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

From: H. Peter Anvin
Date: Tue Sep 29 2015 - 21:32:37 EST

On 09/29/2015 06:20 PM, Eric W. Biederman wrote:
> Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes:
>> On Tue, Sep 29, 2015 at 1:35 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> 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.


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