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spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:37612 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.86_2) (envelope-from ) id 1hePlB-0006iL-NJ for qemu-devel@archiver.kernel.org; Fri, 21 Jun 2019 16:03:45 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:58216) by lists.gnu.org with esmtp (Exim 4.86_2) (envelope-from ) id 1hePiz-0005wd-NP for qemu-devel@nongnu.org; Fri, 21 Jun 2019 16:01:32 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1hePiu-0006om-8O for qemu-devel@nongnu.org; Fri, 21 Jun 2019 16:01:27 -0400 Received: from mx1.redhat.com ([209.132.183.28]:33364) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1hePir-0005dO-5e for qemu-devel@nongnu.org; Fri, 21 Jun 2019 16:01:22 -0400 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 15109C04AC69; Fri, 21 Jun 2019 20:00:50 +0000 (UTC) Received: from x1.home (ovpn-117-35.phx2.redhat.com [10.3.117.35]) by smtp.corp.redhat.com (Postfix) with ESMTP id 8F4C46090E; Fri, 21 Jun 2019 20:00:39 +0000 (UTC) Date: Fri, 21 Jun 2019 14:00:39 -0600 From: Alex Williamson To: Kirti Wankhede Message-ID: <20190621140039.3820dd5d@x1.home> In-Reply-To: <1648d75d-bda3-f5da-2684-312033ae9477@nvidia.com> References: <1561041461-22326-1-git-send-email-kwankhede@nvidia.com> <1561041461-22326-2-git-send-email-kwankhede@nvidia.com> <20190620111848.1bf70e99@x1.home> <20190621090343.3be524f5@x1.home> <1648d75d-bda3-f5da-2684-312033ae9477@nvidia.com> Organization: Red Hat MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.31]); Fri, 21 Jun 2019 20:00:59 +0000 (UTC) X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] X-Received-From: 209.132.183.28 Subject: Re: [Qemu-devel] [PATCH v4 01/13] vfio: KABI for migration interface 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: Zhengxiao.zx@Alibaba-inc.com, kevin.tian@intel.com, yi.l.liu@intel.com, cjia@nvidia.com, eskultet@redhat.com, ziye.yang@intel.com, qemu-devel@nongnu.org, cohuck@redhat.com, shuangtai.tst@alibaba-inc.com, dgilbert@redhat.com, zhi.a.wang@intel.com, mlevitsk@redhat.com, pasic@linux.ibm.com, aik@ozlabs.ru, eauger@redhat.com, felipe@nutanix.com, jonathan.davies@nutanix.com, yan.y.zhao@intel.com, changpeng.liu@intel.com, Ken.Xue@amd.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Sat, 22 Jun 2019 01:05:48 +0530 Kirti Wankhede wrote: > On 6/21/2019 8:33 PM, Alex Williamson wrote: > > On Fri, 21 Jun 2019 11:22:15 +0530 > > Kirti Wankhede wrote: > > > >> On 6/20/2019 10:48 PM, Alex Williamson wrote: > >>> On Thu, 20 Jun 2019 20:07:29 +0530 > >>> Kirti Wankhede wrote: > >>> > >>>> - Defined MIGRATION region type and sub-type. > >>>> - Used 3 bits to define VFIO device states. > >>>> Bit 0 => _RUNNING > >>>> Bit 1 => _SAVING > >>>> Bit 2 => _RESUMING > >>>> Combination of these bits defines VFIO device's state during migration > >>>> _STOPPED => All bits 0 indicates VFIO device stopped. > >>>> _RUNNING => Normal VFIO device running state. > >>>> _SAVING | _RUNNING => vCPUs are running, VFIO device is running but start > >>>> saving state of device i.e. pre-copy state > >>>> _SAVING => vCPUs are stoppped, VFIO device should be stopped, and > >>>> save device state,i.e. stop-n-copy state > >>>> _RESUMING => VFIO device resuming state. > >>>> _SAVING | _RESUMING => Invalid state if _SAVING and _RESUMING bits are set > >>>> - Defined vfio_device_migration_info structure which will be placed at 0th > >>>> offset of migration region to get/set VFIO device related information. > >>>> Defined members of structure and usage on read/write access: > >>>> * device_state: (read/write) > >>>> To convey VFIO device state to be transitioned to. Only 3 bits are used > >>>> as of now. > >>>> * pending bytes: (read only) > >>>> To get pending bytes yet to be migrated for VFIO device. > >>>> * data_offset: (read only) > >>>> To get data offset in migration from where data exist during _SAVING > >>>> and from where data should be written by user space application during > >>>> _RESUMING state > >>>> * data_size: (read/write) > >>>> To get and set size of data copied in migration region during _SAVING > >>>> and _RESUMING state. > >>>> * start_pfn, page_size, total_pfns: (write only) > >>>> To get bitmap of dirty pages from vendor driver from given > >>>> start address for total_pfns. > >>>> * copied_pfns: (read only) > >>>> To get number of pfns bitmap copied in migration region. > >>>> Vendor driver should copy the bitmap with bits set only for > >>>> pages to be marked dirty in migration region. Vendor driver > >>>> should return 0 if there are 0 pages dirty in requested > >>>> range. Vendor driver should return -1 to mark all pages in the section > >>>> as dirty > >>>> > >>>> Migration region looks like: > >>>> ------------------------------------------------------------------ > >>>> |vfio_device_migration_info| data section | > >>>> | | /////////////////////////////// | > >>>> ------------------------------------------------------------------ > >>>> ^ ^ ^ > >>>> offset 0-trapped part data_offset data_size > >>>> > >>>> Data section is always followed by vfio_device_migration_info > >>>> structure in the region, so data_offset will always be none-0. > >>>> Offset from where data is copied is decided by kernel driver, data > >>>> section can be trapped or mapped depending on how kernel driver > >>>> defines data section. If mmapped, then data_offset should be page > >>>> aligned, where as initial section which contain > >>>> vfio_device_migration_info structure might not end at offset which > >>>> is page aligned. > >>>> > >>>> Signed-off-by: Kirti Wankhede > >>>> Reviewed-by: Neo Jia > >>>> --- > >>>> linux-headers/linux/vfio.h | 71 ++++++++++++++++++++++++++++++++++++++++++++++ > >>>> 1 file changed, 71 insertions(+) > >>>> > >>>> diff --git a/linux-headers/linux/vfio.h b/linux-headers/linux/vfio.h > >>>> index 24f505199f83..274ec477eb82 100644 > >>>> --- a/linux-headers/linux/vfio.h > >>>> +++ b/linux-headers/linux/vfio.h > >>>> @@ -372,6 +372,77 @@ struct vfio_region_gfx_edid { > >>>> */ > >>>> #define VFIO_REGION_SUBTYPE_IBM_NVLINK2_ATSD (1) > >>>> > >>>> +/* Migration region type and sub-type */ > >>>> +#define VFIO_REGION_TYPE_MIGRATION (2) > >>>> +#define VFIO_REGION_SUBTYPE_MIGRATION (1) > >>>> + > >>>> +/** > >>>> + * Structure vfio_device_migration_info is placed at 0th offset of > >>>> + * VFIO_REGION_SUBTYPE_MIGRATION region to get/set VFIO device related migration > >>>> + * information. Field accesses from this structure are only supported at their > >>>> + * native width and alignment, otherwise should return error. > >>>> + * > >>>> + * device_state: (read/write) > >>>> + * To indicate vendor driver the state VFIO device should be transitioned > >>>> + * to. If device state transition fails, write to this field return error. > >>>> + * It consists of 3 bits: > >>>> + * - If bit 0 set, indicates _RUNNING state. When its reset, that indicates > >>>> + * _STOPPED state. When device is changed to _STOPPED, driver should stop > >>>> + * device before write returns. > >>>> + * - If bit 1 set, indicates _SAVING state. > >>>> + * - If bit 2 set, indicates _RESUMING state. > >>>> + * > >>>> + * pending bytes: (read only) > >>>> + * Read pending bytes yet to be migrated from vendor driver > >>>> + * > >>>> + * data_offset: (read only) > >>>> + * User application should read data_offset in migration region from where > >>>> + * user application should read data during _SAVING state or write data > >>>> + * during _RESUMING state. > >>>> + * > >>>> + * data_size: (read/write) > >>>> + * User application should read data_size to know data copied in migration > >>>> + * region during _SAVING state and write size of data copied in migration > >>>> + * region during _RESUMING state. > >>>> + * > >>>> + * start_pfn: (write only) > >>>> + * Start address pfn to get bitmap of dirty pages from vendor driver duing > >>>> + * _SAVING state. > >>>> + * > >>>> + * page_size: (write only) > >>>> + * User application should write the page_size of pfn. > >>>> + * > >>>> + * total_pfns: (write only) > >>>> + * Total pfn count from start_pfn for which dirty bitmap is requested. > >>>> + * > >>>> + * copied_pfns: (read only) > >>>> + * pfn count for which dirty bitmap is copied to migration region. > >>>> + * Vendor driver should copy the bitmap with bits set only for pages to be > >>>> + * marked dirty in migration region. > >>>> + * Vendor driver should return 0 if there are 0 pages dirty in requested > >>>> + * range. > >>>> + * Vendor driver should return -1 to mark all pages in the section as > >>>> + * dirty. > >>> > >>> Is the protocol that the user writes start_pfn/page_size/total_pfns in > >>> any order and then the read of copied_pfns is what triggers the > >>> snapshot? > >> > >> Yes. > >> > >>> Are start_pfn/page_size/total_pfns sticky such that a user > >>> can write them once and get repeated refreshes of the dirty bitmap by > >>> re-reading copied_pfns? > >> > >> Yes and that bitmap should be for given range (from start_pfn till > >> start_pfn + tolal_pfns). > >> Re-reading of copied_pfns is to handle the case where it might be > >> possible that vendor driver reserved area for bitmap < total bitmap size > >> for range (start_pfn to start_pfn + tolal_pfns), then user will have to > >> iterate till copied_pfns == total_pfns or till copied_pfns == 0 (that > >> is, there are no pages dirty in rest of the range) > > > > So reading copied_pfns triggers the data range to be