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

[Peter Lieven]

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.


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?


Thanks,

Peter