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

[Vadim Rozenfeld]

On Mon, 2018-12-17 at 18:07 -0500, Michael S. Tsirkin wrote:
> On Mon, Dec 17, 2018 at 04:19:53PM +0100, Peter Lieven wrote:
> > Am 17.12.18 um 15:48 schrieb Stefan Hajnoczi:
> > > On Sun, Dec 16, 2018 at 06:53:44PM +0100, Peter Lieven wrote:
> > > > It turned out that for writes a bounce buffer is indeed always
> > > > necessary. But what I found out is that
> > > > it seems that even for reads it happens that the OS (Windows in
> > > > this case) issues 2 read requests with
> > > > the same buffer in parallel. Is it possible that this is a bug
> > > > in the virtio or has anybone ever seen this before?
> > > 
> > > Is it Windows resume-from-sleep?  I remember a race condition
> > > that
> > > Microsoft fixed.
> > 
> > 
> > Its a running guest, no resume. From the dumped Data it seems that
> > its often (not always) log files.
> > 
> > 
> > > 
> > > The other weird thing I've seen is scatter-gather lists where the
> > > same
> > > buffer address is used multiple times within one request (this
> > > can go
> > > wrong if the host splits up the request, causing races).
> > 
> > 
> > The requests go to different LBAs often several 100MB apart from
> > each other. The only thing I
> > 
> > can see it that the request size always seems to be 4096 byte.

4K is a default cluster size for NTFS partition. 
https://support.microsoft.com/en-au/help/140365/default-cluster-size-fo
r-ntfs-fat-and-exfat

Try reformatting the volume as FAT or change the cluster size and you
should see different numbers.

> > 
> > 
> > I have not seen data corruption so far, but this at all seems very
> > strange.
> > 
> > I only have observed this because the data digest checksum is
> > wrong. After a retry it is correct.
> > 
> > When dumping the buffer addresses (in this case the address from
> > the first buffer in the qiov)
> > 
> > that came from Qemu/the guest the address is the same.
> > 
> > 
> > Actually I don't know for sure that the address comes from the
> > guest. In theory it could be that
> > 
> > the request from the guest was less than 4096 byte and Qemu just
> > assigned a bounce buffer
> > 
> > to read the whole block and copied the relevant part to the guest.
> > Can I see from the
> > 
> > address (0x7fa4354c0000) if it comes from Qemu or the guest?

You can try DiskMon https://docs.microsoft.com/en-us/sysinternals/downl
oads/diskmon to see the HD activity.


Peter, I must be missing something here, but what exactly the problem
is?

Best regards,
Vadim.

> > 
> > 
> > Thanks,
> > 
> > Peter
> > 
> 
> +cc some windows driver developers.
> 
>