Re: -mno-tls-direct-seg-refs support in glibc for i386 PV Xen

[Jan Beulich <jbeulich@xxxxxxxx>]

On 27.05.2020 16:40, Andrew Cooper wrote:
> On 27/05/2020 15:00, Jan Beulich wrote:
>> You can have the descriptor map
>> only the [base,XenBase] part or the [0,base) one. Hence Xen, from its
>> #GP handler, flipped the descriptor between the two options depending
>> on whether the current access was to the positive of negative part of
>> the TLS seg. (An in-practice use of expand down segments, as you'll
>> surely notice.)
> 
> I've found gpf_emulate_4gb() in source history.  It was specific to
> 32bit builds of Xen (now long gone).
> 
> What I can't figure out is why this is unnecessary in 64bit builds of
> Xen.  We still enforce reduced segment limits on the guests descriptors.

Do we? I can't find such - neither boot_compat_gdt[] has any signs
of it, nor check_descriptor(). And we don't have a need to: The
entire range is used for the r/o M2P, i.e. protection is enforced
at the paging layer. 32-bit Xen necessarily had r/w as well as
executable sub-ranges there.

Jan