Re: [Qemu-devel] [PATCH 05/27] qapi: add SIZE type parser to string_input_visitor

[Michael S. Tsirkin]

On Thu, Dec 19, 2013 at 04:14:56PM +0100, Markus Armbruster wrote:
> "Michael S. Tsirkin" <address@hidden> writes:
> 
> > On Mon, Nov 25, 2013 at 09:43:48AM -0700, Eric Blake wrote:
> >> On 11/25/2013 09:32 AM, Paolo Bonzini wrote:
> >> 
> >> >> Yes please. Firing up a calculator to figure out how much is 1G is not
> >> >> friendly, neither is firing it up to figure out what did management do
> >> >> with QMP. It should be a text based interface not a binary one.
> >> 
> >> Right now, QMP takes an 'int', which does not allow a suffix.  Libvirt
> >> prefers passing a value in 'bytes', rather than risking confusion on
> >> whether a value in G was rounded (up, down? to nearest power of 10 or
> >> power of 2?).  Unfortunately, yes, that means you need a calculator when
> >> parsing QMP logs to see whether the 1073741824 passed by libvirt is the
> >> 1G you had in mind.
> >> 
> >> HMP, qtest, and any other decent shell around raw QMP is more than
> >> welcome to provide human-usable wrappers that takes "1G" as a string and
> >> turns it into the raw int used by the underlying QMP.  In fact, I
> >> encourage it.
> >
> > How will it know 1G is not e.g. an ID?
> >
> > We can invent rules like "IDs should not start with a number", but these
> > rules are better enforced in a single place for consistency, and it's
> > likely too late to enforce that in HMP.
> 
> This isn't how the human monitor works.

Yes but we were talking about a friendly shell for qemu using QMP
as a backend.

> Its argument parsing is guided by the command's args_type, which
> declares argument names and type codes.  For instance, type code 's'
> expects a string argument (may be enclosed in quotes), 'i' a 32-bit
> integer argument, 'o' a size argument (may be followed by a suffix such
> as 'G').
> 
> If the current argument has type code 'o', then 1G is interpreted as the
> number 2^30.  With type code 's', it's the string "1G".
> 
> As to rules for IDs: IDs are typically defined via QemuOpts, which
> restricts them to letters, digits, '-', '.', '_', starting with a
> letter.
> 
> >> > This is unfortunately a counter-example to the rule that HMP commands
> >> > should always be implemented in terms of their QMP counterparts.  I do
> >> > not believe this is really a problem.  It can be fixed later; for now, I
> >> > think "perfect is the enemy of good" applies.
> >> 
> >> Hey - I just realized that now that we have anonymous unions, we could
> >> theoretically extend QMP to allow a union between 'int' and 'string' -
> >> if an 'int' is passed, it is in bytes; if a 'string' is passed, then
> >> parse it the way HMP would (so the string "1G" would be equivalent to
> >> the raw int 1073741824).  But I don't know if it will help you (libvirt
> >> will still prefer to use raw ints in any QMP log you read off of libvirt
> >> interactions).
> >
> > Yes, I think that would address the issue.
> 
> I object, because it goes against the design of QMP.