Re: Grant operation batching

[Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>]

On Fri, May 20, 2022 at 08:24:48AM +0200, Jan Beulich wrote:
> On 20.05.2022 01:22, Demi Marie Obenour wrote:
> > It is well known that mapping and unmapping grants is expensive, which
> > is why blkback has persistent grants.  Could this cost be mitigated by
> > batching, and if it was, would it affect the tradeoff of memcpy() vs
> > grant table operations?
> 
> Which backend driver are you thinking about? The in-kernel Linux
> xen-blkback already batches grant operations, afaics. Such
> batching is helpful, but the main cost is assumed (known?) to be
> with the (installing and) tearing down of the actual mappings of
> the guest pages (into/)from backend address space.

My thought was that the expensive part of this is TLB flushes, which are
only needed once per batch.  Also, what do you think about the “unsafe”
mode?  It would only be unsafe if the backend is untrusted, but it is
quite common for the backend to be trusted.

-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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