Re: [RFC PATCH 01/19] OvmfPkg: Reserve the Secrets and Cpuid page for the SEV-SNP guest

["James Bottomley" <jejb@linux.ibm.com>]

On Tue, 2021-04-06 at 14:16 +0200, Laszlo Ersek wrote:
> On 04/06/21 10:11, Xu, Min M wrote:
> > Hi, Singh
> > I have a concern about the sevSnpBlock in ResetVectorVtf0.asm.
> > Actually
> > SEV has inserted 3 blocks in ResetVectorVtf0.asm and the total
> > bytes are
> > (26 + 22 + 20 = 68 bytes). If sevSnpBlock is added, then the total
> > bytes
> > will be (68 +26 = 94 bytes).
> > 
> > I am not sure whether there will be more blocks added in
> > ResetVectorVtf0.asm in the future. But I don't think
> > ResetVectorVtf0.asm
> > is a good place to add these data blobs. Can these data be packed
> > into a
> > single file, for example, SevMetadata.asm, then a pointer is
> > inserted in
> > ResetVectorVtf0.asm which then points to the SevMetadata. In this
> > way we
> > can keep ResetVectorVtf0.asm clean, small and straight forward.
> > 
> > Another reason is that I am working on the Intel TDX which will
> > update the ResetVectorVtf0.asm as well. My change depends on the
> > assumption that the distance between ResetVector(0xfffffff0) and
> > EarlyBspInitReal16 is less than 128 bytes. The blocks in
> > ResetVectorVtf0.asm make it impossible.

Why?  I think the short jump can cover 32k as an offset.

> That's a problem. These info blocks are placed in the reset vector
> because then they can be found by QEMU easily -- they are not
> compressed, and they appear at a known location in the guest physical
> address space. (More precisely, a GUID-ed structure chain starts at a
> known location, and then QEMU can traverse the chain of structures,
> for learning various bits of information about the firmware.)
> 
> Do we absolutely need a short jump?

Yes, I explained this before:

https://listman.redhat.com/archives/edk2-devel-archive/2020-November/msg01063.html

Sorry, it's buried in the middle about why we can only have 32k worth
of entries.  However, the restriction is 32k not 128 bytes unless Intel
is doing something strange.

James