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Thu, 24 Jun 2021 03:16:44 -0700 (PDT) X-Google-Smtp-Source: ABdhPJwnWrym/EqEw5YZ0CBNz49K6QlT9KNGnv4qD5MZLnPtIAu54vfxZGy4aM4hB9f83xB+XurK6w== X-Received: by 2002:a17:906:528b:: with SMTP id c11mr4601373ejm.156.1624529804549; Thu, 24 Jun 2021 03:16:44 -0700 (PDT) Received: from dresden.str.redhat.com ([2a02:908:1e46:160:b272:8083:d5:bc7d]) by smtp.gmail.com with ESMTPSA id j1sm1604831edl.80.2021.06.24.03.16.43 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 24 Jun 2021 03:16:44 -0700 (PDT) Subject: Re: [PATCH v2 3/6] block: Clarify that @bytes is no limit on *pnum To: Vladimir Sementsov-Ogievskiy , qemu-block@nongnu.org References: <20210623150143.188184-1-mreitz@redhat.com> <20210623150143.188184-4-mreitz@redhat.com> From: Max Reitz Message-ID: <3c4ae0ed-a7d7-b6c3-1785-f6e4a9efec7e@redhat.com> Date: Thu, 24 Jun 2021 12:16:43 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.10.1 MIME-Version: 1.0 In-Reply-To: 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-Transfer-Encoding: 8bit Content-Language: en-US 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: -31 X-Spam_score: -3.2 X-Spam_bar: --- X-Spam_report: (-3.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.373, 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_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, 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: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kevin Wolf , Eric Blake , qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 24.06.21 11:15, Vladimir Sementsov-Ogievskiy wrote: > 23.06.2021 18:01, Max Reitz wrote: >> .bdrv_co_block_status() implementations are free to return a *pnum that >> exceeds @bytes, because bdrv_co_block_status() in block/io.c will clamp >> *pnum as necessary. >> >> On the other hand, if drivers' implementations return values for *pnum >> that are as large as possible, our recently introduced block-status >> cache will become more effective. >> >> So, make a note in block_int.h that @bytes is no upper limit for *pnum. >> >> Suggested-by: Eric Blake >> Signed-off-by: Max Reitz >> --- >>   include/block/block_int.h | 5 +++++ >>   1 file changed, 5 insertions(+) >> >> diff --git a/include/block/block_int.h b/include/block/block_int.h >> index fcb599dd1c..f85b96ed99 100644 >> --- a/include/block/block_int.h >> +++ b/include/block/block_int.h >> @@ -347,6 +347,11 @@ struct BlockDriver { >>        * clamped to bdrv_getlength() and aligned to request_alignment, >>        * as well as non-NULL pnum, map, and file; in turn, the driver >>        * must return an error or set pnum to an aligned non-zero value. >> +     * >> +     * Note that @bytes is just a hint on how big of a region the >> +     * caller wants to inspect.  It is not a limit on *pnum. >> +     * Implementations are free to return larger values of *pnum if >> +     * doing so does not incur a performance penalty. > > Worth mention that the cache will benefit of it? Oh, right, absolutely.  Like so: "block/io.c's bdrv_co_block_status() will clamp *pnum before returning it to its caller, but it itself can still make use of the unclamped *pnum value.  Specifically, the block-status cache for protocol nodes will benefit from storing as large a region as possible." ? Max