Re: [PATCH v2 2/4] OvmfPkg/PlatformInitLib: Add PlatformGetLowMemoryCB

["Gerd Hoffmann" <kraxel@redhat.com>]

  Hi,

> > I think it should actually simplify things.  All the inconsistencies we
> > have (as you outlined above) due to the hole punching and edk2
> > supporting only a single range for 32bit mmio should go away, and we
> > will have less address space layout differences between q35 and pc.
> 
> We've tried 0xE000_0000 in the past, in commit 75136b29541b.
> 
> But had to revert it in commit eb4d62b0779c, due to 0xE000_0000 tickling
> a bug in QEMU.
> 
> The bug tickling was actually reported by you :) See
> <https://bugzilla.tianocore.org/show_bug.cgi?id=1859>.

Oh.  I totally forgot about that.  The patch from (I think) 2019 which
added _CRS for the range below the MMCONFIG should have fixed that, and
with recent qemu everything works fine.

I suspect we can't easily detect whenever qemu is broken or not.  Hmm.

Is more than three years being passed enough to just do it
unconditionally and effectively raise the bar for the minimum
supported qemu version?

> (Well, if you mean to keep the same logic for both i440fx adn q35,
> that's OK then.)

Yes, it would be Uc32Base.

LowMemory and Uc32Base are identical anyway most of the time due to qemu
preferring gigabyte pages when possible, you need odd memory sizes like
1.5 or 2.5 GB to see they actually can be different.

take care,
  Gerd