The confidential VMBus is built on the guest talking to the
paravisor only.

Provide functions that allow manipulating the SynIC registers
via paravisor.

Signed-off-by: Roman Kisel <romank@linux.microsoft.com>
---
 arch/arm64/hyperv/mshyperv.c      | 19 +++++++++++++++++++
 arch/arm64/include/asm/mshyperv.h |  3 +++
 arch/x86/include/asm/mshyperv.h   |  3 +++
 arch/x86/kernel/cpu/mshyperv.c    | 28 ++++++++++++++++++++++++++++
 4 files changed, 53 insertions(+)

diff --git a/arch/arm64/hyperv/mshyperv.c b/arch/arm64/hyperv/mshyperv.c
index 4fdc26ade1d7..8778b6831062 100644
--- a/arch/arm64/hyperv/mshyperv.c
+++ b/arch/arm64/hyperv/mshyperv.c
@@ -134,3 +134,22 @@ bool hv_is_hyperv_initialized(void)
 	return hyperv_initialized;
 }
 EXPORT_SYMBOL_GPL(hv_is_hyperv_initialized);
+
+/*
+ * Not supported yet.
+ */
+u64 hv_pv_get_synic_register(unsigned int reg, int *err)
+{
+	*err = -ENODEV;
+	return !0ULL;
+}
+EXPORT_SYMBOL_GPL(hv_pv_get_synic_register);
+
+/*
+ * Not supported yet.
+ */
+int hv_pv_set_synic_register(unsigned int reg, u64 val)
+{
+	return -ENODEV;
+}
+EXPORT_SYMBOL_GPL(hv_pv_set_synic_register);
diff --git a/arch/arm64/include/asm/mshyperv.h b/arch/arm64/include/asm/mshyperv.h
index b721d3134ab6..bce37a58dff0 100644
--- a/arch/arm64/include/asm/mshyperv.h
+++ b/arch/arm64/include/asm/mshyperv.h
@@ -53,6 +53,9 @@ static inline u64 hv_get_non_nested_msr(unsigned int reg)
 	return hv_get_msr(reg);
 }
 
+u64 hv_pv_get_synic_register(unsigned int reg, int *err);
+int hv_pv_set_synic_register(unsigned int reg, u64 val);
+
 /* SMCCC hypercall parameters */
 #define HV_SMCCC_FUNC_NUMBER	1
 #define HV_FUNC_ID	ARM_SMCCC_CALL_VAL(			\
diff --git a/arch/x86/include/asm/mshyperv.h b/arch/x86/include/asm/mshyperv.h
index bab5ccfc60a7..0a4b01c1f094 100644
--- a/arch/x86/include/asm/mshyperv.h
+++ b/arch/x86/include/asm/mshyperv.h
@@ -307,6 +307,9 @@ static __always_inline u64 hv_raw_get_msr(unsigned int reg)
 	return __rdmsr(reg);
 }
 
+u64 hv_pv_get_synic_register(unsigned int reg, int *err);
+int hv_pv_set_synic_register(unsigned int reg, u64 val);
+
 #else /* CONFIG_HYPERV */
 static inline void hyperv_init(void) {}
 static inline void hyperv_setup_mmu_ops(void) {}
diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
index 3e2533954675..4f6e3d02f730 100644
--- a/arch/x86/kernel/cpu/mshyperv.c
+++ b/arch/x86/kernel/cpu/mshyperv.c
@@ -89,6 +89,34 @@ void hv_set_non_nested_msr(unsigned int reg, u64 value)
 }
 EXPORT_SYMBOL_GPL(hv_set_non_nested_msr);
 
+/*
+ * Not every paravisor supports getting SynIC registers, and
+ * this function may fail. The caller has to make sure that this function
+ * runs on the CPU of interest.
+ */
+u64 hv_pv_get_synic_register(unsigned int reg, int *err)
+{
+	if (!hv_is_synic_msr(reg)) {
+		*err = -ENODEV;
+		return !0ULL;
+	}
+	return native_read_msr_safe(reg, err);
+}
+EXPORT_SYMBOL_GPL(hv_pv_get_synic_register);
+
+/*
+ * Not every paravisor supports setting SynIC registers, and
+ * this function may fail. The caller has to make sure that this function
+ * runs on the CPU of interest.
+ */
+int hv_pv_set_synic_register(unsigned int reg, u64 val)
+{
+	if (!hv_is_synic_msr(reg))
+		return -ENODEV;
+	return wrmsrl_safe(reg, val);
+}
+EXPORT_SYMBOL_GPL(hv_pv_set_synic_register);
+
 u64 hv_get_msr(unsigned int reg)
 {
 	if (hv_nested)
-- 
2.43.0

