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From:   Thomas Gleixner <tglx@linutronix.de>
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To:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        mingo@redhat.com, bp@alien8.de, dave.hansen@intel.com,
        luto@kernel.org, peterz@infradead.org
Cc:     sathyanarayanan.kuppuswamy@linux.intel.com, aarcange@redhat.com,
        ak@linux.intel.com, dan.j.williams@intel.com, david@redhat.com,
        hpa@zytor.com, jgross@suse.com, jmattson@google.com,
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        thomas.lendacky@amd.com, brijesh.singh@amd.com, x86@kernel.org,
        linux-kernel@vger.kernel.org,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
Subject: Re: [PATCHv6 24/30] x86/topology: Disable CPU online/offline
 control for TDX guests
In-Reply-To: <20220316020856.24435-25-kirill.shutemov@linux.intel.com>
References: <20220316020856.24435-1-kirill.shutemov@linux.intel.com>
 <20220316020856.24435-25-kirill.shutemov@linux.intel.com>
Date:   Thu, 17 Mar 2022 13:50:08 +0100
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On Wed, Mar 16 2022 at 05:08, Kirill A. Shutemov wrote:
> From: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
>
> Unlike regular VMs, TDX guests use the firmware hand-off wakeup method
> to wake up the APs during the boot process. This wakeup model uses a
> mailbox to communicate with firmware to bring up the APs. As per the
> design, this mailbox can only be used once for the given AP, which means
> after the APs are booted, the same mailbox cannot be used to
> offline/online the given AP. More details about this requirement can be
> found in Intel TDX Virtual Firmware Design Guide, sec titled "AP
> initialization in OS" and in sec titled "Hotplug Device".
>
> Since the architecture does not support any method of offlining the
> CPUs, disable CPU hotplug support in the kernel.
>
> Since this hotplug disable feature can be re-used by other VM guests,
> add a new CC attribute CC_ATTR_HOTPLUG_DISABLED and use it to disable
> the hotplug support.
>
> With hotplug disabled, /sys/devices/system/cpu/cpuX/online sysfs option
> will not exist for TDX guests.

I can't find any code which removes the online file. The write will fail
with -EOPNOTSUPP, right?

> Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
> Reviewed-by: Andi Kleen <ak@linux.intel.com>
> Reviewed-by: Tony Luck <tony.luck@intel.com>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

Other than that:

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>