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Message-ID: <2b6143b6-9db4-05bc-1e8d-c5d129126f99@intel.com>
Date:   Wed, 21 Sep 2022 07:15:07 -0700
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Subject: Re: [PATCH] ACPI: processor_idle: Skip dummy wait for processors
 based on the Zen microarchitecture
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To:     K Prateek Nayak <kprateek.nayak@amd.com>,
        linux-kernel@vger.kernel.org
Cc:     rafael@kernel.org, lenb@kernel.org, linux-acpi@vger.kernel.org,
        linux-pm@vger.kernel.org, dave.hansen@linux.intel.com,
        bp@alien8.de, tglx@linutronix.de, andi@lisas.de, puwen@hygon.cn,
        mario.limonciello@amd.com, peterz@infradead.org,
        rui.zhang@intel.com, gpiccoli@igalia.com,
        daniel.lezcano@linaro.org, ananth.narayan@amd.com,
        gautham.shenoy@amd.com, Calvin Ong <calvin.ong@amd.com>,
        stable@vger.kernel.org, regressions@lists.linux.dev
References: <20220921063638.2489-1-kprateek.nayak@amd.com>
From:   Dave Hansen <dave.hansen@intel.com>
In-Reply-To: <20220921063638.2489-1-kprateek.nayak@amd.com>
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On 9/20/22 23:36, K Prateek Nayak wrote:
> +	/*
> +	 * No delay is needed if we are in guest or on a processor
> +	 * based on the Zen microarchitecture.
> +	 */
> +	if (boot_cpu_has(X86_FEATURE_HYPERVISOR) || boot_cpu_has(X86_FEATURE_ZEN))
>  		return;

In the end, the delay is because of buggy, circa 2006 chipsets?  So, we
use a CPU vendor specific check to approximate that the chipset is
recent and not affected by the bug?  If so, is there no better way to
check for a newer chipset than this?

Do X86_FEATURE_ZEN CPUs just have unusually painful
inl(acpi_fadt.xpm_tmr_blk.address) implementations?  Is that why we
noticed all of a sudden?