Re: [Qemu-devel] Question about the block linking limitation

[Max Filippov]

>>  Doesn't a tb spanning guest pages have above problem? I mean what if the
>> guest page 2 is changed? In this case, the execution flow jump from tb.a to
>> tb.b. It seems it may have the same problem as above to me.
>
> I agree. I see that only i386 and m68k TBs may cross page boundary.
> Maybe it is due to the fact that these CPUs can normally have instructions
> crossing page boundary due to the variable opcode length.
>
> My best guess is that QEMU behavior in such corner cases differs from
> the real CPU behavior, e.g. there'd be no exception in the TB crossing
> page boundary from the present to non-present page. I'll try to make a
> small testcase to prove (or disprove) it.

I've made a test from the grub multiboot sample, you may find it here:
http://jcmvbkbc.spb.ru/git/?p=dumb/qemu-test-kernel.git;a=summary

With it I see that an attempt to execute a TB that spans two pages causes
an exception when the second page is unmapped. It happens because both
tlb_flush and tlb_flush_page invalidate relevant tb_jmp_cache entries:
the former flushes all of them, the latter flushes them for two adjacent pages
around the given address. Later tb_find_fast fails to find a TB in the
tb_jmp_cache and has to call tb_find_slow which retranslates TB, triggering
a pagefault.

-- 
Thanks.
-- Max