Seabios hypercall page

Seabios hypercall page

[Juergen Gross]

While searching for the reason for a HVM domU performance problem
I came across Seabios' hypercall page: it is usually located at the
end of guest memory and marked as "reserved" in the guest E820 map.

This will make it impossible for a Linux guest to use a large page
mapping for the direct map of memory for the last memory chunk.

Would it be possible to put the hypercall page into low memory?
There seems to be plenty of space below 1MB according to /proc/iomem:

# cat /proc/iomem
00000000-00000fff : reserved
00001000-0009fbff : System RAM       <--- could be put in here
0009fc00-0009ffff : reserved
000a0000-000bffff : PCI Bus 0000:00
000c0000-000c95ff : Video ROM
000c9800-000ca5ff : Adapter ROM      <--- or after this chunk?
000f0000-000fffff : reserved
  000f0000-000fffff : System ROM
00100000-7fffefff : System RAM
  01000000-016046e2 : Kernel code
  016046e3-01f5ad7f : Kernel data
  02165000-023edfff : Kernel bss
7ffff000-7fffffff : reserved          <--- Seabios hypercall page
88000000-8fffffff : System RAM
f0000000-fbffffff : PCI Bus 0000:00
  f0000000-f1ffffff : 0000:00:03.0
  f2000000-f2ffffff : 0000:00:02.0
    f2000000-f2ffffff : xen-platform-pci
  f3040000-f304ffff : 0000:00:03.0
  f3050000-f3050fff : 0000:00:03.0
fc000000-ffffffff : reserved
  fec00000-fec003ff : IOAPIC 0
  fed00000-fed003ff : HPET 0
    fed00000-fed003ff : PNP0103:00
  fee00000-fee00fff : Local APIC


Juergen

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Re: Seabios hypercall page

[Wei Liu]

On Mon, Jun 12, 2017 at 11:17:25AM +0200, Juergen Gross wrote:
> While searching for the reason for a HVM domU performance problem
> I came across Seabios' hypercall page: it is usually located at the
> end of guest memory and marked as "reserved" in the guest E820 map.
> 
> This will make it impossible for a Linux guest to use a large page
> mapping for the direct map of memory for the last memory chunk.
> 
> Would it be possible to put the hypercall page into low memory?
> There seems to be plenty of space below 1MB according to /proc/iomem:

The changeset that added xen support doesn't say why high address was
chosen. I suppose you can move it else where just fine.

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