linux-kernel - Re: [Xen-devel] maybe revert commit c275a57f5ec3 "xen/balloon: Set balloon's initial state to number of existing RAM pages"

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Message-ID: <2288a326-752e-85a5-d986-62c86b91e249@oracle.com>
Date:   Tue, 28 Mar 2017 10:27:59 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Jan Beulich <JBeulich@...e.com>,
        Dan Streetman <dan.streetman@...onical.com>
Cc:     xen-devel@...ts.xenproject.org, Juergen Gross <jgross@...e.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [Xen-devel] maybe revert commit c275a57f5ec3 "xen/balloon: Set
 balloon's initial state to number of existing RAM pages"

On 03/28/2017 04:08 AM, Jan Beulich wrote:
>>>> On 28.03.17 at 03:57, <boris.ostrovsky@...cle.com> wrote:
>> I think there is indeed a disconnect between target memory (provided by 
>> the toolstack) and current memory (i.e actual pages available to the guest).
>>
>> For example
>>
>> [    0.000000] BIOS-e820: [mem 0x000000000009e000-0x000000000009ffff] 
>> reserved
>> [    0.000000] BIOS-e820: [mem 0x00000000000e0000-0x00000000000fffff] 
>> reserved
>>
>> are missed in target calculation. The hvmloader marks them as RESERVED 
>> (in build_e820_table()) but target value is not aware of this action.
>>
>> And then the same problem repeats when kernel removes 
>> 0x000a0000-0x000fffff chunk.
> But this is all in-guest behavior, i.e. nothing an entity outside the
> guest (tool stack or hypervisor) should need to be aware of. That
> said, there is still room for improvement in the tools I think:
> Regions which architecturally aren't RAM (namely the
> 0xa0000-0xfffff range) would probably better not be accounted
> for as RAM as far as ballooning is concerned. In the hypervisor,
> otoh, all memory assigned to the guest (i.e. including such backing
> ROMs) needs to be accounted.

On the Linux side we should not include in balloon calculations pages
reserved by trim_bios_range(), i.e. (BIOS_END-BIOS_BEGIN) + 1.

Which leaves hvmloader's special pages (and possibly memory under
0xA0000 which may get reserved). Can we pass this info to guests via
xenstore?

-boris