linux-kernel - Re: [RFC v2 14/32] x86/tdx: Handle port I/O

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Message-ID: <20210511000119.GV4032392@tassilo.jf.intel.com>
Date:   Mon, 10 May 2021 17:01:19 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        Raj Ashok <ashok.raj@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC v2 14/32] x86/tdx: Handle port I/O

On Mon, May 10, 2021 at 04:34:34PM -0700, Dan Williams wrote:
> > > Outside the boot decompression code isn't this branch of the "ifdef
> > > BOOT_COMPRESSED_MISC_H"  handled by #VE? I also don't see any usage of
> > > __{in,out}() in this patch.
> >
> > I thought it was all alternative after decompression, so the #VE code
> > shouldn't be called. We still have it for some reason though.
> 
> Right, I'm struggling to understand where these spurious in/out
> instructions are coming from that are not replaced by the
> alternative's code?

There should be nothing in the main tree at least.

> Shouldn't those be dropped on the floor and warned
> about rather than handled? 

It might be related to eventually handling them in ring 3, but
I believe we disallow that currently too and it's not all that useful
anyways.  So yes it could be forbidden.

> I.e. shouldn't port-io instruction escapes
> that would cause #VE be precluded at build-time?

You mean in objtool? That would seem like overkill for a more theoretical
problem.

> > There used to be SAVE_ALL/SAVE_REGS, but they have been all removed in
> > some past refactorings.
> 
> Not a huge deal, but at a minimum it seems a generic construct that
> deserves to be declared centrally rather than tdx-guest-port-io local.

Yes I agree. We should just bring SAVE_ALL/SAVE_REGS back.

-Andi