Re: [Qemu-devel] [PATCH v10 05/23] tcg: add options for enabling MTTCG

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

Pranith Kumar <bobby.prani@gmail.com> writes:

> Hi Alex,
>
> I just spotted this:
>
> On Mon, Feb 6, 2017 at 10:30 AM, Alex Bennée <alex.bennee@linaro.org> wrote:
>
>> From: KONRAD Frederic <fred.konrad@greensocs.com>
>>
>> We know there will be cases where MTTCG won't work until additional work
>> is done in the front/back ends to support. It will however be useful to
>> be able to turn it on.
>>
>> As a result MTTCG will default to off unless the combination is
>> supported. However the user can turn it on for the sake of testing.
>>
>> Signed-off-by: KONRAD Frederic <fred.konrad@greensocs.com>
>> [AJB: move to -accel tcg,thread=multi|single, defaults]
>> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
>> Reviewed-by: Richard Henderson <rth@twiddle.net>
>> ---
>> v1:
>>   - merge with add mttcg option.
>>   - update commit message
>> v2:
>>   - machine_init->opts_init
>> v3:
>>   - moved from -tcg to -accel tcg,thread=single|multi
>>   - fix checkpatch warnings
>> v4:
>>   - make mttcg_enabled extern, qemu_tcg_mttcg_enabled() now just macro
>>   - qemu_tcg_configure now propagates Error instead of exiting
>>   - better error checking of thread=foo
>>   - use CONFIG flags for default_mttcg_enabled()
>>   - disable mttcg with icount, error if both forced on
>> v7
>>   - explicitly disable MTTCG for TCG_OVERSIZED_GUEST
>>   - use check_tcg_memory_orders_compatible() instead of CONFIG_MTTCG_HOST
>>   - change CONFIG_MTTCG_TARGET to TARGET_SUPPORTS_MTTCG
>> v8
>>   - fix missing include tcg.h
>>   - change mismatched MOs to a warning instead of error
>> v10
>>   - TCG_DEFAULT_MO -> TCG_GUEST_DEFAULT_MO
>> ---
>>  cpus.c                | 72 ++++++++++++++++++++++++++++++
>> +++++++++++++++++++++
>>  include/qom/cpu.h     |  9 +++++++
>>  include/sysemu/cpus.h |  2 ++
>>  qemu-options.hx       | 20 ++++++++++++++
>>  tcg/tcg.h             |  9 +++++++
>>  vl.c                  | 49 ++++++++++++++++++++++++++++++++++-
>>  6 files changed, 160 insertions(+), 1 deletion(-)
>>
>> diff --git a/cpus.c b/cpus.c
>> index 71a82e5004..6a7e5a50d8 100644
>> --- a/cpus.c
>> +++ b/cpus.c
>> @@ -25,6 +25,7 @@
>>  /* Needed early for CONFIG_BSD etc. */
>>  #include "qemu/osdep.h"
>>  #include "qemu-common.h"
>> +#include "qemu/config-file.h"
>>  #include "cpu.h"
>>  #include "monitor/monitor.h"
>>  #include "qapi/qmp/qerror.h"
>> @@ -45,6 +46,7 @@
>>  #include "qemu/main-loop.h"
>>  #include "qemu/bitmap.h"
>>  #include "qemu/seqlock.h"
>> +#include "tcg.h"
>>  #include "qapi-event.h"
>>  #include "hw/nmi.h"
>>  #include "sysemu/replay.h"
>> @@ -150,6 +152,76 @@ typedef struct TimersState {
>>  } TimersState;
>>
>>  static TimersState timers_state;
>> +bool mttcg_enabled;
>> +
>> +/*
>> + * We default to false if we know other options have been enabled
>> + * which are currently incompatible with MTTCG. Otherwise when each
>> + * guest (target) has been updated to support:
>> + *   - atomic instructions
>> + *   - memory ordering primitives (barriers)
>> + * they can set the appropriate CONFIG flags in ${target}-softmmu.mak
>> + *
>> + * Once a guest architecture has been converted to the new primitives
>> + * there are two remaining limitations to check.
>> + *
>> + * - The guest can't be oversized (e.g. 64 bit guest on 32 bit host)
>> + * - The host must have a stronger memory order than the guest
>> + *
>> + * It may be possible in future to support strong guests on weak hosts
>> + * but that will require tagging all load/stores in a guest with their
>> + * implicit memory order requirements which would likely slow things
>> + * down a lot.
>> + */
>> +
>> +static bool check_tcg_memory_orders_compatible(void)
>> +{
>> +#if defined(TCG_GUEST_DEFAULT_MO) && defined(TCG_TARGET_DEFAULT_MO)
>> +    return (TCG_GUEST_DEFAULT_MO & ~TCG_TARGET_DEFAULT_MO) == 0;
>> +#else
>> +    return false;
>> +#endif
>> +}
>> +
>> +static bool default_mttcg_enabled(void)
>> +{
>> +    QemuOpts *icount_opts = qemu_find_opts_singleton("icount");
>> +    const char *rr = qemu_opt_get(icount_opts, "rr");
>> +
>> +    if (rr || TCG_OVERSIZED_GUEST) {
>> +        return false;
>> +    } else {
>> +#ifdef TARGET_SUPPORTS_MTTCG
>> +        return check_tcg_memory_orders_compatible();
>> +#else
>> +        return false;
>> +#endif
>> +    }
>> +}
>> +
>> +void qemu_tcg_configure(QemuOpts *opts, Error **errp)
>> +{
>> +    const char *t = qemu_opt_get(opts, "thread");
>> +    if (t) {
>> +        if (strcmp(t, "multi") == 0) {
>> +            if (TCG_OVERSIZED_GUEST) {
>> +                error_setg(errp, "No MTTCG when guest word size > hosts");
>> +            } else {
>> +                if (!check_tcg_memory_orders_compatible()) {
>> +                    error_report("Guest requires stronger MO that host");
>>
>
> s/MO that host/MO than host/?

Good catch.

>
> Also, this message is a bit cryptic for end users. How about saying "Guest
> and host architectures are incompatible, you may encounter errors!" or
> something similar?

"Guest assumes a stronger memory ordering than the host provides, this
may cause strange/hard to debug errors."

?
>
> Thanks,


--
Alex Bennée