linux-kernel - Re: [PATCH v5 07/22] x86/virt/tdx: Implement SEAMCALL function

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Date:   Wed, 03 Aug 2022 14:37:18 +1200
From:   Kai Huang <kai.huang@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     seanjc@...gle.com, pbonzini@...hat.com, len.brown@...el.com,
        tony.luck@...el.com, rafael.j.wysocki@...el.com,
        reinette.chatre@...el.com, dan.j.williams@...el.com,
        peterz@...radead.org, ak@...ux.intel.com,
        kirill.shutemov@...ux.intel.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com,
        isaku.yamahata@...el.com
Subject: Re: [PATCH v5 07/22] x86/virt/tdx: Implement SEAMCALL function

On Thu, 2022-07-21 at 13:52 +1200, Kai Huang wrote:
> Also, if I understand correctly above, your suggestion is we want to prevent any
> CMR memory going offline so it won't be hot-removed (assuming we can get CMRs
> during boot).  This looks contradicts to the requirement of being able to allow
> moving memory from core-mm to driver.  When we offline the memory, we cannot
> know whether the memory will be used by driver, or later hot-removed.

Hi Dave,

The high level flow of device hot-removal is:

acpi_scan_hot_remove()
	-> acpi_scan_try_to_offline()
		-> acpi_bus_offline()
			-> device_offline()
				-> memory_subsys_offline()
	-> acpi_bus_trim()
		-> acpi_memory_device_remove()

And memory_subsys_offline() can also be triggered via /sysfs:

	echo 0 > /sys/devices/system/memory/memory30/online

After the memory block is offline, my understanding is kernel can theoretically
move it to, i.e. ZONE_DEVICE via memremap_pages().

As you can see memory_subsys_offline() is the entry point of memory device
offline (before it the code is generic for all ACPI device), and it cannot
distinguish whether the removal is from ACPI event, or from /sysfs, so it seems
we are unable to refuse to offline memory in  memory_subsys_offline() when it is
called from ACPI event.

Any comments?

-- 
Thanks,
-Kai