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Re: [Xen-devel] [PATCH for 4.13] x86/shim: don't reserve 640k - 1M region in E820
- To: Jan Beulich <jbeulich@xxxxxxxx>, Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Mon, 28 Oct 2019 11:15:46 +0000
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- Cc: Juergen Gross <jgross@xxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, xen-devel@xxxxxxxxxxxxx
- Delivery-date: Mon, 28 Oct 2019 11:16:03 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 28/10/2019 11:11, Jan Beulich wrote:
> On 28.10.2019 12:08, Sergey Dyasli wrote:
>> On 28/10/2019 09:06, Jan Beulich wrote:
>>> On 28.10.2019 09:56, Sergey Dyasli wrote:
>>>> Converting a guest from PV to PV-in-PVH makes the guest to have 384k
>>>> less memory, which may confuse guest's balloon driver. This happens
>>>> because Xen unconditionally reserves 640k - 1M region in E820 despite
>>>> the fact that it's really a usable RAM in PVH boot mode.
>>> This is a PVH property according to your description and according
>>> to my understanding. Why would you then ...
>>>
>>>> Fix this by skipping the region type change for pv-shim mode.
>>> ... alter behavior for shim mode only, rather than when booted in
>>> PVH mode in general?
>> This is just me being cautious.
>>
>> My original email does have this text after ---
>> "The condition can be relaxed to be just !pvh_boot if there are no
>> plans to support VGA MMIO / ROMs for PVH guests."
> I doubt emulated ones are intended to be supported, but of
> course a graphics card could be passed through. Iirc passthrough
> is a pending work item still anyway for PVH, so dealing with the
> case right now may not even be necessary.
The bug is Xen blindly presuming that a missing hole "must be a firmware
bug".
I can believe that there may have been systems decades ago which got
this wrong, but tbh I doubt it applies to 64bit-capable systems,
considering how standardised things were by then.
I'd just delete this whole piece of logic and call it done.
~Andrew
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