Re: [Xen-devel] [PATCH v2 2/4] xen: introduce grant_map_exists

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Mon, 6 Oct 2014, Jan Beulich wrote:
> >>> On 06.10.14 at 15:46, <stefano.stabellini@xxxxxxxxxxxxx> wrote:
> > On Mon, 6 Oct 2014, Jan Beulich wrote:
> >> >>> On 06.10.14 at 15:08, <stefano.stabellini@xxxxxxxxxxxxx> wrote:
> >> > The other approach to do this, that I avoided because it is slower (2
> >> > spinlocks instead of 1), is to use the existing mapcount function.
> >> > mapcount is based on:
> >> > 
> >> > for ( handle = 0; handle < lgt->maptrack_limit; handle++ )
> >> > 
> >> > do you think it is better?
> >> 
> >> No, that's worse: This is the Dom0 side of things, and the loop
> >> bound would still be controlled by that same command line option.
> >> 
> >> So between the two, the original solution would be better, and
> >> maybe enforcing a lower limit on DomU-s might actually be an
> >> option.
> > 
> > What about keeping track of the highest ref number actually used so far?
> > I could use that as upper limit instead of the theoretical max. It would
> > also make the execution faster.
> 
> But the loops are already using a value on the order of the high
> watermark, not the theoretical one. I.e. you're not alway looping
> for a long time, it is just possible for this to happen.

OK, I understand.

The default max value is 32 pages that lead to 16384 iterations in
grant_map_exists. Should I reduce it?

Given the simple operations in the loop, the function shouldn't actually
be so slow to be a security risk. It should take less that 0.1
milliseconds to complete.

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