hv_set_non_nested_msr() has special handling for SINT MSRs
when a paravisor is present. In addition to updating the MSR on the
host, the mirror MSR in the paravisor is updated, including with the
proxy bit. But with Confidential VMBus, the proxy bit must not be
used, so add a special case to skip it.

Update the hv_set_non_nested_msr() function as well as
vmbus_signal_eom() to trap on access for some synthetic MSRs.

Signed-off-by: Roman Kisel <romank@linux.microsoft.com>
Reviewed-by: Alok Tiwari <alok.a.tiwari@oracle.com>
---
 arch/x86/kernel/cpu/mshyperv.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index 07c60231d0d8..6c5a0a779c02 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/msr.h>
@@ -79,13 +80,21 @@ EXPORT_SYMBOL_GPL(hv_get_non_nested_msr);
 void hv_set_non_nested_msr(unsigned int reg, u64 value)
 {
 	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))
-			wrmsrq(reg, value | 1 << 20);
+		if (hv_is_sint_msr(reg)) {
+			/*
+			 * Write proxy bit in the case of non-confidential VMBus.
+			 * Using wrmsrq instruction so the following goes to the paravisor.
+			 */
+			u32 proxy = vmbus_is_confidential() ? 0 : 1;
+
+			value |= (proxy << 20);
+			native_wrmsrq(reg, value);
+		}
 	} else {
-		wrmsrq(reg, value);
+		native_wrmsrq(reg, value);
 	}
 }
 EXPORT_SYMBOL_GPL(hv_set_non_nested_msr);
-- 
2.43.0

On Tue, Jul 15, 2025 at 6:16 AM Roman Kisel <romank@linux.microsoft.com> wrote:
>
> hv_set_non_nested_msr() has special handling for SINT MSRs
> when a paravisor is present. In addition to updating the MSR on the
> host, the mirror MSR in the paravisor is updated, including with the
> proxy bit. But with Confidential VMBus, the proxy bit must not be
> used, so add a special case to skip it.
>
> Update the hv_set_non_nested_msr() function as well as
> vmbus_signal_eom() to trap on access for some synthetic MSRs.
>
> Signed-off-by: Roman Kisel <romank@linux.microsoft.com>
> Reviewed-by: Alok Tiwari <alok.a.tiwari@oracle.com>
> ---

Reviewed-by: Tianyu Lan <tiala@microsoft.com>

-- 
Thanks
Tianyu Lan

Hi Roman,

kernel test robot noticed the following build errors:

[auto build test ERROR on d9016a249be5316ec2476f9947356711e70a16ec]

url:    https://github.com/intel-lab-lkp/linux/commits/Roman-Kisel/Documentation-hyperv-Confidential-VMBus/20250715-062125
base:   d9016a249be5316ec2476f9947356711e70a16ec
patch link:    https://lore.kernel.org/r/20250714221545.5615-5-romank%40linux.microsoft.com
patch subject: [PATCH hyperv-next v4 04/16] arch/x86: mshyperv: Trap on access for some synthetic MSRs
config: x86_64-buildonly-randconfig-002-20250715 (https://download.01.org/0day-ci/archive/20250716/202507160105.yQ34bnKl-lkp@intel.com/config)
compiler: gcc-12 (Debian 12.2.0-14+deb12u1) 12.2.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20250716/202507160105.yQ34bnKl-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202507160105.yQ34bnKl-lkp@intel.com/

All errors (new ones prefixed by >>):

   ld: vmlinux.o: in function `hv_set_non_nested_msr':
>> arch/x86/kernel/cpu/mshyperv.c:91: undefined reference to `vmbus_is_confidential'


vim +91 arch/x86/kernel/cpu/mshyperv.c

    79	
    80	void hv_set_non_nested_msr(unsigned int reg, u64 value)
    81	{
    82		if (hv_is_synic_msr(reg) && ms_hyperv.paravisor_present) {
    83			/* The hypervisor will get the intercept. */
    84			hv_ivm_msr_write(reg, value);
    85	
    86			if (hv_is_sint_msr(reg)) {
    87				/*
    88				 * Write proxy bit in the case of non-confidential VMBus.
    89				 * Using wrmsrq instruction so the following goes to the paravisor.
    90				 */
  > 91				u32 proxy = vmbus_is_confidential() ? 0 : 1;
    92	
    93				value |= (proxy << 20);
    94				native_wrmsrq(reg, value);
    95			}
    96		} else {
    97			native_wrmsrq(reg, value);
    98		}
    99	}
   100	EXPORT_SYMBOL_GPL(hv_set_non_nested_msr);
   101	

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki