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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Spamd-Result: default: False [-4.29 / 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]; XM_UA_NO_VERSION(0.01)[]; ARC_NA(0.00)[]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; TO_MATCH_ENVRCPT_ALL(0.00)[]; FUZZY_BLOCKED(0.00)[rspamd.com]; FROM_HAS_DN(0.00)[]; RCPT_COUNT_THREE(0.00)[4]; MIME_TRACE(0.00)[0:+]; FROM_EQ_ENVFROM(0.00)[]; RCVD_TLS_ALL(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; RCVD_VIA_SMTP_AUTH(0.00)[]; MID_RHS_MATCH_FROM(0.00)[]; TO_DN_SOME(0.00)[] Received-SPF: pass client-ip=2a07:de40:b251:101:10:150:64:1; envelope-from=lma@suse.de; helo=smtp-out1.suse.de X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 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, 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 在 2025-04-18 23:34，Stefan Hajnoczi 写道： > On Thu, Apr 17, 2025 at 07:27:26PM +0800, lma wrote: >> Hi all, >> >> In case of SCSI passthrough, If the Block Limits VPD device response >> is >> absent from hardware, QEMU handles it. >> >> There are several variables involved in this process as follows: >> * The bl.max_transfer >> * The bl.max_iov that is associated with IOV_MAX. >> * The bl.max_hw_iov that is associated with the max_segments sysfs >> setting >> for the relevant block device on the host. >> * The bl.max_hw_transfer that is associated with the BLKSECTGET ioctl, >> in >> other words related to the current max_sectors_kb sysfs setting of the >> relevant block device on the host. >> >> Then take the smallest value and return it as the result of "Maximum >> transfer length" after relevant calculation, See: >> static uint64_t calculate_max_transfer(SCSIDevice *s) >> { >> uint64_t max_transfer = blk_get_max_hw_transfer(s->conf.blk); >> uint32_t max_iov = blk_get_max_hw_iov(s->conf.blk); >> >> assert(max_transfer); >> max_transfer = MIN_NON_ZERO(max_transfer, >> max_iov * qemu_real_host_page_size()); >> >> return max_transfer / s->blocksize; >> } >> >> >> However, due to the limitation of IOV_MAX, no matter how powerful the >> host >> scsi hardware is, the "Maximum transfer length" that qemu emulates in >> bl vpd >> page is capped at 8192 sectors in case of 4kb page size and 512 bytes >> logical block size. >> For example： >> host:~ # sg_vpd -p bl /dev/sda >> Block limits VPD page (SBC) >> ...... >> Maximum transfer length: 0 blocks [not reported] >> ...... >> >> >> host:~ # cat /sys/class/block/sda/queue/max_sectors_kb >> 16384 >> >> host:~ # cat /sys/class/block/sda/queue/max_hw_sectors_kb >> 32767 >> >> host:~ # cat /sys/class/block/sda/queue/max_segments >> 4096 >> >> >> Expected: >> guest:~ # sg_vpd -p bl /dev/sda >> Block limits VPD page (SBC) >> ...... >> Maximum transfer length: 0x8000 >> ...... >> >> guest:~ # cat /sys/class/block/sda/queue/max_sectors_kb >> 16384 >> >> guest:~ # cat /sys/class/block/sda/queue/max_hw_sectors_kb >> 32767 >> >> >> Actual: >> guest:~ # sg_vpd -p bl /dev/sda >> Block limits VPD page (SBC) >> ...... >> Maximum transfer length: 0x2000 >> ...... >> >> guest:~ # cat /sys/class/block/sda/queue/max_sectors_kb >> 4096 >> >> guest:~ # cat /sys/class/block/sda/queue/max_hw_sectors_kb >> 32767 >> >> >> It seems the current design logic is not able to fully utilize the >> performance of the scsi hardware. I have two questions: >> 1. I'm curious that is it reasonable to drop the logic about IOV_MAX >> limitation, directly use the return value of BLKSECTGET as the maximum >> transfer length when QEMU emulates the block limit page of scsi vpd? >> If we doing so, we will have maximum transfer length in the guest >> that is >> consistent with the capabilities of the host hardware。 >> >> 2. Besides, Assume I set a value(eg: 8192 in kb) to max_sectors_kb in >> guest >> which doesn't exceed the capabilities of the host hardware(eg: 16384 >> in kb) >> but exceeds the limit(eg: 4096 in kb) caused by IOV_MAX, >> Any risks in readv()/writev() of raw-posix? > > Not a definitive answer, but just something to encourage discussion: > > In theory IOV_MAX should not be factored into the Block Limits VPD page > Maximum Transfer Length field because there is already a HBA limit on > the maximum number of segments. For example, virtio-scsi has a seg_max > Configuration Space field that guest drivers honor independently of > Maximum Transfer Length. > > However, I can imagine why MAX_IOV needs to be factored in: > > 1. The maximum number of segments might be hardcoded in guest drivers > for some SCSI HBAs and QEMU has no way of exposing MAX_IOV to the > guest in that case. > > 2. Guest physical RAM addresses translate to host virtual memory. That > means 1 segment as seen by the guest might actually require multiple > physical DMA segments on the host. A conservative calculation that > assumes the worst-case 1 iovec per 4 KB memory page prevents the > host maximum segments limit (note this is not the Maximum Transfer > Length limit!) from being exceeded. > > So there seem to be at least two problems here. If you relax the > calculation there will be corner cases that break because the guest can > send too many segments. > > Stefan The maximum allowed value for /sys/class/block//queue/max_sectors_kb in guest os depends on the smaller of below two items in guest os: the "maximum transfer length of block limits VPD page" and the "/sys/class/block//queue/max_hw_sectors_kb". The "seg_max Configuration Space field" in hw/scsi/virtio-scsi.c: static const Property virtio_scsi_properties[] = { ... DEFINE_PROP_UINT32("max_sectors", VirtIOSCSI, parent_obj.conf.max_sectors, 0xFFFF), ... }; This field determines the value of max_hw_sectors_kb in sysfs in guest os, Eg: In case of Logical block size 512 bytes, 0xFFFF sectors means: max_hw_sectors_kb = 0xFFFF/2 = 32767, I believe many users will keep this default value when using virtio-scsi, rather than customizing it. But by the current design and affected by IOV_MAX, the upper limit of /sys/class/block//queue/max_sectors_kb is 4096 for SCSI passthrough scenario in case of 4kb page size and 512 bytes logical block size. Therefore, the gap between the upper limit of max_sectors_kb and the max_hw_sectors_kb is very large. I think this design logic is a bit strange. Anyway, Thanks for the detailed answer, Lin