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Date: Sat, 26 Sep 2015 22:38:25 +0200
From: Denys Vlasenko <dvlasenk@redhat.com>
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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
References: <1443290440-14930-1-git-send-email-dvlasenk@redhat.com> <706E9982-24E3-442B-808A-172909449DD4@zytor.com>
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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.