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Wed, 8 Jan 2025 14:37:18 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id LicrEp6NfmdLTQAAD6G6ig (envelope-from ); Wed, 08 Jan 2025 14:37:18 +0000 From: Fabiano Rosas To: Peter Xu Cc: qemu-devel@nongnu.org, Thomas Huth Subject: Re: [PATCH 3/7] migration: Document the effect of vmstate_info_nullptr In-Reply-To: References: <20250107195025.9951-1-farosas@suse.de> <20250107195025.9951-4-farosas@suse.de> <87frlt4eli.fsf@suse.de> Date: Wed, 08 Jan 2025 11:37:15 -0300 Message-ID: <87a5c14bj8.fsf@suse.de> MIME-Version: 1.0 Content-Type: text/plain X-Spamd-Result: default: False [-4.30 / 50.00]; BAYES_HAM(-3.00)[100.00%]; NEURAL_HAM_LONG(-1.00)[-1.000]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; FROM_HAS_DN(0.00)[]; TO_MATCH_ENVRCPT_ALL(0.00)[]; FUZZY_BLOCKED(0.00)[rspamd.com]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; MIME_TRACE(0.00)[0:+]; ARC_NA(0.00)[]; RCVD_TLS_ALL(0.00)[]; RCPT_COUNT_THREE(0.00)[3]; MISSING_XM_UA(0.00)[]; FROM_EQ_ENVFROM(0.00)[]; TO_DN_SOME(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; RCVD_VIA_SMTP_AUTH(0.00)[]; MID_RHS_MATCH_FROM(0.00)[]; DBL_BLOCKED_OPENRESOLVER(0.00)[imap1.dmz-prg2.suse.org:helo, suse.de:mid, suse.de:email] Received-SPF: pass client-ip=2a07:de40:b251:101:10:150:64:2; envelope-from=farosas@suse.de; helo=smtp-out2.suse.de X-Spam_score_int: -43 X-Spam_score: -4.4 X-Spam_bar: ---- X-Spam_report: (-4.4 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_MED=-2.3, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Peter Xu writes: > On Wed, Jan 08, 2025 at 10:31:05AM -0300, Fabiano Rosas wrote: >> Peter Xu writes: >> >> > On Tue, Jan 07, 2025 at 04:50:21PM -0300, Fabiano Rosas wrote: >> >> The migration stream lacks magic numbers at some key points. It's easy >> >> to mis-parse data. Unfortunately, the VMS_NULLPTR_MARKER continues >> >> with the trend. A '0' byte is ambiguous and could be interpreted as a >> >> valid 0x30. >> >> >> >> It is maybe not worth trying to change this while keeping backward >> >> compatibility, so add some words of documentation to clarify. >> >> >> >> Signed-off-by: Fabiano Rosas >> >> --- >> >> migration/vmstate-types.c | 6 ++++++ >> >> scripts/analyze-migration.py | 9 +++++++++ >> >> 2 files changed, 15 insertions(+) >> >> >> >> diff --git a/migration/vmstate-types.c b/migration/vmstate-types.c >> >> index e83bfccb9e..08ed059f87 100644 >> >> --- a/migration/vmstate-types.c >> >> +++ b/migration/vmstate-types.c >> >> @@ -339,6 +339,12 @@ static int put_nullptr(QEMUFile *f, void *pv, size_t size, >> >> >> >> const VMStateInfo vmstate_info_nullptr = { >> >> .name = "uint64", >> > >> > Ouch.. So I overlooked this line and this explains why it didn't go via >> > VMSDFieldGeneric already. >> >> Yes, actually I overlooked as well that it should match the size of the >> data being handled in the get/put functions. >> >> My comment below is about NULL -> 0x30 that I think should instead be >> NULL -> 0x3030303030303030 so we have any chance of looking at this and >> identifying it's a NULL pointer. When we write 0x30 it might become >> confusing for people reading the scripts output that their stream has a >> bunch of '0' in the place where pointers should be. If the MAGIC number >> were more identifiable, I could change the script to output (null) or 0x0ULL. > > I suppose we can? If we want, by renaming this from "uint64" to "nullptr", > then add an entry for it in Python's vmsd_field_readers. That would be a nice alternative because it maps NULL to something, just like the actual stream does. NULL -> '0' in the stream, NULL -> nullptr in the JSON. I'll give it a try, thanks. >> >> We also don't really have the concept of a pointer, which I suspect >> might be the real reason behind all this mess. So we'll see: >> >> 0x30 >> 0x30 >> { >> .some >> .struct >> .here >> } >> 0x30 >> >> So all this patch was trying to do is document this situation somehow. > > Yes, more docs makes sense, though just to mention it's nothing better here > to use a full size of pointer: firstly it's not possible I think as 32/64 > bits have different size of pointers... > > More importantly, we're not sending the pointer but a marker, in this case > the size of the real pointer doesn't really matter, IMHO. A marker would > make sense in saving some bytes when / if the array is large and sparse. Right, it's just that a larger data type allows for a more unique marker, which can be detected more reliably by the consumers of the stream. The smaller data type is too ambiguous. > > Said that, let's try above idea, maybe it's optimal as you said the script > can show things like "nullptr" (or any better name, I think that's better > than "null" at least to show it's not a real pointer, otherwise it's weird > to see any pointer in a migration stream..). Yes, the script is just presenting the data, we can use what's more informative. > >> >> > >> > Instead of below comment, do we still have chance to change this to >> > something like "uint8"? Then I suppose the script will be able to identify >> > this properly.