From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: "Philippe Mathieu-Daudé" <philmd@linaro.org>, qemu-devel@nongnu.org
Cc: Bernhard Beschow <shentey@gmail.com>,
Nicholas Piggin <npiggin@gmail.com>,
bharata.rao@gmail.com, Chinmay Rath <rathc@linux.ibm.com>,
qemu-ppc@nongnu.org
Subject: Re: [PATCH 2/3] target/ppc: Have gen_pause() actually pause vCPUs
Date: Mon, 29 Sep 2025 14:28:57 +1000 [thread overview]
Message-ID: <4b0138bc76c60385de71c26eb55a4aecd8d1786d.camel@kernel.crashing.org> (raw)
In-Reply-To: <20250924173028.53658-3-philmd@linaro.org>
On Wed, 2025-09-24 at 19:30 +0200, Philippe Mathieu-Daudé wrote:
> gen_pause() sets CPUState::halted = 0, effectively unhalting
> (a.k.a. "running") the cpu. Correct by setting the '1' value
> to really halt the cpu.
What will resume it though ? The smt_low() case isn't meant to *halt*
the CPUs permanently. smt_*() levels are about SMT thread priorities.
Using a "pause" that just gets out of TCG (and back in), is a way to
"yield" to another thread, thus enabling more forward progress when a
thread is spinning on an smt_low() loop. This happens in firmware and
in some spinlock cases.
This isn't about stopping until some external event resumes it.
Cheers,
Ben.
next prev parent reply other threads:[~2025-09-29 4:32 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-09-24 17:30 [PATCH 0/3] target/ppc: Have gen_pause() actually pause vCPUs Philippe Mathieu-Daudé
2025-09-24 17:30 ` [PATCH 1/3] hw/ppc: Do not open-code cpu_resume() in spin_kick() Philippe Mathieu-Daudé
2025-09-24 17:58 ` Richard Henderson
2025-09-24 17:30 ` [PATCH 2/3] target/ppc: Have gen_pause() actually pause vCPUs Philippe Mathieu-Daudé
2025-09-24 17:58 ` Richard Henderson
2025-09-29 4:28 ` Benjamin Herrenschmidt [this message]
2025-09-29 7:51 ` Philippe Mathieu-Daudé
2025-09-29 22:59 ` Benjamin Herrenschmidt
2025-09-30 3:00 ` Philippe Mathieu-Daudé
2025-09-24 17:30 ` [PATCH 3/3] target/ppc: Re-use gen_pause() in gen_wait() Philippe Mathieu-Daudé
2025-09-24 18:05 ` Richard Henderson
2025-10-07 8:21 ` [PATCH 0/3] target/ppc: Have gen_pause() actually pause vCPUs Philippe Mathieu-Daudé
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