Re: [PATCH v12 19/37] x86/fred: Update MSR_IA32_FRED_RSP0 during task switch

Borislav Petkov
2023-11-13T09:38:20Z

On Mon, Oct 02, 2023 at 11:24:40PM -0700, Xin Li wrote:
> From: "H. Peter Anvin (Intel)" <hpa@zytor.com>
>
> MSR_IA32_FRED_RSP0 is used during ring 3 event delivery, and needs to
> be updated to point to the top of next task stack during task switch.
>
> Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
> Tested-by: Shan Kang <shan.kang@intel.com>
> Signed-off-by: Xin Li <xin3.li@intel.com>
> ---
> arch/x86/include/asm/switch_to.h | 8 ++++++--
> 1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/include/asm/switch_to.h b/arch/x86/include/asm/switch_to.h
> index f42dbf17f52b..c3bd0c0758c9 100644
> --- a/arch/x86/include/asm/switch_to.h
> +++ b/arch/x86/include/asm/switch_to.h
> @@ -70,9 +70,13 @@ static inline void update_task_stack(struct task_struct *task)
> #ifdef CONFIG_X86_32
> this_cpu_write(cpu_tss_rw.x86_tss.sp1, task->thread.sp0);
> #else
> - /* Xen PV enters the kernel on the thread stack. */
> - if (cpu_feature_enabled(X86_FEATURE_XENPV))
> + if (cpu_feature_enabled(X86_FEATURE_FRED)) {
> + /* WRMSRNS is a baseline feature for FRED. */
> + wrmsrns(MSR_IA32_FRED_RSP0, (unsigned long)task_stack_page(task) + THREAD_SIZE);

If this non-serializing write happens now and, AFAICT, the CR3 write
during the task switch has already happened in switch_mm* earlier, what
is the serialization point that's going to make sure that write is
committed before the new task starts executing?

Thx.

--
Regards/Gruss,
Boris.