linux-kernel - Re: [RFC PATCH 23/32] x86/fred: update MSR_IA32_FRED

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Date:   Tue, 20 Dec 2022 10:48:44 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Xin Li <xin3.li@...el.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, kvm@...r.kernel.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com,
        andrew.cooper3@...rix.com, seanjc@...gle.com, pbonzini@...hat.com,
        ravi.v.shankar@...el.com
Subject: Re: [RFC PATCH 23/32] x86/fred: update MSR_IA32_FRED_RSP0 during
 task switch

On Mon, Dec 19, 2022 at 10:36:49PM -0800, Xin Li wrote:
> From: "H. Peter Anvin (Intel)" <hpa@...or.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@...or.com>
> Signed-off-by: Xin Li <xin3.li@...el.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 c08eb0fdd11f..c28170d4fbba 100644
> --- a/arch/x86/include/asm/switch_to.h
> +++ b/arch/x86/include/asm/switch_to.h
> @@ -71,9 +71,13 @@ static inline void update_task_stack(struct task_struct *task)
>  	else
>  		this_cpu_write(cpu_tss_rw.x86_tss.sp1, task->thread.sp0);
>  #else
> -	/* Xen PV enters the kernel on the thread stack. */
> -	if (static_cpu_has(X86_FEATURE_XENPV))
> +	if (cpu_feature_enabled(X86_FEATURE_FRED)) {
> +		wrmsrl(MSR_IA32_FRED_RSP0,
> +		       task_top_of_stack(task) + TOP_OF_KERNEL_STACK_PADDING);

Urgh, I'm assuming this is a *fast* MSR ?

> +	} else if (static_cpu_has(X86_FEATURE_XENPV)) {
> +		/* Xen PV enters the kernel on the thread stack. */
>  		load_sp0(task_top_of_stack(task));
> +	}
>  #endif