Re: [PATCH 5/9] i386: Add address spaces for fs/gs segments

[Richard Henderson <rth at redhat dot com>]

On 10/16/2015 05:28 AM, Paolo Bonzini wrote:
> On 08/10/2015 06:59, Richard Henderson wrote:
> > +/* Address space support.
> > +
> > +   This is not "far pointers" in the 16-bit sense, but an easy way
> > +   to use %fs and %gs segment prefixes.  Therefore:
> > +
> > +    (a) All address spaces have the same modes,
> > +    (b) All address spaces have the same addresss forms,
> > +    (c) While %fs and %gs are technically subsets of the generic
> > +        address space, they are probably not subsets of each other.
> > +    (d) Since we have no access to the segment base register values
> > +        without resorting to a system call, we cannot convert a
> > +        non-default address space to a default address space.
> > +        Therefore we do not claim %fs or %gs are subsets of generic.
>
> rdfsbase and rdgsbase are potentially accessible to userspace too, so I
> think %fs or %gs should be considered subsets of generic.

Potentially, yes.  Though in practice not enabled for any current operating 
system that I can find.  Thus at present we still require some sort of syscall.

I suppose we could add a libgcc interface for it, but it certainly wouldn't be 
quick.

r~