On 12-05-2025 04:37, Roman Kisel wrote:
> +/*
> + * Not every paravisor supports getting SynIC registers, and
> + * this function may fail. The caller has to make sure that this function
> + * runs on the CPU of interest.
> + */

Title and Intent: Clearly state the purpose of the function in the first 
sentence
/*
  * Attempt to get the SynIC register value.
  *
  * Not all paravisors support reading SynIC registers, so this function
  * may fail. The caller must ensure that it is executed on the target
  * CPU.
  *
  * Returns: The SynIC register value or ~0ULL on failure.
  * Sets err to -ENODEV if the provided register is not a valid SynIC
  * MSR.
  */

> +u64 hv_pv_get_synic_register(unsigned int reg, int *err)
> +{
> +	if (!hv_is_synic_msr(reg)) {
> +		*err = -ENODEV;
> +		return !0ULL;
> +	}
> +	return native_read_msr_safe(reg, err);
> +}
> +EXPORT_SYMBOL_GPL(hv_pv_get_synic_register);
> +
> +/*
> + * Not every paravisor supports setting SynIC registers, and
> + * this function may fail. The caller has to make sure that this function
> + * runs on the CPU of interest.
> + */

ditto.

> +int hv_pv_set_synic_register(unsigned int reg, u64 val)
> +{
> +	if (!hv_is_synic_msr(reg))
> +		return -ENODEV;
> +	return wrmsrl_safe(reg, val);
> +}
> +EXPORT_SYMBOL_GPL(hv_pv_set_synic_register);

Thanks,
Alok

On 5/12/2025 2:39 AM, ALOK TIWARI wrote:
> 
> 
> On 12-05-2025 04:37, Roman Kisel wrote:
>> +/*
>> + * Not every paravisor supports getting SynIC registers, and
>> + * this function may fail. The caller has to make sure that this 
>> function
>> + * runs on the CPU of interest.
>> + */
> 
> Title and Intent: Clearly state the purpose of the function in the first 
> sentence
> /*
>   * Attempt to get the SynIC register value.
>   *
>   * Not all paravisors support reading SynIC registers, so this function
>   * may fail. The caller must ensure that it is executed on the target
>   * CPU.
>   *
>   * Returns: The SynIC register value or ~0ULL on failure.
>   * Sets err to -ENODEV if the provided register is not a valid SynIC
>   * MSR.
>   */
> 
>> +u64 hv_pv_get_synic_register(unsigned int reg, int *err)
>> +{
>> +    if (!hv_is_synic_msr(reg)) {
>> +        *err = -ENODEV;
>> +        return !0ULL;
>> +    }
>> +    return native_read_msr_safe(reg, err);
>> +}
>> +EXPORT_SYMBOL_GPL(hv_pv_get_synic_register);
>> +
>> +/*
>> + * Not every paravisor supports setting SynIC registers, and
>> + * this function may fail. The caller has to make sure that this 
>> function
>> + * runs on the CPU of interest.
>> + */
> 
> ditto.
> 
>> +int hv_pv_set_synic_register(unsigned int reg, u64 val)
>> +{
>> +    if (!hv_is_synic_msr(reg))
>> +        return -ENODEV;
>> +    return wrmsrl_safe(reg, val);
>> +}
>> +EXPORT_SYMBOL_GPL(hv_pv_set_synic_register);
> 

Indeed, I wrote a bit of a novel in the comments which might be
distracting and making it harder to find the point :)

Ought to be more conscious of the reader's perhaps constrained
time budget. I'll restructure that as you suggested!

> Thanks,
> Alok
> 

-- 
Thank you,
Roman