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

[Samuel Thibault <samuel.thibault@ens-lyon.org>]

Hello,

Andrew Cooper via Libc-alpha, le mer. 27 mai 2020 14:39:00 +0100, a ecrit:
> Why does the MSB make any difference?  %gs still needs to remain intact
> so the thread pointer can be pulled out, so there is nothing that Xen or
> Linux can do in the way of lazy loading.
> 
> Beyond that, its straight up segment base semantics in x86.  There will
> be a 1-cycle AGU delay from a non-zero base, but that nothing to do with
> Xen and applies to all segment based TLS accesses on x86, and you'll win
> that back easily through reduced register pressure.
> 
> Are there any further details on the perf problem claim?  I find it
> suspicious.

The concern is not about the indirection.

The concern is that to keep safe from the guest, the hypervisor has to
restrict the size of the segment, and thus negative offsets, used in the
i386 TLS model, are rejected by the processor, and the hypervisor has to
emulate these access, thus a high cost.

Samuel