Re: [Qemu-devel] [PATCH v5 05/10] ACPI: Add Virtual Machine Generation ID support

[Igor Mammedov]

On Wed, 8 Feb 2017 12:17:54 +0100
Laszlo Ersek <address@hidden> wrote:

> On 02/08/17 12:04, Igor Mammedov wrote:
> > On Wed, 8 Feb 2017 01:48:42 +0100
> > Laszlo Ersek <address@hidden> wrote:  
> 
> [snip]
> 
> >> (2) Structure of the vmgenid fw_cfg blob, AKA "why that darn offset 40
> >> decimal".
> >>
> >> I explained it under points (6) and (7) in the following message:
> >>
> >> Message-Id: <address@hidden>
> >> URL: https://www.mail-archive.com/address@hidden/msg425226.html
> >>
> >> The story is that *wherever* an ADD_POINTER command points to -- that
> >> is, at the *exact* target address --, OVMF will look for an ACPI table
> >> header, somewhat heuristically. If it finds a byte pattern that (a) fits
> >> into the remaining blob and (b) passes some superficial ACPI table
> >> header checks, such as length and checksum, then OVMF assumes that the
> >> blob contains an ACPI table there, and passes the address (again, the
> >> exact, relocated, absolute target address of ADD_POINTER) to
> >> EFI_ACPI_TABLE_PROTOCOL.InstallAcpiTable().
> >>
> >> We want to disable this heuristic for the vmgenid blob. *If* the blob
> >> contained only 16 bytes (for the GUID), then the heuristic would
> >> automatically disable itself, because the ACPI table header (36 bytes)
> >> is larger than 16 bytes, so OVMF wouldn't even look. However, for the
> >> caching and other VMGENID requirements, we need to allocate a full page
> >> with the blob (4KB), hence OVMF *will* look. If we placed the GUID right
> >> at the start of the page, then OVMF would sanity-check it as an ACPI
> >> table header. The check would *most likely* not pass, so things would be
> >> fine in practice, but we can do better than that: just put 40 zero bytes
> >> at the front of the blob.
> >>
> >> And this is why the ADD_POINTER command has to point to the beginning of
> >> the blob (i.e., 36 zero bytes), to "suppress the OVMF ACPI SDT header
> >> detection". (The other 4 bytes are for arriving at an address divisible
> >> by 8, which is a VMGENID requirement for the GUID.)  
> > that's deserves a comment in code/doc as it's non obvious to someone
> > who is not familiar with OVMF.  
> 
> It is mentioned in the document "docs/specs/vmgenid.txt", added in patch 3.
> 
> I didn't want to push for more details there, but if you think it's
> helpful or even required, then Ben should please simply add a
> 
> Rationale for padding at the front (OVMF SDT Header probe suppressor)
> ---------------------------------------------------------------------
> 
> subsection under the
> 
> GUID Storage Format
> -------------------
> 
> section, and just dump the above into it.
> 
> Maybe tone down the over-use of *bold*. :)
+ comment/reference to doc in the code where this 'suppressor' is used,
otherwise it would be easy to forget why pointer points to the blob start
instead of directly to GUID.

> 
> Thanks!
> Laszlo
> 
>