Re: [PATCHv4 29/30] ACPICA: Avoid cache flush on TDX guest

Josh Poimboeuf
2022-02-28T17:02:42Z

On Mon, Feb 28, 2022 at 07:37:13PM +0300, Kirill A. Shutemov wrote:
> On Sun, Feb 27, 2022 at 05:34:45PM -0800, Dan Williams wrote:
> > On Sun, Feb 27, 2022 at 2:05 PM Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> > >
> > > On Thu, Feb 24, 2022 at 06:56:29PM +0300, Kirill A. Shutemov wrote:
> > > > +/*
> > > > + * ACPI_FLUSH_CPU_CACHE() flushes caches on entering sleep states.
> > > > + * It is required to prevent data loss.
> > > > + *
> > > > + * While running inside TDX guest, the kernel can bypass cache flushing.
> > > > + * Changing sleep state in a virtual machine doesn't affect the host system
> > > > + * sleep state and cannot lead to data loss.
> > > > + *
> > > > + * TODO: Is it safe to generalize this from TDX guests to all guest kernels?
> > > > + */
> > > > +#define ACPI_FLUSH_CPU_CACHE() \
> > > > +do { \
> > > > + if (!cpu_feature_enabled(X86_FEATURE_TDX_GUEST)) \
> > > > + wbinvd(); \
> > > > +} while (0)
> > >
> > > If it's safe, why not do it for all VMs? Is there something specific
> > > about TDX which makes this more obviously known to be safe than for
> > > regular VMs?
> > >
> > > The patch description and the above comment make it sound like "we're
> > > not really sure this is safe, so we'll just use TDX as a testing ground
> > > for the idea." Which doesn't really inspire a lot of confidence in the
> > > stability of TD sleep states.
> >
> > Agree, why is this marked as "TODO"? The cache flushes associated with
> > ACPI sleep states are to flush cache before bare metal power loss to
> > CPU caches and bare metal transition of DDR in self-refresh mode. If a
> > cache flush is required it is the responsibility of the hypervisor.
> > Either it is safe for all guests or it is unsafe for all guests, not
> > TD specific.
>
> Do we have "any VM" check? I can't find it right away.

X86_FEATURE_HYPERVISOR

--
Josh