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bh=yFxuVXRhC9dGoUvtXt1yMEw6BspsBanUe+daanGdViQ=; b=GTx3WsiKxGOjOUunODA2BuiXWH27zhZPBLF4tLfOJzs/xlgS2xvH1iO9yLaDtpgy67AENM 6smyiiINFOtL5i4d0JtWfpF4NiZi8MInQZB0j8YuQn6f0HXqDlzVr0tc2FhIXq3dzHeBEI jyYT/HX0X8y7lPi8YyG52ZK873uL0zg= 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-8-VTjHZYs1O9iVwEXPxLZHhw-1; Thu, 11 Mar 2021 07:15:28 -0500 X-MC-Unique: VTjHZYs1O9iVwEXPxLZHhw-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id DECC980D6AA; Thu, 11 Mar 2021 12:15:27 +0000 (UTC) Received: from work-vm (ovpn-115-29.ams2.redhat.com [10.36.115.29]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 7D84260C5F; Thu, 11 Mar 2021 12:15:12 +0000 (UTC) Date: Thu, 11 Mar 2021 12:15:09 +0000 From: "Dr. David Alan Gilbert" To: Chirantan Ekbote Subject: Re: [Virtio-fs] [PATCH 07/24] DAX: virtio-fs: Add vhost-user slave commands for mapping Message-ID: References: <20210209190224.62827-1-dgilbert@redhat.com> <20210209190224.62827-8-dgilbert@redhat.com> MIME-Version: 1.0 In-Reply-To: User-Agent: Mutt/2.0.5 (2021-01-21) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=dgilbert@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=170.10.133.124; envelope-from=dgilbert@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, 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: mst@redhat.com, qemu-devel@nongnu.org, virtio-fs-list , Stefan Hajnoczi , marcandre.lureau@redhat.com, Vivek Goyal Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * Chirantan Ekbote (chirantan@chromium.org) wrote: > On Wed, Feb 10, 2021 at 4:04 AM Dr. David Alan Gilbert (git) > wrote: > > > > From: "Dr. David Alan Gilbert" > > + > > +typedef struct { > > + /* Offsets within the file being mapped */ > > + uint64_t fd_offset[VHOST_USER_FS_SLAVE_ENTRIES]; > > + /* Offsets within the cache */ > > + uint64_t c_offset[VHOST_USER_FS_SLAVE_ENTRIES]; > > + /* Lengths of sections */ > > + uint64_t len[VHOST_USER_FS_SLAVE_ENTRIES]; > > + /* Flags, from VHOST_USER_FS_FLAG_* */ > > + uint64_t flags[VHOST_USER_FS_SLAVE_ENTRIES]; > > +} VhostUserFSSlaveMsg; > > + > > Is it too late to change this? This struct allocates space for up to > 8 entries but most of the time the server will only try to set up one > mapping at a time so only 32 out of the 256 bytes in the message are > actually being used. We're just wasting time memcpy'ing bytes that > will never be used. Is there some reason this can't be dynamically > sized? Something like: > > typedef struct { > /* Number of mapping requests */ > uint16_t num_requests; > /* `num_requests` mapping requests */ > MappingRequest requests[]; > } VhostUserFSSlaveMsg; > > typedef struct { > /* Offset within the file being mapped */ > uint64_t fd_offset; > /* Offset within the cache */ > uint64_t c_offset; > /* Length of section */ > uint64_t len; > /* Flags, from VHOST_USER_FS_FLAG_* */ > uint64_t flags; > } MappingRequest; > > The current pre-allocated structure both wastes space when there are > fewer than 8 requests and requires extra messages to be sent when > there are more than 8 requests. I realize that in the grand scheme of > things copying 224 extra bytes is basically not noticeable but it just > irks me that we could fix this really easily before it gets propagated > to too many other places. So this has come out as: typedef struct { /* Offsets within the file being mapped */ uint64_t fd_offset; /* Offsets within the cache */ uint64_t c_offset; /* Lengths of sections */ uint64_t len; /* Flags, from VHOST_USER_FS_FLAG_* */ uint64_t flags; } VhostUserFSSlaveMsgEntry; typedef struct { /* Generic flags for the overall message */ uint32_t flags; /* Number of entries */ uint16_t count; /* Spare */ uint16_t align; VhostUserFSSlaveMsgEntry entries[]; } VhostUserFSSlaveMsg; which seems to work OK. I've still got a: #define VHOST_USER_FS_SLAVE_MAX_ENTRIES 8 to limit the size VhostUserFSSlaveMsg can get to. The variable length array makes the union in the reader a bit more hairy, but it's OK. Dave > Chirantan > > > -- > > 2.29.2 > > > > _______________________________________________ > > Virtio-fs mailing list > > Virtio-fs@redhat.com > > https://www.redhat.com/mailman/listinfo/virtio-fs > > > -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK