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

[Dan Williams]

On Tue, Jun 8, 2021 at 4:38 PM Dave Hansen <dave.hansen@xxxxxxxxx> 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.