Re: [PATCH V2 1/1] x86/tdx: Route safe halt execution via tdx_safe_halt()

[Kirill A. Shutemov]

On Fri, Jan 31, 2025 at 06:32:04PM -0800, Vishal Annapurve wrote:
> On Fri, Jan 31, 2025 at 12:13 AM Kirill A. Shutemov
> <kirill@xxxxxxxxxxxxx> wrote:
> >
> > On Thu, Jan 30, 2025 at 11:45:01AM -0800, Vishal Annapurve wrote:
> > > On Thu, Jan 30, 2025 at 10:48 AM Kirill A. Shutemov
> > > <kirill@xxxxxxxxxxxxx> wrote:
> > > > ...
> > > > > >
> > > > > > I think it is worth to putting this into a separate patch and not
> > > > > > backport. The rest of the patch is bugfix and this doesn't belong.
> > > > > >
> > > > > > Otherwise, looks good to me:
> > > > > >
> > > > > > Reviewed-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>@linux.intel.com>
> > > > > >
> > > > > > --
> > > > > >   Kiryl Shutsemau / Kirill A. Shutemov
> > > > >
> > > > > Thanks Kirill for the review.
> > > > >
> > > > > Thinking more about this fix, now I am wondering why the efforts [1]
> > > > > to move halt/safe_halt under CONFIG_PARAVIRT were abandoned. Currently
> > > > > proposed fix is incomplete as it would not handle scenarios where
> > > > > CONFIG_PARAVIRT_XXL is disabled. I am tilting towards reviving [1] and
> > > > > requiring CONFIG_PARAVIRT for TDX VMs. WDYT?
> > > > >
> > > > > [1] https://lore.kernel.org/lkml/20210517235008.257241-1-sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx/
> > > >
> > > > Many people dislike paravirt callbacks. We tried to avoid relying on them
> > > > for core TDX enabling.
> > > >
> > > > Can you explain the issue you see with CONFIG_PARAVIRT_XXL being disabled?
> > > > I don't think I follow.
> > >
> > > Relevant callers of *_safe_halt() are:
> > > 1) kvm_wait() -> safe_halt() -> raw_safe_halt() -> arch_safe_halt()
> >
> > Okay, I didn't realized that CONFIG_PARAVIRT_SPINLOCKS doesn't depend on
> > CONFIG_PARAVIRT_XXL.
> >
> > It would be interesting to check if paravirtualized spinlocks make sense
> > for TDX given the cost of TD exit.
> >
> > Maybe we should avoid advertising KVM_FEATURE_PV_UNHALT to the TDX guests?
> >
> 
> Are you hinting towards a model where TDX guest prohibits such call
> sites from being configured? I am not sure if it's a sustainable model
> if we just rely on the host not advertising these features as the
> guest kernel can still add new paths that are not controlled by the
> host that lead to *_safe_halt().

I've asked TDX module folks to provide additional information in ve_info
to help handle STI shadow correctly. They will implement it, but it will
take some time.

So we need some kind of stopgap until we have it.

I am reluctant to commit to paravirt calls for this workaround. They will
likely stick forever. It is possible, I would like to avoid them. If not,
oh well.

> > > 2) acpi_safe_halt() -> safe_halt() -> raw_safe_halt() -> arch_safe_halt()
> >
> > Have you checked why you get there? I don't see a reason for TDX guest to
> > get into ACPI idle stuff. We don't have C-states to manage.
> 
> Apparently userspace VMM is advertising pblock_address through SSDT
> tables in my configuration which causes guests to enable ACPI cpuidle
> drivers. Do you know if future generations of TDX hardware will not
> support different c-states for TDX VMs?

I have very limited understanding of power management, but I don't see how
C-states can be meaningfully supported by any virtualized environment.
To me, C-states only make sense for baremetal.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov