linux-kernel - Re: [PATCH 2/5] x86/pvh: Use correct size value in GDT descriptor

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Message-ID: <58eb8a15-4369-40c6-90fe-71999cffcb63@citrix.com>
Date: Thu, 26 Sep 2024 15:29:02 +0100
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: Ard Biesheuvel <ardb+git@...gle.com>, linux-kernel@...r.kernel.org
Cc: Ard Biesheuvel <ardb@...nel.org>, Jason Andryuk <jason.andryuk@....com>,
 Juergen Gross <jgross@...e.com>, Boris Ostrovsky
 <boris.ostrovsky@...cle.com>, x86@...nel.org, xen-devel@...ts.xenproject.org
Subject: Re: [PATCH 2/5] x86/pvh: Use correct size value in GDT descriptor

On 26/09/2024 11:41 am, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@...nel.org>
>
> The size field in a GDT descriptor is offset by 1, so subtract 1 from
> the calculated range.
>
> Signed-off-by: Ard Biesheuvel <ardb@...nel.org>

I realise this is probably nitpicking, but the GDT descriptor has a
limit field, which is (intentionally) not a size field.

This is why there's a difference of 1 between them.

Working in terms of limits rather than sizes avoids needing a 16th bit
on the segment limit circuitry, which mattered for the 286.

~Andrew