linux-kernel - Re: [RFC v2-fix-v3 1/1] x86/tdx: Skip WBINVD instruction for TDX guest

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Date:   Tue, 8 Jun 2021 17:07:02 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Tony Luck <tony.luck@...el.com>,
        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>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux ACPI <linux-acpi@...r.kernel.org>
Subject: Re: [RFC v2-fix-v3 1/1] x86/tdx: Skip WBINVD instruction for TDX guest

On Tue, Jun 8, 2021 at 4:38 PM Dave Hansen <dave.hansen@...el.com> wrote:
>
> On 6/8/21 4:32 PM, Dan Williams wrote:
> >> Persistent memory is also currently not supported. Another code
> >> path that uses WBINVD is the MTRR driver, but EPT/virtualization
> >> always disables MTRRs so those are not needed. This all implies
> >> WBINVD is not needed with current TDX.
> > Let's drop the last three paragraphs and just say something like:
> > "This is one of a series of patches to usages of wbinvd for protected
> > guests. For now this just addresses the one known path that TDX
> > executes, ACPI reboot. Its usage can be elided because FOO reason and
> > all the other ACPI_FLUSH_CPU_CACHE usages can be elided because BAR
> > reason"
>
> A better effort at transparency can be made here:
>
>         This patches the one WBINVD instance which has been encountered
>         in practice: ACPI reboot.  Assume no other instance will be
>         encountered.
>

That works too, but I assume if ACPI_FLUSH_CPU_CACHE() itself is going
to be changed rather than sprinkling protected_guest_has() checks in a
few places it will need to assert why changing all of those at once is
correct. Otherwise I expect Rafael to ask why this global change of
the ACPI_FLUSH_CPU_CACHE() policy is ok.