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

Kirill A. Shutemov
2022-02-28T16:37:07Z

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.

--
Kirill A. Shutemov