-devel@nongnu.org; Fri, 28 Oct 2011 05:19:13 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1RJiak-0007bB-Lr for qemu-devel@nongnu.org; Fri, 28 Oct 2011 05:19:11 -0400 Message-ID: <4EAA7441.8010608@redhat.com> Date: Fri, 28 Oct 2011 11:22:09 +0200 From: Kevin Wolf MIME-Version: 1.0 References: <1319658678-18355-1-git-send-email-sunshine@sunshineco.com> <4EA91C16.6050908@redhat.com> <4EA982D0.10902@weilnetz.de> <4EAA6116.3050308@redhat.com> <8399BFA7-3D21-444E-BB1B-C9E9821B7922@sunshineco.com> In-Reply-To: <8399BFA7-3D21-444E-BB1B-C9E9821B7922@sunshineco.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH] Teach block/vdi about "discarded" (no longer allocated) blocks List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Eric Sunshine Cc: qemu-trivial@nongnu.org, Stefan Weil , qemu-devel@nongnu.org Am 28.10.2011 10:15, schrieb Eric Sunshine: > > On Oct 28, 2011, at 4:00 AM, Kevin Wolf wrote: > >> Am 27.10.2011 18:12, schrieb Stefan Weil: >>> Am 27.10.2011 10:53, schrieb Kevin Wolf: >>>> Am 26.10.2011 21:51, schrieb Eric Sunshine: >>>>> An entry in the VDI block map will hold an offset to the actual >>>>> block if >>>>> the block is allocated, or one of two specially-interpreted >>>>> values if >>>>> not allocated. Using VirtualBox terminology, value >>>>> VDI_IMAGE_BLOCK_FREE >>>>> (0xffffffff) represents a never-allocated block (semantically >>>>> arbitrary >>>>> content). VDI_IMAGE_BLOCK_ZERO (0xfffffffe) represents a >>>>> "discarded" >>>>> block (semantically zero-filled). block/vdi knows only about >>>>> VDI_IMAGE_BLOCK_FREE. Teach it about VDI_IMAGE_BLOCK_ZERO. >>>>> >>>>> Signed-off-by: Eric Sunshine >>>> >>>> Thanks, applied to the block branch. >>>> >>>> Kevin >>> >>> >>> Kevin, I don't want to block improvements. Nevertheless >>> I'd like to see a small modification in this patch: >>> both #defines should be implemented without a type cast. >>> Please change them or wait until Eric sends an update. >>> >>> My favorite is this: >>> >>> #define VDI_UNALLOCATED UINT32_MAX >>> #define VDI_DISCARD (VDI_UNALLOCATED - 1) >>> >>> This would also be ok: >>> >>> #define VDI_UNALLOCATED 0xffffffffU >>> #define VDI_DISCARD 0xfffffffeU >>> >>> Using the macro names and the definitions (with type cast) >>> from the original VirtualBox code would also be ok. >> >> I did see your comments, and I waited for the endianness thing to be >> answered. However, how the definition of these constants is written is >> really not a functional defect, but simply a matter of taste. It's an >> old rule that whoever does the work also decides on the details. >> >> I really think it's wasting our time if we need to discuss if a type >> cast in the constant definition is only allowed after typedefing >> uint32_t to something else like in VBox. >> >> So my preferred way is to leave the patch as it is. The code is simple >> and clear and objectively seen it won't get any better with your taste >> applied. If Eric prefers, I can update it to use 0xffffffffU, though. > > The 0xffffffffU notation has the benefit of being explicit, whereas > the ((uint32_t)~0) notation, taken from the VirtualBox source, is > somewhat magical for a reader who does not perform an automatic > ((uint32_t)~0) == 0xffffffffU conversion in his head. Consequently, > the 0xffffffffU notation might a better choice, if it's not too much > bother for you to amend the patch. I'll amend it with this change: diff --git a/block/vdi.c b/block/vdi.c index 25790c4..523a640 100644 --- a/block/vdi.c +++ b/block/vdi.c @@ -115,10 +115,10 @@ void uuid_unparse(const uuid_t uu, char *out); #define VDI_TEXT "<<< QEMU VM Virtual Disk Image >>>\n" /* A never-allocated block; semantically arbitrary content. */ -#define VDI_UNALLOCATED ((uint32_t)~0) +#define VDI_UNALLOCATED 0xffffffffU /* A discarded (no longer allocated) block; semantically zero-filled. */ -#define VDI_DISCARDED ((uint32_t)~1) +#define VDI_DISCARDED 0xfffffffeU #define VDI_IS_ALLOCATED(X) ((X) < VDI_DISCARDED)