linux-kernel - [PATCH hyperv-next v3 04/15] arch/x86: mshyperv: Trap on access for some synthetic MSRs

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Message-ID: <20250604004341.7194-5-romank@linux.microsoft.com>
Date: Tue,  3 Jun 2025 17:43:30 -0700
From: Roman Kisel <romank@...ux.microsoft.com>
To: alok.a.tiwari@...cle.com,
	arnd@...db.de,
	bp@...en8.de,
	corbet@....net,
	dave.hansen@...ux.intel.com,
	decui@...rosoft.com,
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	hpa@...or.com,
	kys@...rosoft.com,
	mingo@...hat.com,
	mhklinux@...look.com,
	tglx@...utronix.de,
	wei.liu@...nel.org,
	linux-arch@...r.kernel.org,
	linux-doc@...r.kernel.org,
	linux-hyperv@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	x86@...nel.org
Cc: apais@...rosoft.com,
	benhill@...rosoft.com,
	bperkins@...rosoft.com,
	sunilmut@...rosoft.com
Subject: [PATCH hyperv-next v3 04/15] arch/x86: mshyperv: Trap on access for some synthetic MSRs

To set up and run the confidential VMBus, the guest needs the paravisor
to intercept access to some synthetic MSRs. In the non-confidential case,
the guest continues using the vendor-specific guest-host communication
protocol.

Update the hv_set_non_nested_msr() function to trap access to some
synthetic MSRs.

Signed-off-by: Roman Kisel <romank@...ux.microsoft.com>
Reviewed-by: Alok Tiwari <alok.a.tiwari@...cle.com>
---
 arch/x86/kernel/cpu/mshyperv.c | 23 +++++++++++++++++++----
 1 file changed, 19 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index 83a85d94bcb3..db6f3e3db012 100644
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c
@@ -28,6 +28,7 @@
 #include <asm/apic.h>
 #include <asm/timer.h>
 #include <asm/reboot.h>
+#include <asm/msr.h>
 #include <asm/nmi.h>
 #include <clocksource/hyperv_timer.h>
 #include <asm/numa.h>
@@ -77,14 +78,28 @@ EXPORT_SYMBOL_GPL(hv_get_non_nested_msr);
 
 void hv_set_non_nested_msr(unsigned int reg, u64 value)
 {
+	if (reg == HV_X64_MSR_EOM && vmbus_is_confidential()) {
+		/* Reach out to the paravisor. */
+		native_wrmsrl(reg, value);
+		return;
+	}
+
 	if (hv_is_synic_msr(reg) && ms_hyperv.paravisor_present) {
+		/* The hypervisor will get the intercept. */
 		hv_ivm_msr_write(reg, value);
 
-		/* Write proxy bit via wrmsl instruction */
-		if (hv_is_sint_msr(reg))
-			wrmsrl(reg, value | 1 << 20);
+		if (hv_is_sint_msr(reg)) {
+			/*
+			 * Write proxy bit in the case of non-confidential VMBus.
+			 * Using wrmsl instruction so the following goes to the paravisor.
+			 */
+			u32 proxy = vmbus_is_confidential() ? 0 : 1;
+
+			value |= (proxy << 20);
+			native_wrmsrl(reg, value);
+		}
 	} else {
-		wrmsrl(reg, value);
+		native_wrmsrl(reg, value);
 	}
 }
 EXPORT_SYMBOL_GPL(hv_set_non_nested_msr);
-- 
2.43.0