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

["H. Peter Anvin" <hpa@zytor.com>]

On 09/29/2015 06:20 PM, Eric W. Biederman wrote:
> 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.
> 

This is totally different from a TLB violation.  In your case, the write
goes through as far as the CPU is concerned, but when the data is
fetched back, it hasn't changed.  A write to a TLB-protected location
will #PF.

	-hpa