Re: [Xen-devel] [PATCH for-4.11] libxl: fix memory map reported to PVH guests

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Fri, Apr 20, 2018 at 08:25:41AM -0600, Jan Beulich wrote:
> >>> On 20.04.18 at 16:15, <roger.pau@xxxxxxxxxx> wrote:
> > On Fri, Apr 20, 2018 at 08:01:35AM -0600, Jan Beulich wrote:
> >> >>> On 20.04.18 at 15:52, <roger.pau@xxxxxxxxxx> wrote:
> >> > PVH guests with 4GB of RAM or more get a memory map like the
> >> > following:
> >> > 
> >> > 0x00000000000000 - 0x000000fee00000 RAM
> >> > 0x000000fee00000 - 0x00000100000000 RESERVED
> >> > 0x000000fc009000 - 0x000000fc009040 ACPI
> >> > 0x000000fc000000 - 0x000000fc001000 ACPI
> >> > 0x000000fc001000 - 0x000000fc009000 ACPI
> >> > 0x00000100000000 - 0x000001fb200400 RAM
> >> > 
> >> > This is wrong because ACPI regions are also reported as RAM regions.
> >> > The cause of this issue is not setting a big enough MMIO hole, current
> >> > libxl code only takes into account the address of the local APIC page
> >> > and the reserved pages in order to set the size of the MMIO hole, when
> >> > it should also take into account the location of the ACPI tables.
> >> > 
> >> > After the fix the layout reported for the same guest is:
> >> > 
> >> > 0x00000000000000 - 0x000000fc000000 RAM
> >> > 0x000000fc000000 - 0x00000100000000 RESERVED
> >> > 0x000000fc009000 - 0x000000fc009040 ACPI
> >> > 0x000000fc000000 - 0x000000fc001000 ACPI
> >> > 0x000000fc001000 - 0x000000fc009000 ACPI
> >> > 0x00000100000000 - 0x000001fe000400 RAM
> >> 
> >> But this is still wrong - no two regions may overlap, regardless of type.
> > 
> > It's going to be more complicated to fix that. I can give it a try,
> > but I think this is strictly better than what we do now.
> > 
> > Maybe instead of marking the whole MMIO hole as reserved we should
> > only mark as reserved the lapic page and the special pages? That
> > should avoid any overlaps.
> 
> Well, if nothing else is in that range (or can be placed there dynamically at
> runtime) I don't see why all of it is reserved. Marking a range reserved
> prevents, for example, PCI device BARs to be put there by the OS. The
> way it looks there's no MMIO window left available at all below 4Gb ...

Right, this will change when PVH gets devices with BARs, then it's
going to need a proper MMIO hole below 4GB

AFAICT the only reserved pages for PVH are the HVM special pages, so
I've changed the memory map builder to only report those as reserved,
here is the resulting memory map:

0x00000000000000 - 0x000000fc000000 RAM
0x000000feff8000 - 0x000000ff000000 RESERVED
0x000000fc009000 - 0x000000fc009040 ACPI
0x000000fc000000 - 0x000000fc001000 ACPI
0x000000fc001000 - 0x000000fc009000 ACPI
0x00000100000000 - 0x000001fe000400 RAM

I see no reason to mark the local APIC page as reserved in the memory
map. If that seems like a sensible approach I can send the updated
patch.

Thanks, Roger.

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel