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([2400:4050:a840:1e00:32ed:25ae:21b1:72d6]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-210bbf87e0fsm50570785ad.113.2024.10.28.07.02.03 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 28 Oct 2024 07:02:09 -0700 (PDT) Message-ID: <27ced2db-472d-47ae-9047-9efc0b589a1e@daynix.com> Date: Mon, 28 Oct 2024 23:02:02 +0900 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 02/15] hw/display/apple-gfx: Introduce ParavirtualizedGraphics.Framework support To: Phil Dennis-Jordan Cc: qemu-devel@nongnu.org, agraf@csgraf.de, peter.maydell@linaro.org, pbonzini@redhat.com, rad@semihalf.com, quic_llindhol@quicinc.com, marcin.juszkiewicz@linaro.org, stefanha@redhat.com, mst@redhat.com, slp@redhat.com, richard.henderson@linaro.org, eduardo@habkost.net, marcel.apfelbaum@gmail.com, gaosong@loongson.cn, jiaxun.yang@flygoat.com, chenhuacai@kernel.org, kwolf@redhat.com, hreitz@redhat.com, philmd@linaro.org, shorne@gmail.com, palmer@dabbelt.com, alistair.francis@wdc.com, bmeng.cn@gmail.com, liwei1518@gmail.com, dbarboza@ventanamicro.com, zhiwei_liu@linux.alibaba.com, jcmvbkbc@gmail.com, marcandre.lureau@redhat.com, berrange@redhat.com, qemu-arm@nongnu.org, qemu-block@nongnu.org, qemu-riscv@nongnu.org, Alexander Graf References: <20241024102813.9855-1-phil@philjordan.eu> <20241024102813.9855-3-phil@philjordan.eu> <9e310d5e-ab73-47b9-b9ed-5a16d4db3fb9@daynix.com> <6a989d04-6416-4bd9-98ac-e1230a1095a9@daynix.com> Content-Language: en-US From: Akihiko Odaki In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Received-SPF: none client-ip=2607:f8b0:4864:20::635; envelope-from=akihiko.odaki@daynix.com; helo=mail-pl1-x635.google.com X-Spam_score_int: -18 X-Spam_score: -1.9 X-Spam_bar: - X-Spam_report: (-1.9 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_NONE=0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On 2024/10/28 22:31, Phil Dennis-Jordan wrote: > > > On Mon, 28 Oct 2024 at 10:00, Phil Dennis-Jordan > wrote: > > > >      > > >      > Hmm. I think if we were to use that, we would need to > create a new > >      > QemuEvent for every job and destroy it afterward, > which seems > >     expensive. > >      > We can't rule out multiple concurrent jobs being > submitted, and the > >      > QemuEvent system only supports a single producer as > far as I can > >     tell. > >      > > >      > You can probably sort of hack around it with just one > QemuEvent by > >      > putting the qemu_event_wait into a loop and turning > the job.done > >     flag > >      > into an atomic (because it would now need to be > checked outside the > >      > lock) but this all seems unnecessarily complicated > considering the > >      > QemuEvent uses the same mechanism QemuCond/QemuMutex > internally > >     on macOS > >      > (the only platform relevant here), except we can use it as > >     intended with > >      > QemuCond/QemuMutex rather than having to work against the > >     abstraction. > > > >     I don't think it's going to be used concurrently. It > would be difficult > >     to reason even for the framework if it performs memory > >     unmapping/mapping/reading operations concurrently. > > > > > > I've just performed a very quick test by wrapping the job > submission/ > > wait in the 2 mapMemory callbacks and the 1 readMemory > callback with > > atomic counters and logging whenever a counter went above 1. > > > >   * Overall, concurrent callbacks across all types were > common (many per > > second when the VM is busy). It's not exactly a "thundering > herd" (I > > never saw >2) but it's probably not a bad idea to use a separate > > condition variable for each job type. (task map, surface map, > memory read) > >   * While I did not observe any concurrent memory mapping > operations > > *within* a type of memory map (2 task mappings or 2 surface > mappings) I > > did see very occasional concurrent memory *read* callbacks. > These would, > > as far as I can tell, not be safe with QemuEvents, unless we > placed the > > event inside the job struct and init/destroyed it on every > callback > > (which seems like excessive overhead). > > I think we can tolerate that overhead. init/destroy essentially > sets the > fields in the data structure and I estimate its total size is > about 100 > bytes. It is probably better than waking an irrelevant thread > up. I also > hope that keeps the code simple; it's not worthwhile adding code to > optimize this. > > > At least pthread_cond_{init,destroy} and > pthread_mutex_{init,destroy} don't make any syscalls, so yeah it's > probably an acceptable overhead. > > > I've just experimented with QemuEvents created on-demand and ran into > some weird deadlocks, which then made me sit down and think about it > some more. I've come to the conclusion that creating (and crucially, > destroying) QemuEvents on demand in this way is not safe. > > Specifically, you must not call qemu_event_destroy() - which > transitively destroys the mutex and condition variable - unless you can > guarantee that the qemu_event_set() call on that event object has completed. > > In qemu_event_set, the event object's value is atomically set to EV_SET. > If the previous value was EV_BUSY, qemu_futex_wake() is called. All of > this is outside any mutex, however, so apart from memory coherence > (there are barriers) this can race with the waiting thread. > qemu_event_wait() reads the event's value. If EV_FREE, it's atomically > set to EV_BUSY. Then the mutex is locked, the value is checked again, > and if it's still EV_BUSY, it waits for the condition variable, > otherwise the mutex is immediately unlocked again. If the trigger > thread's qemu_event_set() flip to EV_SET occurs between the waiting > thread's two atomic reads of the value, the waiting thread will never > wait for the condition variable, but the trigger thread WILL try to > acquire the mutex and signal the condition variable in > qemu_futex_wake(), by which  time the waiting thread may have advanced > outside of qemu_event_wait(). Sorry if I'm making a mistake again, but the waiting thread won't set to EV_BUSY unless the value is EV_FREE on the second read so the trigger thread will not call qemu_futex_wake() if it manages to set to EV_SET before the second read, will it? > > This is all fine usually, BUT if you destroy the QemuEvent immediately > after the qemu_event_wait() call, qemu_futex_wake() may try to lock a > mutex that has been destroyed, or signal a condition variable which has > been destroyed. I don't see a reasonable way of making this safe other > than using long-lived mutexes and condition variables. And anyway, we > have much, MUCH bigger contention/performance issues coming from almost > everything being covered by the BQL. (If waking these callbacks can even > be considered an issue: I haven't seen it show up in profiling, whereas > BQL contention very much does.) > > I'll submit v5 of this patch set with separate condition variables for > each job type. This should make the occurrence of waking the wrong > thread quite rare, while reasoning about correctness is pretty > straightforward. I think that's good enough. > > >