Re: [PATCH v3 23/75] x86/boot/compressed/64: Setup GHCB Based VC Exception handler

Borislav Petkov
2020-05-13T08:59:34Z

On Tue, May 12, 2020 at 11:08:12PM +0200, Joerg Roedel wrote:
> No, this is a recent addition, otherwise this breaks out-of-tree builds
> (make O=/some/path ...) because inat-tables.c (included from inat.c) is
> generated during build and ends up in the $(objtree).

Please add a blurb about this above it otherwise no one would have a
clue why it is there.

> Because the immediate is the last part of the instruction which is
> decoded (even if there is no immediate). The .got field is set when
> either the immediate was decoded successfully or, in case the
> instruction has no immediate, when the rest of the instruction was
> decoded successfully. So testing immediate.got is the indicator whether
> decoding was successful.

Hm, whether the immediate was parsed correctly or it wasn't there and
using that as the sign whether the instruction was decoded successfully
sounds kinda arbitrary.

@Masami: shouldn't that insn_get_length() thing return a value to denote
whether the decode was successful or not instead of testing arbitrary
fields?

Pasting for you the code sequence again:

...
insn_get_length(&ctxt->insn);

ret = ctxt->insn.immediate.got ? ES_OK : ES_DECODE_FAILED;
...

I wonder if one would be able to do instead:

if (insn_get_length(&ctxt->insn))
return ES_OK;

return ES_DECODE_FAILED;

to have it straight-forward...

> Probably better rename the function, it also does the sev-es protocol
> version negotiation and all other related setup tasks. Maybe
> sev_es_setup() is a better name?

Sure.

> If the above call fails its the end of the systems lifetime, because we
> can't continue to boot an SEV-ES guest when we have no GHCB.
>
> BUG_ON() and friends are also not available in the pre-decompression
> boot stage.

Oh ok, error() does hlt the system.

Thx.

--
Regards/Gruss,
Boris.