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

[H. Peter Anvin]

On November 13, 2023 4:37:42 AM EST, Borislav Petkov <bp@xxxxxxxxx> wrote:
>On Mon, Oct 02, 2023 at 11:24:40PM -0700, Xin Li wrote:
>> From: "H. Peter Anvin (Intel)" <hpa@xxxxxxxxx>
>> 
>> 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@xxxxxxxxx>
>> Tested-by: Shan Kang <shan.kang@xxxxxxxxx>
>> Signed-off-by: Xin Li <xin3.li@xxxxxxxxx>
>> ---
>>  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.
>

A resource cannot be consumed after the value has been written; this is the only necessary level of serialization, equivalent to, say, RAX.

A serializing instruction stops the entire pipeline until everything has retired and any stores have become globally visible.