Re: [edk2-devel] [PATCH v4 4/7] OvmfPkg: Generate CloudHv as a PVH ELF binary

["Boeuf, Sebastien" <sebastien.boeuf@intel.com>]

On Mon, 2022-02-28 at 08:08 +0100, Gerd Hoffmann wrote:
>   Hi,
> 
> > > Oh.  So the ELF header *replaces* the firmware volume header.
> > > Didn't notice that before.
> > > 
> > > Hmm.  With that the varstore initialization most likely fails.
> > > There is a fallback path though, with ovmf storing variables
> > > elsewhere (in normal ram instead of firmware flash I think).
> > > Persistent variables don't work in that case.
> > > 
> > > When the persistent varstore doesn't work anyway (and you are
> > > ok with that) you probably can drop the space reserved for it
> > > from the firmware image and use just a single page for the pvh
> > > elf header (and sharing VarStore.fdf doesn't make sense then).
> > 
> > Sorry it's not entirely clear to me what should be done here.
> > Should I
> > remove the VARS section since we're not using it anyway?
> > I'm lacking some knowledge here to understand what this is used
> > for,
> > and how I actually broke it. I mean with the define and the
> > include, I
> > ended up modifying exclusively the [FD.CLOUDHV] which is exactly
> > what
> > was done for Xen. Do you think the Xen VARS section is broken as
> > well?
> 
> Probably.  Not sure how xen uses the generated firmware images
> though.
> 
> OVMF has three images.  The OVMF_VARS.fd is the variable store.
> OVMF_CODE.fd is the firmware code.  Both combined is OVMF.fd.  The
> split
> into VARS and CODE exists to simplify firmware updates:  On the host
> machine you'll have a per-guest VARS file with the persistent uefi
> variables, and the shared CODE file.  Firmware upgrades work by
> simply
> updating the shared CODE file.
> 
> The address space layout is the same in both cases
> (combined/splitted).
> The firmware takes 2M or 4M below 4G.  The upper part is covered by
> the
> CODE, the lower part is covered by VARS.  OVMF expects the varstore
> at
> a fixes address, but as mentioned can cope with the varstore not
> being
> there or not being backed by flash.

Thanks for the explanation, it helps :)

> 
> So, do you use the CODE or VARS file for cloudhv?

No we don't.

> Does cloudhv emulate flash?

No it doesn't.

> 
> take care,
>   Gerd
> 
> 
> 
> 
> 
> 

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