Re: [RFC v2-fix-v4 1/1] x86/tdx: Skip WBINVD instruction for TDX guest

[Dan Williams]

On Wed, Jun 9, 2021 at 10:28 AM Kuppuswamy, Sathyanarayanan
<sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx> wrote:
>
>
>
> On 6/9/21 9:12 AM, Andy Lutomirski wrote:
> > On 6/9/21 8:09 AM, Dan Williams wrote:
> >> On Tue, Jun 8, 2021 at 9:27 PM Andi Kleen <ak@xxxxxxxxxxxxxxx> wrote:
> >>>
> >>>
> >>> here is no resume path.
> >>>
> >>>> Host is free to go into S3 independent of any guest state.
> >>>
> >>> Actually my understanding is that none of the systems which support TDX
> >>> support S3. S3 has been deprecated for a long time.
> >>
> >> Ok, I wanted to imply any power state that might power-off caches.
> >>
> >>>
> >>>
> >>>>    A hostile
> >>>> host is free to do just enough cache management so that it can resume
> >>>> from S3 while arranging for TDX guest dirty data to be lost. Does a
> >>>> TDX guest go fatal if the cache loses power?
> >>>
> >>> That would be a machine check, and yes it would be fatal.
> >>
> >> Sounds good, so incorporating this and Andy's feedback:
> >>
> >> "TDX guests, like other typical guests, use standard ACPI mechanisms
> >> to signal sleep state entry (including reboot) to the host. The ACPI
> >> specification mandates WBINVD on any sleep state entry with the
> >> expectation that the platform is only responsible for maintaining the
> >> state of memory over sleep states, not preserving dirty data in any
> >> CPU caches. ACPI cache flushing requirements pre-date the advent of
> >> virtualization. Given guest sleep state entry does not affect any host
> >> power rails it is not required to flush caches. The host is
> >> responsible for maintaining cache state over its own bare metal sleep
> >> state transitions that power-off the cache. A TDX guest, unlike a
> >> typical guest, will machine check if the CPU cache is powered off."
> >>
> >> Andi, is that machine check behavior relative to power states
> >> mentioned in the docs?
> >
> > I don't think there's anything about power states.  There is a general
> > documented mechanism to integrity-check TD guest memory, but it is *not*
> > replay-resistant.  So, if the guest dirties a cache line, and the cache
> > line is lost, it seems entirely plausible that the guest would get
> > silently corrupted.
> >
> > I would argue that, if this happens, it's a host, TD module, or
> > architecture bug, and it's not the guest's fault.
>
> If you want to apply this fix for all hypervisors (using boot_cpu_has
> (X86_FEATURE_HYPERVISOR) check), then we don't need any TDX specific
> reference in commit log right? It can be generalized for all VM guests.
>
> agree?

No, because there is a note needed about the integrity implications in
the TDX case that makes it distinct from typical hypervisor enabling.