linux-kernel - [RFC PATCH v3 23/32] x86/fred: update MSR_IA32_FRED

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Date:   Thu, 23 Feb 2023 23:01:36 -0800
From:   Xin Li <xin3.li@...el.com>
To:     linux-kernel@...r.kernel.org, x86@...nel.org, kvm@...r.kernel.org
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, peterz@...radead.org,
        andrew.cooper3@...rix.com, seanjc@...gle.com, pbonzini@...hat.com,
        ravi.v.shankar@...el.com
Subject: [RFC PATCH v3 23/32] x86/fred: update MSR_IA32_FRED_RSP0 during task switch

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.

Update MSR_IA32_FRED_RSP0 with WRMSR instruction for now, and will use
WRMSRNS/WRMSRLIST for performance once it gets upstreamed.

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 | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/switch_to.h b/arch/x86/include/asm/switch_to.h
index 5c91305d09d2..00fd85abc1d2 100644
--- a/arch/x86/include/asm/switch_to.h
+++ b/arch/x86/include/asm/switch_to.h
@@ -68,9 +68,16 @@ 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)) {
+		/*
+		 * Will use WRMSRNS/WRMSRLIST for performance once it's upstreamed.
+		 */
+		wrmsrl(MSR_IA32_FRED_RSP0,
+		       task_top_of_stack(task) + TOP_OF_KERNEL_STACK_PADDING);
+	} else if (cpu_feature_enabled(X86_FEATURE_XENPV)) {
+		/* Xen PV enters the kernel on the thread stack. */
 		load_sp0(task_top_of_stack(task));
+	}
 #endif
 }
 
-- 
2.34.1