linux-kernel - Re: [PATCH v2 1/1] x86/tdx: Add __tdcall() and _

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Message-ID: <CAPcyv4hmA=V+wQA7JsvBNYnm+dHkRZtJSSxSZM8b2Xwnjq2kWQ@mail.gmail.com>
Date:   Tue, 20 Apr 2021 16:53:48 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     "Kuppuswamy, Sathyanarayanan" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Andi Kleen <ak@...ux.intel.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: [PATCH v2 1/1] x86/tdx: Add __tdcall() and __tdvmcall() helper functions

On Tue, Apr 20, 2021 at 4:12 PM Kuppuswamy, Sathyanarayanan
<sathyanarayanan.kuppuswamy@...ux.intel.com> wrote:
[..]
> >>> Also, do you *REALLY* need to do this from assembly?  Can't it be done
> >>> in the C wrapper?
> >> Its common for all use cases of TDVMCALL (vendor specific, in/out, etc).
> >> so added
> >> it here.
> >

Can I ask a favor?

Please put a line break between quoted lines and your reply.

> > That's not a good reason.  You could just as easily have a C wrapper
> > which all uses of TDVMCALL go through.

...because this runs together when reading otherwise.

> Any reason for not preferring it in assembly code?
> Also, using wrapper will add more complication for in/out instruction
> substitution use case. please check the use case in following patch.
> https://github.com/intel/tdx/commit/1b73f60aa5bb93554f3b15cd786a9b10b53c1543

This commit still has open coded assembly for the TDVMCALL? I thought
we talked about it being unified with the common definition, or has
this patch not been reworked with that feedback yet? I expect there is
no performance reason why in/out need to get their own custom coded
TDVMCALL implementation. It should also be the case the failure should
behave the same as native in/out failure i.e. all ones on read
failure, and silent drops on write failure.