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

From: Denys Vlasenko <dvlasenk@redhat.com>
To: "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@kernel.org>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	David Vrabel <david.vrabel@citrix.com>,
	Joerg Roedel <joro@8bytes.org>, Gleb Natapov <gleb@kernel.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	kvm@vger.kernel.org, x86@kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] x86: Use entire page for the per-cpu GDT only if paravirt-enabled
Date: Sat, 26 Sep 2015 22:38:25 +0200	[thread overview]
Message-ID: <56070241.2030407@redhat.com> (raw)
In-Reply-To: <706E9982-24E3-442B-808A-172909449DD4@zytor.com>

On 09/26/2015 09:50 PM, H. Peter Anvin wrote:
> NAK.  We really should map the GDT read-only on all 64 bit systems,
> since we can't hide the address from SLDT.  Same with the IDT.

Sorry, I don't understand your point.

> On September 26, 2015 11:00:40 AM PDT, Denys Vlasenko <dvlasenk@redhat.com> wrote:
>> We have our GDT in a page-sized per-cpu structure, gdt_page.
>>
>> On x86_64 kernel, GDT is 128 bytes - only ~3% of that page is used.
>>
>> It is page-sized because of paravirt. Hypervisors need to know when
>> GDT is changed, so they remap it read-only and handle write faults.
>> If it's not in its own page, other writes nearby will cause
>> those faults too.
>>
>> In other words, we need GDT to live in a separate page
>> only if CONFIG_HYPERVISOR_GUEST=y.
>>
>> This patch reduces GDT alignment to cacheline-aligned
>> if CONFIG_HYPERVISOR_GUEST is not set.
>>
>> Patch also renames gdt_page to cpu_gdt (mimicking naming of existing
>> cpu_tss per-cpu variable), since now it is not always a full page.