Re: [PATCH v3 09/23] hw/uefi: add var-service-core.c

[Gerd Hoffmann]

  Hi,

> > How do your patches pick the transfer mode?  Is that dictated by the
> > host?  Or is the guest free to choose?  In case of the latter:  How does
> > the guest decide what to do?
> 
> In our version, the guest gets to pick. It defaults to the DMA interface
> unless it detects that it's running either the macOS logic (a case you can
> ignore for now) or is running with SEV-SNP.
> 
> I think for the upstream interface, it would be best to have the host
> indicate which one it recommends the guest to use. That way you can force
> the fallback path without requiring tedious edk2 changes.

I'm more thinking about a hard switch, i.e. the host would support only
the one or the other.  That way we'll go need less register space,
because we'll need either the buffer location register (dma mode) or the
pio transfer register (pio mode) but not both at the same time so they
can share the location.

> > 0xfef1000 seems to be free, which is kida fun b/c of the 'ef1' in the
> > address.
> 
> True, I love it! :)
> 
> It's not enough address space to fit the full 64k buffer though, right?
> Would all of 0xfef00000 be free by chance? Then you could just direct map
> the transfer buffer there.

All of 0xfef00000 is 1M, i.e. 16 x 64k.

take care,
  Gerd