Re: [PATCHv4 15/30] x86/boot: Allow to hook up alternative port I/O helpers

Josh Poimboeuf
2022-02-28T16:44:41Z

On Mon, Feb 28, 2022 at 07:33:53PM +0300, Kirill A. Shutemov wrote:
> On Sun, Feb 27, 2022 at 02:02:19PM -0800, Josh Poimboeuf wrote:
> > On Thu, Feb 24, 2022 at 06:56:15PM +0300, Kirill A. Shutemov wrote:
> > > Port I/O instructions trigger #VE in the TDX environment. In response to
> > > the exception, kernel emulates these instructions using hypercalls.
> > >
> > > But during early boot, on the decompression stage, it is cumbersome to
> > > deal with #VE. It is cleaner to go to hypercalls directly, bypassing #VE
> > > handling.
> > >
> > > Add a way to hook up alternative port I/O helpers in the boot stub.
> > >
> > > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> >
> > I think you missed my comment from v3.
>
> I did not missed it, but I failed to acknowledge it.
>
> To me it is a judgement call. Either way has right to live.
> I talked to Borislav on this and we suggested to keep it as. Rework later
> as needed.
>
> > Repeating it here:
> >
> > At least from reading the commit message it's not self-evident why #VE
> > handling would be worse, especially since there's already #VC support in
> > boot. It would help to give more info about that in the commit message.
> >
> > The current approach also seems fragile, doesn't it require all future
> > code to remember to not do i/o directly? How do we make sure that
> > doesn't happen going forward?
> >
> > How does it fail if some code accidentally does i/o directly? Or
> > triggers #VE some other way? Is the error understandable and
> > actionable?
>
> Dealing with failure in decompression code is a pain. We don't have usual
> infrastructure there. The patch deals with port I/O which is the only way
> to communicate issue to the user. If it fails for whatever reason we are
> screwed. And it doesn't depend on how it was implemented.

In the patch description, please address all of my concerns and
questions.

--
Josh