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Thu, 25 Nov 2021 16:33:57 +0000 (UTC) Received: from localhost (unknown [10.39.193.107]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 648AE60BF4; Thu, 25 Nov 2021 16:33:54 +0000 (UTC) From: Cornelia Huck To: Yishai Hadas , alex.williamson@redhat.com Subject: Re: [PATCH] vfio/migration: Improve to read/write full migration region per chunk In-Reply-To: <20211111095040.183977-1-yishaih@nvidia.com> Organization: Red Hat GmbH References: <20211111095040.183977-1-yishaih@nvidia.com> User-Agent: Notmuch/0.33.1 (https://notmuchmail.org) Date: Thu, 25 Nov 2021 17:33:52 +0100 Message-ID: <871r34jitr.fsf@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=cohuck@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain Received-SPF: pass client-ip=170.10.133.124; envelope-from=cohuck@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -34 X-Spam_score: -3.5 X-Spam_bar: --- X-Spam_report: (-3.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.7, 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_H2=-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: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: kwankhede@nvidia.com, yishaih@nvidia.com, maorg@nvidia.com, qemu-devel@nongnu.org, jgg@nvidia.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Thu, Nov 11 2021, Yishai Hadas wrote: > Upon reading/writing the migration data there is no real reason to limit > the read/write system call from the file to be 8 bytes. > > In addition, there is no reason to depend on the file offset alignment. > The offset is just some logical value which depends also on the region > index and has nothing to do with the amount of data that can be > accessed. > > Move to read/write the full region size per chunk, this reduces > dramatically the number of the systems calls that are needed and improve > performance. > > Signed-off-by: Yishai Hadas > --- > hw/vfio/migration.c | 36 ++---------------------------------- > 1 file changed, 2 insertions(+), 34 deletions(-) > > diff --git a/hw/vfio/migration.c b/hw/vfio/migration.c > index ff6b45de6b5..b5f310bb831 100644 > --- a/hw/vfio/migration.c > +++ b/hw/vfio/migration.c > @@ -62,40 +62,8 @@ static inline int vfio_mig_access(VFIODevice *vbasedev, void *val, int count, > return 0; > } > > -static int vfio_mig_rw(VFIODevice *vbasedev, __u8 *buf, size_t count, > - off_t off, bool iswrite) > -{ > - int ret, done = 0; > - __u8 *tbuf = buf; > - > - while (count) { > - int bytes = 0; > - > - if (count >= 8 && !(off % 8)) { > - bytes = 8; > - } else if (count >= 4 && !(off % 4)) { > - bytes = 4; > - } else if (count >= 2 && !(off % 2)) { > - bytes = 2; > - } else { > - bytes = 1; > - } > - > - ret = vfio_mig_access(vbasedev, tbuf, bytes, off, iswrite); > - if (ret) { > - return ret; > - } > - > - count -= bytes; > - done += bytes; > - off += bytes; > - tbuf += bytes; > - } > - return done; > -} > - > -#define vfio_mig_read(f, v, c, o) vfio_mig_rw(f, (__u8 *)v, c, o, false) > -#define vfio_mig_write(f, v, c, o) vfio_mig_rw(f, (__u8 *)v, c, o, true) > +#define vfio_mig_read(f, v, c, o) vfio_mig_access(f, (__u8 *)v, c, o, false) > +#define vfio_mig_write(f, v, c, o) vfio_mig_access(f, (__u8 *)v, c, o, true) > > #define VFIO_MIG_STRUCT_OFFSET(f) \ > offsetof(struct vfio_device_migration_info, f) I've been looking at this patch and it doesn't look wrong to me in any obvious way. The question is: why had it been done like that in the first place? I dug through the old mailing list discussions, and it seems it had been introduced in v26, but I don't see any explanation for that. Kirti, do you remember why you added this?