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

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

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.

-- 
 Kirill A. Shutemov