Re: [Qemu-devel] Re: [PATCH 2/6] qemu-kvm: Modify and introduce wrapper functions to access phys_ram_dirty.

Avi Kivity
2010-03-17T04:07:50Z

On 03/16/2010 10:10 PM, Blue Swirl wrote:
>
>> Yes, and is what tlb_protect_code() does and it's called from
>> tb_alloc_page() which is what's code when a TB is created.
>>
> Just a tangential note: a long time ago, I tried to disable self
> modifying code detection for Sparc. On most RISC architectures, SMC
> needs explicit flushing so in theory we need not track code memory
> writes. However, during exceptions the translator needs to access the
> original unmodified code that was used to generate the TB. But maybe
> there are other ways to avoid SMC tracking, on x86 it's still needed
>

On x86 you're supposed to execute a serializing instruction (one of
INVD, INVEPT, INVLPG, INVVPID, LGDT, LIDT, LLDT, LTR, MOV (to control
register, with the exception of MOV CR8), MOV (to debug register),
WBINVD, WRMSR, CPUID, IRET, and RSM) before running modified code.

> but I suppose SMC is pretty rare.
>

Every time you demand load a code page from disk, you're running self
modifying code (though it usually doesn't exist in the tlb, so there's
no previous version that can cause trouble).

--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.