Re: [Xen-devel] [RFC] netif: staging grants for requests

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 9 Jan 2017, Paul Durrant wrote:
> > Wouldn't it be better to introduce the new memcpy based scheme for RX
> > only?
> 
> It would certainly be reasonable to tackle RX first. But I don't want to 
> force a copy in the TX path when it may be possible to avoid it.
> 
> > 
> > This suggestion increases the total amount of grants and the amount of
> > non-packet data simultaneously shared from frontend to backend by
> > "accident" (because it happens to be on the same pages that are granted
> > to the backend). Am I right? If so, it could be a security concern.
> > Let's keep in mind that if we trust the backend then we might as well go
> > all the way and assume that the backend is in Dom0 and can map any mfn
> > in memory without requiring grants. That would be a simple extension
> > guaranteed to have great performance (I am in favor of this FYI).
> 
> I'm not sure that we want to do away with grants altogether. In my scheme the 
> frontend can ask for its grants back so that it doesn't leak, but if it's 
> willing to place more trust in the backend then it can grant pages from the 
> stack which may contain other information. Certainly bypassing grants would 
> be a quick performance boost but it's an all-or-nothing approach, whereas I'm 
> shooting for something more flexible.

I understand the intent but this is a suboptimal choice. If we do trust
the backend, then we can do better by using MFNs instead of grants. It
is simpler and more efficient. I prefer to make decisions based on
numbers, rather than gut feeling, but I bet that the MFN solution would
be significantly faster. If we do not trust the backend, then we would
be better off with a pure memcpy approach. This half-way proposal tries
to please both the security crowd and the performance crowd but leaves
them both wanting.

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