linux-kernel - Re: [PATCHv5 15/30] x86/boot: Port I/O: allow to hook up alternative helpers

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Date:   Wed, 2 Mar 2022 11:41:53 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        luto@...nel.org, peterz@...radead.org,
        sathyanarayanan.kuppuswamy@...ux.intel.com, aarcange@...hat.com,
        ak@...ux.intel.com, dan.j.williams@...el.com, david@...hat.com,
        hpa@...or.com, jgross@...e.com, jmattson@...gle.com,
        joro@...tes.org, knsathya@...nel.org, pbonzini@...hat.com,
        sdeep@...are.com, seanjc@...gle.com, tony.luck@...el.com,
        vkuznets@...hat.com, wanpengli@...cent.com,
        thomas.lendacky@....com, brijesh.singh@....com, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCHv5 15/30] x86/boot: Port I/O: allow to hook up alternative
 helpers

On 3/2/22 09:42, Josh Poimboeuf wrote:
> At the very least, please remove the ability for future code to> accidentally bypass 'pio_ops'.  Going forward, are we really expected
to> just remember to always use pio_ops for i/o?  Or else TDX will just>
silently break?  That's just not acceptable.
What did you have in mind here?  The in/out() instruction wrappers could
be moved to a spot where they're impossible to call directly, for instance.

I guess we could get really fancy and use objtool to look for any I/O
instructions that show up outside of the "official" pio_ops copies.
That would prevent anyone using inline assembly.

In the end, though, TDX *is* a new sub-architecture.  There are lots of
ways it's going to break silently and nobody will notice on bare metal.
 SEV is the same way with things like the C (encryption) bit in the page
tables.  Adding more safeguards sounds like a good idea but, in the end,
we're going to have to find the non-obvious issues with testing.