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

From: Peter Lieven
Subject: Re: [Qemu-block] Virtio-BLK/SCSI write requests and data payload checksums
Date: Sun, 16 Dec 2018 18:53:44 +0100

Von: Paolo Bonzini <address@hidden>

> > 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.
> okay, so it's no bug. and there is no way to detect that the page is dirty
> (again). so i need a bounce buffer if i have data checksums?

I think so, yes.

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?


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