[PATCH 4/4] KVM: PPC: Bookehv: Get vcpu's last instruction for emulation

Alexander Graf agraf at suse.de
Tue Apr 1 16:47:17 EST 2014

> Am 01.04.2014 um 01:03 schrieb Scott Wood <scottwood at freescale.com>:
>> On Mon, 2014-03-31 at 15:41 +0200, Alexander Graf wrote:
>>> On 03/26/2014 10:17 PM, Scott Wood wrote:
>>>> On Thu, 2014-02-20 at 18:30 +0200, Mihai Caraman wrote:
>>>> +    /*
>>>> +     * Another thread may rewrite the TLB entry in parallel, don't
>>>> +     * execute from the address if the execute permission is not set
>>>> +     */
>> What happens when another thread rewrites the TLB entry in parallel? 
>> Does tlbsx succeed? Does it fail? Do we see failure indicated somehow? 
>> Are the contents of the MAS registers consistent at this point or 
>> inconsistent?
> If another thread rewrites the TLB entry, then we use the new TLB entry,
> just as if it had raced in hardware.  This check ensures that we don't
> execute from the new TLB entry if it doesn't have execute permissions
> (just as hardware would).
> What happens if the new TLB entry is valid and executable, and the
> instruction pointed to is valid, but doesn't trap (and thus we don't
> have emulation for it)?
>> There has to be a good way to detect such a race and deal with it, no?
> How would you detect it?  We don't get any information from the trap
> about what physical address the instruction was fetched from, or what
> the instruction was.

Ah, this is a guest race where the guest modifies exactly the TLB in question. I see.

Why would this ever happen in practice (in a case where we're not executing malicious code)?


> -Scott

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