[Qemu-devel] Re: tcg/{ppc, s390, sparc}: branch target and code retranslation

[Aurelien Jarno <aurelien@aurel32.net>]

On Mon, Jan 10, 2011 at 03:07:52PM +0100, Alexander Graf wrote:
> 
> On 10.01.2011, at 15:00, Aurelien Jarno wrote:
> 
> > On Mon, Jan 10, 2011 at 01:13:25PM +0100, Alexander Graf wrote:
> >> 
> >> On 06.01.2011, at 23:12, Aurelien Jarno wrote:
> >> 
> >>> Hi,
> >>> 
> >>> I have just sent a tcg/arm patch concerning code retranslation. You
> >>> might want to look at the description (copied below), as from a first
> >>> glance ppc, s390 and sparc TCG targets might be affected. If you see
> >>> guest kernel panics, some segmentation fault of qemu or in the guest,
> >>> strange behaviors, that happen randomly and that looks difficult to
> >>> debug it might be the issue.
> >>> 
> >>> Aurelien
> >>> 
> >>> 
> >>> | QEMU uses code retranslation to restore the CPU state when an exception
> >>> | happens. For it to work the retranslation must not modify the generated
> >>> | code. This is what is currently implemented in ARM TCG.
> >>> |
> >>> | However on CPU that don't have icache/dcache/memory synchronised like
> >>> | ARM, this requirement is stronger and code retranslation must not modify
> >>> | the generated code "atomically", as the cache line might be flushed
> >>> | at any moment (interrupt, exception, task switching), even if not
> >>> | triggered by QEMU. The probability for this to happen is very low, and
> >>> | depends on cache size and associativiy, machine load, interrupts, so the
> >>> | symptoms are might happen randomly.
> >>> |
> >>> | This requirement is currently not followed in tcg/arm, for the
> >>> | load/store code, which basically has the following structure:
> >>> |   1) tlb access code is written
> >>> |   2) conditional fast path code is written
> >>> |   3) branch is written with a temporary target
> >>> |   4) slow path code is written
> >>> |   5) branch target is updated
> >>> | The cache lines corresponding to the retranslated code is not flushed
> >>> | after code retranslation as the generated code is supposed to be the
> >>> | same. However if the cache line corresponding to the branch instruction
> >>> | is flushed between step 3 and 5, and is not flushed again before the
> >>> | code is executed again, the branch target is wrong. In the guest, the
> >>> | symptoms are MMU page fault at a random addresses, which leads to
> >>> | kernel page fault or segmentation faults.
> >> 
> >> I don't see the problem. If you have separate icache from dcache, the code in question doesn't get executed during the rewrite, so all should be fine. If you have both combined, the data write should automatically modify the cache line, so all is great too.
> > 
> > As far as I understand, this is what happens to the branch target
> > instruction, or at least one possibility:
> > 
> > operation                                 | icache | dcache | mem/L2 |
> > ------------------------------------------+--------+--------+--------+
> > tlb access code is written                | absent | absent |   ok   |
> > conditional fast path code is written     | absent | absent |   ok   |
> > branch is written with a temporary target | absent | wrong  |   ok   |
> > cache line is flushed to memory           | absent | absent |  wrong |
> > slow path code is written                 | absent | absent |  wrong |
> > branch target is updated                  | absent |   ok   |  wrong |
> > TB is re-executed                         | wrong  |   ok   |  wrong |
> > 
> > Note that the issue is real, the patch really fixes the issue on ARM and
> > MIPS. It's not impossible that I don't fully understand it given I can't
> > analyse the cache values at a given time. However, when QEMU crashes 
> > because of that, we have seen that the executed code doesn't match what
> > GDB says, and changing the temporary branch value also changes the way 
> > QEMU or the guest crash.
> > 
> > The problem doesn't happens very often though (for sure less than 1 
> > every few millions). The other way to fix it is to issue a cache flush
> > after a code retranslation, however, it is something very costly on some
> > architectures.
> 
> I'd guess it only happens when code is overwritten. Only then icache can still be stale in that code region. Can't we just invalidate the icache on every tb flush? That should also fix the issue I'd guess.

That's a solution that works (tested). However making sure that the
retranslation doesn't change the code looks better.

> > 
> >> As long as the code in question doesn't get run in between, I don't see where breakage could occur?
> > 
> > The breakage can occur, because the same TB can be re-executed later,
> > and we don't have an explicit cache flush after the retranslation.
> 
> Oh, I think I'm starting to understand the issue. Have you seen this with system or user emulation? I'd guess with user it makes sense:
> 
> [T1] trap
> [T2] run code
> [T1] write branch code to find guest IP
> [T2] run exactly that branch code
> [T1] rewrite branch code
> [T2] has invalid code in its cache
> 
> If that's the case, a global lock on retranslation should do the trick, no?
>

I haven't seen that during user emulation, however your scenario, which
is different is also possible. Note however that they are far less
retranslation on linux-user code. Also far less code being translated.

In any case, a global lock also doesn't seems to be the best solution,
just making sure to not change the code during retranslation seems a
better solution.

> If it was during system emulation, I'm still puzzled :).
> 

Yes, it was during system emulation.

-- 
Aurelien Jarno	                        GPG: 1024D/F1BCDB73
aurelien@aurel32.net                 http://www.aurel32.net