Re: [PATCH 1/1] OvmfPkg/PlatformPei: support >=1TB high RAM, and discontiguous high RAM

[Laszlo Ersek <lersek@redhat.com>]

On 07/11/17 10:38, Igor Mammedov wrote:
> On Tue, 11 Jul 2017 05:22:31 +0200
> Laszlo Ersek <lersek@redhat.com> wrote:
> 
>> In OVMF we currently get the upper (>=4GB) memory size with the
>> GetSystemMemorySizeAbove4gb() function.
>>
>> The GetSystemMemorySizeAbove4gb() function is used in two places:
>>
>> (1) It is the starting point of the calculations in GetFirstNonAddress().
>>     GetFirstNonAddress() in turn
>>     - determines the placement of the 64-bit PCI MMIO aperture,
> that's probably broken in memory hotplug case as
> the first usable PCI MMIO address is at fwcfg("etc/reserved-memory-end")

No, it's not broken :) I wrote above, "starting point of the
calculations". The fw_cfg file "etc/reserved-memory-end" is specifically
considered already. (You explained that file to me when I previously
worked on this code, in 2016Q1 or so.) It's just a later point in the
calculation.

> though the file exists only when memory hotplug is enabled
> ( to make life of FW writers harder :/ )

No problem, OVMF deals with "etc/reserved-memory-end" being either
present or absent.

Basically the GetFirstNonAddress() function performs a "cascade of
increases" upwards, for determining the highest address plus one (i.e.,
the first non-address).

If you'd like to see the full function (with this patch applied), it
starts at:

https://github.com/lersek/edk2/blob/highram1tb/OvmfPkg/PlatformPei/MemDetect.c#L292

It is heavily commented.

Thanks!
Laszlo