Re: [Qemu-devel] [PATCH v9 04/26] target: [tcg] Add generic translation framework

[Richard Henderson]

On 06/26/2017 03:14 AM, Alex Bennée wrote:
> > +        if ((db->pc_first & TARGET_PAGE_MASK)
> > +            != (db->pc_next & TARGET_PAGE_MASK)) {
> > +            db->is_jmp = DJ_TOO_MANY;
> > +            break;
> > +        }
>
> How does the first insn avoid this check? And if it does is that right?
>
> I mean I understand you can construct weird multi-byte instructions
> (especially on x86) that cross the boundary but even if it is the first
> in a TB shouldn't it error if there are no contiguous pages?
>
> Also isn't the page crossing issue different for SoftMMU and linux-user?

It could be, though I don't believe that many front-ends take advantage.  I 
don't think it makes *that* much difference.

But if it comes to that, there are other edge conditions that can be used, e.g. 
the KSEG non-paged addressing segment for Alpha or MIPS kernel mode.


r~