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

[ebiederm@xmission.com (Eric W. Biederman)]

Linus Torvalds <torvalds@linux-foundation.org> writes:

> On Tue, Sep 29, 2015 at 1:35 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>>
>> Does anyone know what happens if you stick a non-accessed segment in
>> the GDT, map the GDT RO, and access it?
>
> You should get a #PF, as you guess, but go ahead and test it if you
> want to make sure.

I tested this by accident once when workinng on what has become known
as coreboot.  Early in boot with your GDT in a EEPROM switching from
real mode to 32bit protected mode causes a write and locks up the
machine when the hardware declines the write to the GDT to set the
accessed bit.  As I recall the write kept being retried and retried and
retried...

Setting the access bit in the GDT cleared up the problem and I did not
look back.

Way up in 64bit mode something might be different, but I don't know why
cpu designeres would waste the silicon.

Eric