Re: [Qemu-devel] virtio scsi host draft specification, v2

[Avi Kivity]

On 06/02/2011 01:42 PM, Michael S. Tsirkin wrote:
> On Wed, Jun 01, 2011 at 05:51:54PM +0300, Avi Kivity wrote:
> >  On 06/01/2011 05:36 PM, Michael S. Tsirkin wrote:
> >  >>
> >  >>   So, if I am going to give this liberty with buffers to the driver, I
> >  >>   _have_ to keep the size information.  Otherwise, I agree that it is
> >  >>   redundant and I will remove it.  What poison do you prefer?
> >  >>
> >  >
> >  >Ah, I think I understand now. Both sense and data have in
> >  >fields that might only be used partially?
> >  >In that case I think I agree: it's best to require the use of separate
> >  >buffers for them, in this way used len will give you useful information
> >  >and you won't need sense_len and data_len: just a flag to
> >  >mark the fact that there *is* a sense buffer following.
> >  >And the num field does that.
> >
> >
> >  Do you mean to use the virtio iovec length to determine information
> >  about the message (like splitting it into buffers)?
>
> Exactly the reverse :)

They're both equally bad.

> >  I think that's a bad idea.  Splitting into buffers is a function of
> >  memory management.  For example, a driver in userspace (or a nested
> >  guest) will have additional fragmentation into 4K pages after it
> >  passes through the iommu.
> >
> >  Let's not mix layers here.
>
> Right. If there are two buffers of variable length there
> should be two add_buf calls.

No.  The guest should be free to use one large continuous buffer of size 
N, of N buffers of size 1.

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