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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>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Mon, 28 Oct 2019 11:33:41 +0000
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- Cc: Juergen Gross <jgross@xxxxxxxx>, Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, xen-devel@xxxxxxxxxxxxx
- Delivery-date: Mon, 28 Oct 2019 11:33:54 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 28/10/2019 11:30, Jan Beulich wrote:
> On 28.10.2019 12:15, Andrew Cooper wrote:
>> 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.
> Well, I could see use being less aggressive here, but firmware (if
> there is firmware, i.e. everything except PVH) claiming there to
> be RAM immediately below the 1M boundary is a pretty good sign of
> something being wrong. There _ought_ to be ROM at that address.
> Otoh there were even ways in older chipsets to make RAM appear at
> address ranges not used by option ROMs, so the logic we currently
> have goes too far potentially even on bare metal.
>
> So while I'm all for relaxing, I don't think I can support
> outright deletion.
For now, how about cpu_has_hypervisor ?
Whatever the virtual environment, we should trust the provided memory map.
~Andrew
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