Re: [Xen-devel] [PATCH for-4.11] x86/dom0: add extra RAM regions as RESERVED for PVH memory map

[Alexey G <x1917x@xxxxxxxxx>]

On Thu, 3 May 2018 12:15:18 +0100
Roger Pau Monné <roger.pau@xxxxxxxxxx> wrote:

>On Thu, May 03, 2018 at 08:55:14PM +1000, Alexey G wrote:
>> On Thu, 3 May 2018 10:56:40 +0100
>> Roger Pau Monne <roger.pau@xxxxxxxxxx> wrote:
>>   
>> >When running as PVH Dom0 the native memory map is used in order to
>> >craft a tailored memory map for Dom0 taking into account it's memory
>> >limit.
>> >
>> >Dom0 memory is always going to be smaller than the total amount
>> >of memory present on the host, so in order to prevent Dom0 from
>> >relocating PCI BARs over RAM regions mark all the RAM regions not
>> >available to Dom0 as RESERVED in the memory map.
>> >---
>> >NB: I haven't seen any system where Dom0 would relocate the BARs
>> >over RAM regions, but AFAICT given the current memory map provided
>> >to Dom0 this is a possibility that should be avoided.  
>> 
>> Guest OSes typically use information from ACPI to learn where PCI
>> BARs can (or cannot) be relocated.  
>
>I think it's better to be safe than sorry, so IMO the host RAM regions
>should be added to the memory map as RESERVED.
>
>Roger.

I assume host's DSDT passed through to PVH Dom0 as is? In this case
Dom0 will see PCI holes matching those of the host and shouldn't make
any attempts to place BARs outside provided PCI holes (except "nocrs"
given).

As long as Dom0 P2M map prevent using these host ranges we shouldn't
worry if it is marked as reserved in e820 map I think. It's kinda
excessive information for Dom0 -- knowing about host RAM ranges which
he cannot touch anyway due to lack of corresponding p2m mappings.

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