Re: [Qemu-block] Virtio-BLK/SCSI write requests and data payload checksums

[Paolo Bonzini]

----- Original Message -----
> From: "Peter Lieven" <address@hidden>
> To: "qemu block" <address@hidden>
> Cc: "Paolo Bonzini" <address@hidden>, address@hidden, address@hidden, "Max 
> Reitz" <address@hidden>,
> "Michael S. Tsirkin" <address@hidden>
> Sent: Tuesday, June 6, 2017 10:13:51 PM
> Subject: Virtio-BLK/SCSI write requests and data payload checksums
> 
> Hi,
> 
> 
> ich have spend several hours debugging a strange checksum error issue and
> finally found the cause, but I am totally unsure if whats happening
> is correct or not.
> 
> Imagine a Protocol like iSCSI which has a Data Digest and which receives its
> data via zero copy straight from the guest kernel through Qemu and
> 
> then out of the socket to the network. It calculates the CRC32C data digest
> from the buffer it gets from Qemu. It totally relies thereby that the buffer
> is not mangled after the write request has been submitted. But it seems this
> assumption is not true.
> 
> [...] the contents of the buffer submitted to Qemu change while Qemu processes
> the write.
> In the end several write requests follow and the /etc/network/interfaces file
> has the right content, but
> is this at all legal?

Yes, it's ugly but it's legal.  It probably doesn't happen on real hardware
that computes the checksum after or during DMA and has some kind of buffer
inside the board.  But on virt there is only one copy until we reach the actual
physical hardware.

Paolo

> If yes, checksum calculation of payload and zero copy
> would not work together.