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RE: [Xen-devel] [PATCH] 32on64 call gate support

  • To: "Keir Fraser" <Keir.Fraser@xxxxxxxxxxxx>
  • From: "Petersson, Mats" <Mats.Petersson@xxxxxxx>
  • Date: Fri, 26 Jan 2007 11:18:53 +0100
  • Cc: Xen Development Mailing List <xen-devel@xxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Fri, 26 Jan 2007 02:18:49 -0800
  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>
  • Thread-index: AcdBMTKDcN+Ii60kEduhMgANk04WTAAAFCIwAABPQvoAAAwCoA==
  • Thread-topic: [Xen-devel] [PATCH] 32on64 call gate support


> -----Original Message-----
> From: Keir Fraser [mailto:Keir.Fraser@xxxxxxxxxxxx] 
> Sent: 26 January 2007 10:14
> To: Petersson, Mats
> Cc: Xen Development Mailing List
> Subject: Re: [Xen-devel] [PATCH] 32on64 call gate support
> On 26/1/07 10:07 am, "Petersson, Mats" <Mats.Petersson@xxxxxxx> wrote:
> >> Do we know that anyone uses 64-bit call gates? I would 
> have thought it
> >> unlikely.
> > 
> > I agree, it's VERY unlikely that anyone uses call-gates in 
> 64-bit mode.
> > There are other, better, ways to do system calls, including the
> > dedicated SYSCALL instruction. Since 64-bit code is "new", 
> it's unlikely
> > to use old ways of doing things "just because there was nothing else
> > around at the time".
> Of course, it begs the question why call gates weren't 
> stripped from long
> mode along with other ancient architectural baggage.

Yeah, well... Actually, there may be a good reason for call-gates: If
you use call-gates correctly, you could avoid having one arugment that
is "which system call", but rather set up three dozen call-gates for
your three-dozen system calls - so that may be what they were preserving
it for, along with your suggestion below about old-OS support. There are
a few more possible call-gates than other methods of system-call, even
if they are not quite "unlimited". [Bear in mind that I'm NOT a
chip-architect, so I don't know why they kept some things and removed
others, aside from the "prefix" clean-up]

> Perhaps the idea was to allow compat-mode applications 
> relying on call gates
> to be supported? So the OS would be responsible for 
> allocating 64-bit call
> gates instead of 32-bit call gates?
> Anyhow, this is a bridge we should probably only cross if we 
> come to it.

Definitely my opinion too. 

>  -- Keir

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