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Re: [Qemu-arm] [RFC PATCH 0/4] ARM virt: ACPI memory hotplug support

From: Laszlo Ersek
Subject: Re: [Qemu-arm] [RFC PATCH 0/4] ARM virt: ACPI memory hotplug support
Date: Fri, 1 Mar 2019 14:49:45 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1

On 02/28/19 15:02, Shameerali Kolothum Thodi wrote:

> Ah..I missed the fact that, firmware indeed sees an update in the blob len 
> here
> (rounded or not) after reboot. So don’t think x86 has the same issue and 
> padding
> is not the right solution as Igor explained in his reply.
> I will try to debug this further. Any pointers welcome.

How about this.

(1) The firmware looks up the fw_cfg file called "etc/table-loader" in
the fw_cfg file directory (identified by constant selector key 0x0019,

(2) The directory entry, once found, tells the firmware two things
simultaneously. The selector key, and the size of the blob.

(3) The firmware selects the selector key from step (2).

(4) QEMU regenerates the ACPI payload (as a select callback).

(5) The firmware reads the number of bytes from the fw_cfg blob that it
learned in step (2).

Here's the problem. As long as QEMU used to perform step (4) only for
the purpose of refreshing PCI resources in the ACPI payload, step (4)
wouldn't *resize* the blob.

However, if step (4) enlarges the blob, then the byte count that step
(5) uses -- from step (2) -- for reading, is obsolete.


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