Re: re-writing on powerpc

[Hollis Blanchard <hollis_blanchard@mentor.com>]

On 12/14/2010 12:48 AM, Avi Kivity wrote:
> On 12/13/2010 07:17 PM, Hollis Blanchard wrote:
>>> Rewriting is dangerous if the guest is unaware of it.  As soon as it 
>>> is made aware of it, it might as well actually do it in the best way 
>>> that suits it.
>>
>> Can you list some examples of dangerous scenarios?
>>
Perhaps I should rephrase... any real-world dangerous scenarios? :) I 
was hoping you could share some traps you've hit with Linux or Windows 
on x86.
> - guest checksums own kernel pages
For runtime intrusion detection? Such guests can simply not ask the 
hypervisor to enable the rewriting feature.
> - clever compiler reuses code for constant pool
Not sure what you mean here. Anyways I think clever compilers are 
irrelevant, since a compiler will not ordinarily emit a supervisor-mode 
instruction. The hypervisor has no need to patch normal user-mode 
instructions.
> - guest patches itself (a la linux alternatives), surprised when it 
> sees a different instruction
PowerPC Linux does patch itself, which is a write-only operation.
> - guest jits own kernel code (like Singularity), gets confused when it 
> reads back something it didn't write
This is getting really hypothetical, but why would a JIT need to read 
the generated code?

Hollis Blanchard
Mentor Graphics, Embedded Systems Division