Subject: Re: [PATCH] xen/balloon: Mark unallocated host memory as UNUSABLE
To: Juergen Gross <jgross@suse.com>, xen-devel@lists.xen.org,
        linux-kernel@vger.kernel.org
References: <1513119066-10748-1-git-send-email-boris.ostrovsky@oracle.com>
 <14852f9f-f234-3226-f32b-3080ca114332@suse.com>
 <a8495745-b56a-83ff-993f-a745c8edf2c0@oracle.com>
 <1385e130-7300-66a4-3995-838244600746@suse.com>
Cc: helgaas@kernel.org, christian.koenig@amd.com
From: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Message-ID: <9f7c7d7b-e8a7-d851-472d-972a815bf8e2@oracle.com>
Date: Fri, 15 Dec 2017 09:58:49 -0500
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On 12/15/2017 09:47 AM, Juergen Gross wrote:
> On 15/12/17 15:24, Boris Ostrovsky wrote:
>>>> +
>>>> +	hostmem_resource->start = max_addr;
>>>> +	hostmem_resource->end = entry->addr + entry->size;
>>>> +	for (; i < memmap.nr_entries; i++) {
>>>> +		entry = &xen_e820_table->entries[i];
>>>> +		if (entry->type == E820_TYPE_RAM)
>>> Shouldn't that be != ?
>> No, the idea here is to populate hostmem_resource with ranges already
>> taken by things other than RAM, leaving memory regions as available for
>> the balloon hotplug. This will allow us to use allocate_resource(),
>> which searches for a free range, in the balloon driver.
> But why says the comment "Host memory not allocated to dom0" then?

hostmem_resource is created starting from the end of dom0 RAM and ending
at the end of host RAM. If these two are the same (or if the former is
larger, which I don't think is possible) then there is nothing to do, as
the hostmem_resource will be empty. That's what the comment is referring
to.

> And why are you trying to allocate from this resource in case of
> hotplugging memory (and fall back to iomem_resource in case of
> failure)?

Because that area (end of dom0 RAM through end of host RAM) is not going
to be used by anyone else and thus is available. That was the idea
behind f5775e0b6116.

-boris


>
> Either the comment is completely wrong and I don't understand the
> logic here, or your code is wrong.