updated, but the > > caller cannot assume it to be synchronous and uses total_pfns to poll > > that the update is complete? How does the vendor driver differentiate > > the user polling for the previous update to finish versus requesting a > > new update? > > > > Write on start_pfn/page_size/total_pfns, then read on copied_pfns > indicates new update, where as sequential read on copied_pfns indicates > polling for previous update. Hmm, this seems to contradict the answer to my question above where I ask if the write fields are sticky so a user can trigger a refresh via copied_pfns. Does it really make sense that this is asynchronous? Are we going to need to specify polling intervals and completion eventfds? data_size is synchronous, right? Thanks, Alex > >>> What's the advantage to returning -1 versus > >>> returning copied_pfns == total_pfns? > >>> > >> > >> If all bits in bitmap are 1, then return -1, that is, all pages in the > >> given range to be marked dirty. > >> > >> If all bits in bitmap are 0, then return 0, that is, no page to be > >> marked dirty in given range or rest of the range. > >> > >> Otherwise vendor driver should return copied_pfns == total_pfn and > >> provide bitmap for total_pfn, which means that bitmap copied for given > >> range contains information for all pages where some bits are 0s and some > >> are 1s. > > > > Given that the vendor driver can indicate zero dirty pfns and all dirty > > pfns, I interpreted copied_pfns as a synchronous operation where the > > return value could indicate the number of dirty pages within the > > requested range. > > > >>> If the user then wants to switch back to reading device migration > >>> state, is it a read of data_size that switches the data area back to > >>> making that address space available? > >> > >> No, Its not just read(data_size), before that there is a > >> read(data_offset). If Vendor driver wants to have different sub-regions > >> for device data and dirty page bitmap, vendor driver should return > >> corresponding offset on read(data_offset). > > > > The dynamic use of data_offset was not at all evident to me until I got > > further into the QEMU series. The usage model needs to be well > > specified in the linux header. I infer this behavior is such that the > > vendor driver can effectively identity map portions of device memory > > and the user will restore to the same offset. I suppose this is a > > valid approach but it seems specifically tuned to devices which allow > > full direct mapping, whereas many devices have more device memory than > > is directly map'able and state beyond simple device memory. Does this > > model unnecessarily burden such devices? It is a nice feature that > > they data range can contain both mmap'd sections and trapped sections > > and by adjusting data_offset the vendor driver can select which is > > currently being used, but we really need to formalize all these details. > > > >>> In each case, is it the user's > >>> responsibility to consume all the data provided before triggering the > >>> next data area?> For example, if I ask for a range of dirty bitmap, the > >>> vendor driver will provide that range and and clear it, such that the > >>> pages are considered clean regardless of whether the user consumed the > >>> data area. > >> > >> Yes. > >> > >>> Likewise if the user asks for data_size, that would be > >>> deducted from pending_bytes regardless of the user reading the data > >>> area. > >> > >> User should read data before deducting data_size from pending_bytes. > > > > The user deducts data_size form pending_bytes? pending_bytes is > > read-only, how does this work? > > Pending_bytes is readonly from migration region. User should read device > data while pending_bytes > 0. How User would decide to iterate or not? > User will have to check if previously read pending_bytes - data_size is > still > 0, if yes then iterate. Before iterating, its users > responsibility to read data from data section. > > > > >> From vendor driver point of view, data_size will be deducted from > >> pending_bytes once data is copied to data region. > > > > If the data is entirely from an mmap'd range, how does the vendor > > driver know when the data is copied? > > > >>> Are there any read side-effects to pending_bytes? > >> > >> No, its query to vendor driver about pending bytes yet to be > >> migrated/read from vendor driver. > >> > >>> Are there > >>> read side-effects to the data area on SAVING? > >> > >> No. > > > > So the vendor driver must make an assumption somewhere in the usage > > protocol that it's the user's responsibility, this needs to be > > specified. > > > > Ok. > > >>> Are there write > >>> side-effects on RESUMING, or is it only the write of data_size that > >>> triggers the buffer to be consumed? > >> > >> Its write(data_size) triggers the buffer to be consumed, if region is > >> mmaped, then data is already copied to region, if its trapped then > >> following writes from data_offset is data to be consumed. > >> > >>> Is it the user's responsibility to > >>> write only full "packets" on RESUMING? For example if the SAVING side > >>> provides data_size X, that full data_size X must be written to the > >>> RESUMING side, the user cannot write half of it to the data area on the > >>> RESUMING side, write data_size with X/2, write the second half, and > >>> again write X/2. IOW, the data_size "packet" is indivisible at the > >>> point of resuming. > >>> > >> > >> If source and destination are compatible or of same driver version, then > >> if user is reading data_size X at source/SAVING, destination should be > >> able to consume data_size X at restoring/RESUMING. Then why should user > >> write X/2 and iterate? > > > > Because users do things we don't expect ;) Maybe they decide to chunk > > the data into smaller packets over the network, but the receiving side > > would rather write the packet immediately rather than queuing it. > > OTOH, does it necessarily matter so long as data_size is written on > > completion of a full "packet"? > > > > Doesn't matter. As long as data is written in same order as it was read, > size doesn't matter. > > >>> What are the ordering requirements? Must the user write data_size > >>> packets in the same order that they're read, or is it the vendor > >>> driver's responsibility to include sequence information and allow > >>> restore in any order? > >>> > >> > >> For user, data is opaque. User should write data in the same order as he > >> received. > > > > Let's make sure that's specified. > > > > Ok. > > Thanks, > Kirti > > >>>> + */ > >>>> + > >>>> +struct vfio_device_migration_info { > >>>> + __u32 device_state; /* VFIO device state */ > >>>> +#define VFIO_DEVICE_STATE_STOPPED (0) > >>> > >>> We need to be careful with how this is used if we want to leave the > >>> possibility of using the remaining 29 bits of this register. Maybe we > >>> want to define VFIO_DEVICE_STATE_MASK and be sure that we only do > >>> read-modify-write ops within the mask (ex. set_bit and clear_bit > >>> helpers). > >> > >> Makes sense, I'll do changes in next iteration. > >> > >>> Also, above we define STOPPED to indicate simply > >>> not-RUNNING, but here it seems STOPPED means not-RUNNING, not-SAVING, > >>> and not-RESUMING. > >>> > >> > >> That's correct. > >> > >>>> +#define VFIO_DEVICE_STATE_RUNNING (1 << 0) > >>>> +#define VFIO_DEVICE_STATE_SAVING (1 << 1) > >>>> +#define VFIO_DEVICE_STATE_RESUMING (1 << 2) > >>>> +#define VFIO_DEVICE_STATE_INVALID (VFIO_DEVICE_STATE_SAVING | \ > >>>> + VFIO_DEVICE_STATE_RESUMING) > >>>> + __u32 reserved; > >>>> + __u64 pending_bytes; > >>>> + __u64 data_offset; > >>> > >>> Placing the data more than 4GB into the region seems a bit absurd, so > >>> this could probably be a __u32 and take the place of the reserved field. > >>> > >> > >> Is there a maximum limit on VFIO region size? > >> There isn't any such limit, right? Vendor driver can define region of > >> any size and then place data section anywhere in the region. I prefer to > >> keep it __u64. > > > > We have a single file descriptor for all accesses to the device, which > > gives us quite a bit of per device address space. As I mention above, > > it wasn't clear to me that data_offset is used dynamically until I got > > further into the series, so it seemed strange to me that we'd choose > > such a large offset, but given my new understanding I agree it requires > > a __u64 currently. Thanks, > > > > Alex > > > >>>> + __u64 data_size; > >>>> + __u64 start_pfn; > >>>> + __u64 page_size; > >>>> + __u64 total_pfns; > >>>> + __s64 copied_pfns; > >>> > >>> If this is signed so that we can get -1 then the user could > >>> theoretically specify total_pfns that we can't represent in > >>> copied_pfns. Probably best to use unsigned and specify ~0 rather than > >>> -1. > >>> > >> > >> Ok. > >> > >>> Overall this looks like a good interface, but we need to more > >>> thoroughly define the protocol with the data area and set expectations > >>> we're placing on the user and vendor driver. There should be no usage > >>> assumptions, it should all be spelled out. Thanks, > >>> > >> > >> Thanks for your feedback. I'll update comments above to be more specific. > >> > >> Thanks, > >> Kirti > >> > >>> Alex > >>> > >>>> +} __attribute__((packed)); > >>>> + > >>>> /* > >>>> * The MSIX mappable capability informs that MSIX data of a BAR can be mmapped > >>>> * which allows direct access to non-MSIX registers which happened to be within > >>> > >