From: Peter Maydell <peter.maydell@linaro.org>
To: Stefan Hajnoczi <stefanha@redhat.com>
Cc: "Richard Henderson" <richard.henderson@linaro.org>,
"QEMU Developers" <qemu-devel@nongnu.org>,
"Marc-André Lureau" <marcandre.lureau@gmail.com>,
"Paolo Bonzini" <pbonzini@redhat.com>,
"Samuel Thibault" <samuel.thibault@ens-lyon.org>,
"Philippe Mathieu-Daudé" <philmd@redhat.com>
Subject: Re: [PULL 0/2] Libslirp patches
Date: Mon, 5 Jul 2021 17:31:05 +0100 [thread overview]
Message-ID: <CAFEAcA9JnYri2TiB-zE1J0w26TQnUzzMe_z2XUD9YxBugJm6tg@mail.gmail.com> (raw)
In-Reply-To: <YOMyh9fu5ZEAlGHN@stefanha-x1.localdomain>
On Mon, 5 Jul 2021 at 17:25, Stefan Hajnoczi <stefanha@redhat.com> wrote:
>
> On Mon, Jul 05, 2021 at 12:31:01PM +0200, Philippe Mathieu-Daudé wrote:
> > +Richard/Stefan for "atomic" error.
> ...
> > > [2363/9207] Linking target qemu-system-aarch64
> > > FAILED: qemu-system-aarch64
> > > clang++ @qemu-system-aarch64.rsp
> > > libqemu-aarch64-softmmu.fa.p/accel_tcg_cputlb.c.o: In function
> > > `helper_atomic_cmpxchgq_le_mmu':
> > > /tmp/tmp.VkWONZ62bA/build/../accel/tcg/atomic_template.h:86: undefined
> > > reference to `__atomic_compare_exchange_8'
> > > libqemu-aarch64-softmmu.fa.p/accel_tcg_cputlb.c.o: In function
> > > `helper_atomic_xchgq_le_mmu':
> > > /tmp/tmp.VkWONZ62bA/build/../accel/tcg/atomic_template.h:134: undefined
> > > reference to `__atomic_exchange_8'
> > > libqemu-aarch64-softmmu.fa.p/accel_tcg_cputlb.c.o: In function
> > > `helper_atomic_fetch_addq_le_mmu':
>
> According to docs/devel/atomics.rst:
>
> These operations are polymorphic; they operate on any type that is as
> wide as a pointer or smaller.
>
> It looks like the compiler doesn't support 8-bit atomics here?
8 here means "8 bytes", not "8 bits". And indeed on i386 you can't
do 8-byte atomics with simple insns. The compiler's answer to this
is "emit a call to a helper in libatomic, which will emulate an
atomic access by taking some kind of lock". We don't ever want to
fall back to "take a lock" because sometimes our accesses to the
atomic variables are from TCG generated code -- this is why we
don't link against libatomic. The problem is that we have not
correctly detected that this compiler can't do inline atomics
for 64-bit values and avoided using them. But at least we have
made this a compile failure rather than a silently-wrong-code bug :-)
thanks
-- PMM
next prev parent reply other threads:[~2021-07-05 16:33 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-29 18:55 [PULL 0/2] Libslirp patches marcandre.lureau
2021-05-29 18:55 ` [PULL 1/2] Update libslirp to v4.5.0 marcandre.lureau
2021-06-18 16:01 ` Doug Evans
2021-06-18 18:14 ` Marc-André Lureau
2021-07-27 20:05 ` Doug Evans
2021-05-29 18:55 ` [PULL 2/2] build-sys: make libslirp a meson subproject marcandre.lureau
2021-06-01 9:10 ` [PULL 0/2] Libslirp patches Peter Maydell
2021-06-01 11:01 ` Marc-André Lureau
2021-06-07 12:16 ` Peter Maydell
2021-06-08 15:55 ` Marc-André Lureau
2021-06-08 16:55 ` Peter Maydell
2021-06-08 20:35 ` Marc-André Lureau
2021-07-05 10:31 ` Philippe Mathieu-Daudé
2021-07-05 16:25 ` Stefan Hajnoczi
2021-07-05 16:31 ` Peter Maydell [this message]
2021-07-05 16:43 ` Paolo Bonzini
2021-07-05 16:47 ` Paolo Bonzini
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