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

[H. Peter Anvin]

No, it is a natural result of an implemention which treats setting the A bit as an abnormal flow (e.g. in microcode as opposed to hardware).

On September 29, 2015 7:11:59 PM PDT, ebiederm@xxxxxxxxxxxx wrote:
>"H. Peter Anvin" <hpa@xxxxxxxxx> writes:
>
>> 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.
>
>The key point is that a write is generated when the cpu needs to set
>the
>access bit.  I agree the failure points are different.  A TLB fault vs
>a
>case where the hardware did not accept the write.
>
>The idea of a cpu reading back data (and not trusting it's cache
>coherency controls) to verify the access bit gets set seems mind
>boggling.  That is slow, stupid, racy and incorrect.  Incorrect as the
>cpu should not only set the access bit once per segment register load.
>
>In my case I am pretty certain it was something very weird with the
>hardware not acceppting the write and either not acknowledging the bus
>transaction or cancelling it.  In which case the cpu knew the write had
>not made it to the ``memory'' and was trying to cope.
>
>Eric

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