linux-kernel - Re: [PATCH] x86: Use entire page for the per-cpu GDT only if paravirt-enabled

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Message-ID: <FEA1724A-1E6F-47EE-87C7-CC6A7EBBBB66@zytor.com>
Date:	Tue, 29 Sep 2015 11:27:01 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>
CC:	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Vrabel <david.vrabel@...rix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Brian Gerst <brgerst@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Gleb Natapov <gleb@...nel.org>,
	Kees Cook <keescook@...omium.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Joerg Roedel <joro@...tes.org>, X86 ML <x86@...nel.org>,
	kvm list <kvm@...r.kernel.org>
Subject: Re: [PATCH] x86: Use entire page for the per-cpu GDT only if paravirt-enabled

Ugh.  Didn't realize that.

On September 29, 2015 11:22:04 AM PDT, Andy Lutomirski <luto@...capital.net> wrote:
>On Tue, Sep 29, 2015 at 11:18 AM, H. Peter Anvin <hpa@...or.com> wrote:
>> SGDT would be easy to use, and it is logical that it is faster since
>it reads an internal register.  SIDT does too but unlike the GDT has a
>secondary limit (it can never be larger than 4096 bytes) and so all
>limits in the range 4095-65535 are exactly equivalent.
>>
>
>Using the IDT limit would have been a great ideal if Intel hadn't
>decided to clobber it on every VM exit.
>
>--Andy

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