On Thu, 24 Sep 2020, Sudhakar Panneerselvam wrote: > Hello Eric, > > > -----Original Message----- > > From: Eric Biggers [mailto:ebiggers@xxxxxxxxxx] > > Sent: Wednesday, September 23, 2020 11:14 PM > > To: Mike Snitzer <snitzer@xxxxxxxxxx> > > Cc: Sudhakar Panneerselvam <sudhakar.panneerselvam@xxxxxxxxxx>; > > Damien.LeMoal@xxxxxxx; ssudhakarp@xxxxxxxxx; Martin Petersen > > <martin.petersen@xxxxxxxxxx>; dm-crypt@xxxxxxxx; dm-devel@xxxxxxxxxx; > > Shirley Ma <shirley.ma@xxxxxxxxxx>; mpatocka@xxxxxxxxxx; Milan Broz > > <gmazyland@xxxxxxxxx>; agk@xxxxxxxxxx > > Subject: Re: [RFC PATCH 0/2] dm crypt: Allow unaligned buffer > > lengths for skcipher devices > > > > On Wed, Sep 23, 2020 at 09:27:32PM -0400, Mike Snitzer wrote: > > > You've clearly done a nice job with these changes. Looks clean. > > > > > > BUT, I'm struggling to just accept that dm-crypt needs to go to these > > > extra lengths purely because of one bad apple usecase. > > > > > > These alignment constraints aren't new. Are there other portions of > > > Linux's crypto subsystem that needed comparable fixes in order to work > > > with Microsfot OS initiated IO through a guest? > > > > > > You forecast that these same kinds of changes are needed for AEAD and > > > dm-integrity... that's alarming. > > > > > > Are we _certain_ there is no other way forward? > > > (Sorry I don't have suggestions.. I'm in "fact finding mode" ;) > > > > > > > I don't understand why this is needed, since dm-crypt already sets its > > logical_block_size to its crypto sector_size. Isn't it expected that I/O that > > isn't aligned to logical_block_size fails? It's the I/O submitter's > > responsibility to ensure logical_block_size alignment of all I/O segments. > > Exactly how is the misaligned I/O actually being submitted here? > > You are right that each I/O size should be a multiple of the block > device's sector size, but I am not sure if there is any constraint that > individual segment lengths should be aligned to its sector size, could > you help me with how this is enforced in block layer? The closest I see > is "dma_alignment" member in "struct request_queue" of the low-level > block device driver and as mentioned in the patch description, iSCSI, > MegaRaid, qla2xxx, nvme and others have much relaxed constraint. > > To your other question, the IO stack looks like this: > > Windows Guest <--> Vhost-Scsi <--> LIO(scsi/target/blockio) <--> dm-crypt <--> iSCSI block device > > One real example out of my debugging: Windows sends a I/O request with > 6656 bytes to vhost-scsi interface. Vhost-scsi uses translate_desc() in > drivers/vhost/vhost.c to convert windows user space memory buffers to > kernel iovecs. Vhost-scsi then converts the iovecs to sg entries in > vhost_scsi_mapal() which is then handed over to "target" subsystem and > eventually submitted to dm-crypt. This 6656 bytes IO has got 3 segments, > first segment had 1584, second 4096 and the last had 976 bytes. Dm-crypt > rejects the I/O after seeing the first segment length 1584 which is not > a 512 byte multiple. > > Let me know if there are further questions. > > Thanks > Sudhakar Hi I think it should be fixed in vhost-scsi. Mikulas -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
