Re: [PATCHv6 29/30] ACPICA: Avoid cache flush inside virtual machines

[Thomas Gleixner]

On Wed, Mar 16 2022 at 05:08, Kirill A. Shutemov wrote:

> While running inside virtual machine, 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.
>
> Before entering sleep states, the ACPI code flushes caches to prevent
> data loss using the WBINVD instruction.  This mechanism is required on
> bare metal.
>
> But, any use WBINVD inside of a guest is worthless.  Changing sleep
> state in a virtual machine doesn't affect the host system sleep state
> and cannot lead to data loss, so most hypervisors simply ignore it.
> Despite this, the ACPI code calls WBINVD unconditionally anyway.
> It's useless, but also normally harmless.
>
> In TDX guests, though, WBINVD stops being harmless; it triggers a
> virtualization exception (#VE).  If the ACPI cache-flushing WBINVD
> were left in place, TDX guests would need handling to recover from
> the exception.
>
> Avoid using WBINVD whenever running under a hypervisor.  This both
> removes the useless WBINVDs and saves TDX from implementing WBINVD
> handling.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>