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

From: Denys Vlasenko
Date: Sat Sep 26 2015 - 16:39:04 EST

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@xxxxxxxxxx> 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
>> This patch reduces GDT alignment to cacheline-aligned
>> 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.

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