Re: [Qemu-devel] [PATCH v2 00/45] tcg: support for multiple TCG contexts

["Alex Bennée" <alex.bennee@linaro.org>]

Richard Henderson <rth@twiddle.net> writes:

> On 07/18/2017 02:22 PM, jiang.biao2@zte.com.cn wrote:
>> Seeing your work on multiple TCG, it seems that it has some kind of
>> connection with the  MTTCG feature,
>>
>> but I do not figure out how they are connected in detail.
>>
>> Could you pls help to confirm the following questions:
>>
>>  1.
>>
>>     what is the relationship between your patches and the MTTCG feature
>>     mentioned by https://lwn.net/Articles/697265/?
>
>
> The current MTTCG feature is in QEMU mainline.  It allows parallel
> execution of translated code in both system mode.  It does *not* allow
> parallel translation -- all translation is done with tb_lock held.
>
> Note that we *always* have parallel execution in user mode.  However,
> this can and does lead to problems.  See below.
>
> This patch set allows parallel translation in system mode.  This is
> shown to improve the overall throughput.  It does *not* allow parallel
> translation in user mode.  Firstly because user mode already shares
> more translations between threads (because it is running a single
> executable), and so the translation routines are not high in the
> profile.  Secondly because there are additional locking problems due
> to the fact that we have no bound on the number of user threads.
>
>
>>  2.
>>
>>     What is the current status of the development of the MTTCG feature?
>
> MTTCG has only been enabled on a few targets: alpha, arm, ppc64.
> Look for "mttcg=yes" in configure.
>
> In order for MTTCG to be enabled, the target must be adjusted so that
> (1) all atomic instructions are implemented with atomic tcg operations,
> (2) define TCG_GUEST_DEFAULT_MO to indicate any barriers implied by
>     normal memory operations by the target architecture.
>
> For target/mips, neither of these things are complete.
>
> MTTCG has only been enabled on one host: i386.
> Look for TCG_TARGET_DEFAULT_MO in tcg/*/tcg-target.h.
>
> In order for MTTCG to be enabled, the target memory order must not be
> stronger than the host memory order.  Since i386 has a very strong
> host memory order, it is easy for it to emulate any guest.  When the
> host has a weak memory order, we need to add the additional barriers
> that are implied by the target.  This is work that has not been done.
>
> I am not sure why we have not already added this definition to all of
> the other tcg hosts.  I think this is just oversight, since almost
> everyone uses x86_64 linux as the host for testing tcg.  However,
> since all of the supported targets have weak memory orders we ought to
> be able to support them with any host.

Yeah the MO definitions can be added to the other guests/backends. I was
hoping it would be done by those who have a better understanding of each
guests micro-architecture so as to avoid any silly mistakes. As you say
I think they will all mostly be 0 anyway ;-)

>>  3.
>>
>>     Is there any problem with the multithread programme running with linux-user
>>     qemu mode? would the situation be improved with  the MTTCG feature?
>>
>>     We need to use linux-user mode qemu to run multithread app, but there seems
>>     to be many problem.
>
> For user mode, we should still follow the rules for MTTCG, but we do
> not. Instead we take it on faith that they have been and execute the
> code in parallel anyway.  This faith is often misplaced and it does
> mean that unsupported targets execute user mode code incorrectly.

Certainly for user-mode once the appropriate changes are made to atomic
and barrier support instructions user-mode stability should improve
(modulo unsupported/buggy syscall translation).

System mode emulation usually requires a bit more work and system-wide
instructions, usually ones which trigger interrupts and TLB flushes, to
make sure they are done in a safe way.

--
Alex Bennée