linux-kernel - Re: [PATCH entry v2 5/6] x86/ldt: Disable 16-bit segments on Xen PV

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Message-ID: <87fc68cf-540a-85e1-4dde-80cb3e82c3a5@citrix.com>
Date:   Fri, 3 Jul 2020 20:00:34 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Andy Lutomirski <luto@...nel.org>, <x86@...nel.org>
CC:     Juergen Gross <jgross@...e.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH entry v2 5/6] x86/ldt: Disable 16-bit segments on Xen PV

On 03/07/2020 18:02, Andy Lutomirski wrote:
> Xen PV doesn't implement ESPFIX64, so they don't work right.  Disable
> them.  Also print a warning the first time anyone tries to use a
> 16-bit segment on a Xen PV guest that would otherwise allow it
> to help people diagnose this change in behavior.
>
> This gets us closer to having all x86 selftests pass on Xen PV.

Do we know exactly how much virtual address space ESPFIX64 takes up?

Are people going to scream in horror if Linux needs to donate some
virtual address space back to Xen to get this working?  Linux's current
hypervisor range (8TB, 16 pagetable slots) really isn't enough for some
systems these days.

~Andrew