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

On Thu, May 03, 2018 at 10:02:47PM +1000, Alexey G wrote:
> 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.

Dom0 could attempt to relocate a BAR over a RAM region and Xen won't
prevent it, because Dom0 is trusted. The same could happen with a PV
Dom0, but in the PV case Dom0 is provided with the unmodified host
memory map.

I don't see how providing this UNUSABLE/RESERVED ranges is going to
cause any issues to Dom0, so I think we should just do it.

Thanks, Roger.

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