RE: [Xen-devel] xen and tls (fwd)

["Barry Silverman" <barry@xxxxxxxxx>]

As I recall, tls maps negative offsets >0xFF000000 (from segment registers
FS, GS) into the kernel address space (IE >0xa8000000), and this conflicts
with xen's virtual address mapping.

Xen lives mapped into high virtual memory but inaccessible to the guest
kernels - however the whole guest kernel address space is accessible to xen.

Keir Fraser developed an instruction emulator/code patcher in the page fault
code that detected these large offsets from FS/GS, and then fixed them up...

I recall that the emulator ended up getting called way for often than anyone
wanted and ran slowly. The "not-to-use" TLS recommendation followed from
that.

Barry Silverman

-----Original Message-----
From: xen-devel-admin@xxxxxxxxxxxxxxxxxxxxx
[mailto:xen-devel-admin@xxxxxxxxxxxxxxxxxxxxx]On Behalf Of Ronald G.
Minnich
Sent: Monday, February 14, 2005 11:53 AM
To: Himanshu Raj
Cc: xen-devel@xxxxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-devel] xen and tls (fwd)




On Mon, 14 Feb 2005, Himanshu Raj wrote:

> I believe it is due to the use of both +ve and -ve offsets by TLS. It
becomes
> a nightmare to manage it with segmentation, and system has to emulate -ve
> offsets by converting to +ve ones etc. I am still learning more on this.

I vaguely remember this discussion, I just wish I could remember more.

ron


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