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bh=0R5QGBaLoowQUhwx37lPPp4B+Y0kLBA7iDJi4GPKSuw=; b=LMVX7wguLFC7mFSn7KnzsSQa6TmSwG4gkS3gOHnj/EOXLcLUX3CQzccPtQvUzVrWpOJNuf 6yfPTZ+1yRgwsHnohKA/aQslHfhQ2UY9VTRihbPJh8iZjXuW/fsmF8hmExShV4i/ZoR/gJ kwDHMeS/1znKy1dFu43a2/qOUoMg2Fw= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-279-_mQt-ffNOSuBrCiM5Gv5wg-1; Thu, 11 Mar 2021 07:11:28 -0500 X-MC-Unique: _mQt-ffNOSuBrCiM5Gv5wg-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 56014100C618; Thu, 11 Mar 2021 12:11:27 +0000 (UTC) Received: from dresden.str.redhat.com (ovpn-112-143.ams2.redhat.com [10.36.112.143]) by smtp.corp.redhat.com (Postfix) with ESMTPS id BFCFC196E3; Thu, 11 Mar 2021 12:11:25 +0000 (UTC) Subject: Re: [PATCH] block: Introduce zero-co:// and zero-aio:// To: Fam Zheng References: <20210310141752.5113-1-fam@euphon.net> <8aeb8f65-d467-958d-6434-152d758c3d7a@redhat.com> From: Max Reitz Message-ID: Date: Thu, 11 Mar 2021 13:11:23 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.0 MIME-Version: 1.0 In-Reply-To: X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=mreitz@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=216.205.24.124; envelope-from=mreitz@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -29 X-Spam_score: -3.0 X-Spam_bar: --- X-Spam_report: (-3.0 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.25, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Fam Zheng , Kevin Wolf , =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= , qemu-devel@nongnu.org, qemu-block@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 10.03.21 17:35, Fam Zheng wrote: > > > On Wed, 10 Mar 2021 at 15:02, Max Reitz > wrote: > > On 10.03.21 15:17, fam@euphon.net wrote: > > From: Fam Zheng > > > > > null-co:// has a read-zeroes=off default, when used to in security > > analysis, this can cause false positives because the driver doesn't > > write to the read buffer. > > > > null-co:// has the highest possible performance as a block driver, so > > let's keep it that way. This patch introduces zero-co:// and > > zero-aio://, largely similar with null-*://, but have > read-zeroes=on by > > default, so it's more suitable in cases than null-co://. > > > > Signed-off-by: Fam Zheng > > > --- > >   block/null.c | 91 > ++++++++++++++++++++++++++++++++++++++++++++++++++++ > >   1 file changed, 91 insertions(+) > > You’ll also need to make all tests that currently use null-{co,aio} use > zero-{co,aio}, because none of those are performance tests (as far as > I’m aware), so they all want a block driver that memset()s. > > (And that’s basically my problem with this approach; nearly everyone > who > uses null-* right now probably wants read-zeroes=on, so keeping null-* > as it is means all of those users should be changed.  Sure, they were > all wrong to not specify read-zeroes=on, but that’s how it is.  So > while > technically this patch is a compatible change, in contrast to the one > making read-zeroes=on the default, in practice it absolutely isn’t.) > > Another problem arising from that is I can imagine that some > distributions might have whitelisted null-co because many iotests > implicitly depend on it, so the iotests will fail if they aren’t > whitelisted.  Now they’d need to whitelist zero-co, too.  Not > impossible, sure, but it’s work that would need to be done. > > > My problem is this: We have a not-really problem, namely “valgrind and > other tools complain”.  Philippe (and I guess me on IRC) proposed a > simple solution whose only drawbacks (AFAIU) are: > > (1) When writing performance tests, you’ll then need to explicitly > specify read-zeroes=off.  Existing performance tests using null-co will > probably wrongly show degredation.  (Are there such tests, though?) > > (2) null will not quite conform to its name, strictly speaking. > Frankly, I don’t know who’d care other than people who write those > performance tests mentioned in (1).  I know I don’t care. > > (Technically: (3) We should look into all qemu tests that use > null-co to > see whether they test performance.  In practice, they don’t, so we > don’t > need to.) > > So AFAIU change the read-zeroes default would affect very few > people, if > any.  I see you care about (2), and you’re the maintainer, so I can’t > say that there is no problem.  But it isn’t a practical one. > > So on the practical front, we get a small benefit (tools won’t > complain) > for a small drawback (having to remember read-zeroes=off for > performance > tests). > > > Now you propose a change that has the following drawbacks, as I see it: > > (1) All non-performance tests using null-* should be changed to zero-*. >   And those are quite a few tests, so this is some work. > > (2) Distributions that have whitelisted null-co now should whitelist > zero-co, too. > > Not impossible, but I consider these much bigger practical drawbacks > than making read-zeroes=on the default.  It’s actual work that must be > done.  To me personally, these drawbacks far outweigh the benefit of > having valgrind and other tools not complain. > > > I can’t stop you from updating this patch to do (1), but it doesn’t > make > sense to me from a practical perspective.  It just doesn’t seem > worth it > to me. > > > But using null-co:// in tests is not wrong, the problem is the caller > failed to initialize its buffer. Then I don’t see why we’d need zero-co://. Max