[libvirt] [RFC PATCH 6/6] qemu: Format pseries.cap-hpt-mps on the command line

[David Gibson]

On Thu, May 24, 2018 at 07:34:25AM +0200, Peter Krempa wrote:
> On Wed, May 23, 2018 at 19:09:59 +0200, Andrea Bolognani wrote:
> > On Wed, 2018-05-23 at 18:40 +0200, Peter Krempa wrote:
> > > On Wed, May 23, 2018 at 18:18:02 +0200, Andrea Bolognani wrote:
> > > > +            /* QEMU expects the argument to be a number of left shifts:
> > > > +             * for example, if you wanted to limit the guest to 4 KiB pages,
> > > > +             * since 4096 == 1 << 12, you would need to add cap-hpt-mps=12
> > > > +             * to the command line.
> > > 
> > > So basically you need to pass the exponent of a power of 2 that yields
> > > this number. The number of left shifts may be slightly confusing ...
> > 
> > I guess it depends on the reader; the two definitions are
> > equivalent anyway, so no harm in having both in the comment :)
> 
> Using both are fine, but without mentioning that it is in fact a power of
> two-only thing I was thinking that the interface is off by one or
> something:
> 
> 4096 == 1 << 12 == 2 << 11
> 
> > In general, I'd say it's not the most user-friendly interface on
> > QEMU's side, but I believe it's dictated by hardware / emulator
> > details, given how it ends up being used: see
> > 
> >   http://lists.nongnu.org/archive/html/qemu-devel/2018-04/msg02822.html
> > 
> > To be fair, it would perhaps make sense to perform the conversion
> > directly inside QEMU, in order to make it more convenient not only
> > for libvirt but for for people driving it directly as well.
> 
> If strictly only powers of two make sense for this knob then this gives
> you input validation for free. On the other hand, specifying a big
> number can overflow internally if it is ever used in the non-exponent
> form. I think the format does not matter much, since libvirt's job is to
> shield users from such weirdness.

Yeah, the above is basically my reasoning for using the exponent, not
the final page size.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson
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