Re: [Qemu-devel] KVM call minutes for Feb 15

[Anthony Liguori]

On 02/17/2011 07:25 AM, Avi Kivity wrote:
> On 02/17/2011 03:10 PM, Anthony Liguori wrote:
> > On 02/17/2011 06:23 AM, Avi Kivity wrote:
> > > On 02/17/2011 02:12 PM, Anthony Liguori wrote:
> > > > > (btw what happens in a non-UTF-8 locale? I guess we should just 
> > > > > reject unencodable strings).
> > > >
> > > >
> > > > While QEMU is mostly ASCII internally, for the purposes of the JSON 
> > > > parser, we always encode and decode UTF-8.  We reject invalid UTF-8 
> > > > sequences.  But since JSON is string-encoded unicode, we can always 
> > > > decode a JSON string to valid UTF-8 as long as the string is well 
> > > > formed.
> > >
> > > That is wrong.  If the user passes a Unicode filename it is expected 
> > > to be translated to the current locale encoding for the purpose of, 
> > > say, filename lookup.
> >
> > QEMU does not support anything but UTF-8.
>
> Since when?
>
> AFAICT, JSON string conversion is the only place where there is any 
> dependency on UTF-8.  Anything else should just work.
>
> > That's pretty common with Unix software.  I don't think any modern 
> > Unix platform actually uses UCS2 or UTF-16.  It's either ascii or UTF-8.
>
> Most/all Linux distributions support UTF-8 as well as a zillion other 
> encodings (single-byte ASCII + another charset, or multi-byte charsets 
> for languages with many characters.

Maybe there's some confusion here.  UTF-8 is an encoding, not a locale.

The common encodings are ASCII, UTF-8, UCS2, UTF-16, and UTF-32.

An application has to explicitly support an encoding.  It is not 
transparent.  UCS2/UTF-16 means that strings are not 'const char *'s but 
'const wchar_t *' where typedef unsigned short wchar_t;.

QEMU assumes, in lots of places that strings are single-byte NUL 
terminated.  Basically, any use of snprintf, printf, strcpy, strlen, 
etc. pretty much tie you to ASCII/UTF-8.  You can have a single NUL byte 
as part of a valid UCS2 string.

> > The only place it even matters is Windows and Windows has ASCII and 
> > UTF-16 versions of their APIs.  So on Windows, non-ASCII characters 
> > won't be handled correctly (yet another one of the many issues with 
> > Windows support in QEMU).  UTF-8 is self-recovering though so it 
> > degrades gracefully.
>
> It matters on Linux with el_GR.iso88597, for example.

The whole series of iso8859 (8-bit encodings) are officially abandoned 
in favor of UCS and encodings that support the full UCS code page 
(UTF-8/UTF-16).

I see no strong reason to try and support deprecated encodings when 
there are perfectly valid replacements like el_GR.utf8.

Regards,

Anthony Liguori