From: Roman Kisel <romank@xxxxxxxxxxxxxxxxxxx> Sent: Wednesday, August 27, 2025 6:06 PM
>
> The confidential VMBus is supported starting from the protocol
> version 6.0 onwards.
>
> Update the relevant definitions, provide the x86-specific way to
> detect availability of the Confidential VMBus and provide a
> function that returns whether VMBus is confidential or not.
> No functional changes.
>
> Signed-off-by: Roman Kisel <romank@xxxxxxxxxxxxxxxxxxx>
> Reviewed-by: Alok Tiwari <alok.a.tiwari@xxxxxxxxxx>
> Reviewed-by: Michael Kelley <mhklinux@xxxxxxxxxxx>
> ---
> arch/x86/kernel/cpu/mshyperv.c | 19 ++++++++++
> drivers/hv/hv_common.c | 6 +++
> drivers/hv/hyperv_vmbus.h | 2 +
> drivers/hv/vmbus_drv.c | 12 ++++++
> include/asm-generic/mshyperv.h | 1 +
> include/hyperv/hvgdk_mini.h | 1 +
> include/linux/hyperv.h | 69 ++++++++++++++++++++++++----------
> 7 files changed, 91 insertions(+), 19 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
> index 25773af116bc..95cd78004b11 100644
> --- a/arch/x86/kernel/cpu/mshyperv.c
> +++ b/arch/x86/kernel/cpu/mshyperv.c
> @@ -90,6 +90,25 @@ void hv_set_non_nested_msr(unsigned int reg, u64 value)
> }
> EXPORT_SYMBOL_GPL(hv_set_non_nested_msr);
>
> +/*
> + * Detect if the confidential VMBus is available.
> + */
> +bool hv_confidential_vmbus_available(void)
> +{
> + u32 eax;
> +
> + eax = cpuid_eax(HYPERV_CPUID_VIRT_STACK_INTERFACE);
> + if (eax != HYPERV_VS_INTERFACE_EAX_SIGNATURE)
> + return false;
> +
> + eax = cpuid_eax(HYPERV_CPUID_VIRT_STACK_PROPERTIES);
> +
> + /*
> + * The paravisor may set the bit in the hardware confidential VMs.
> + */
> + return eax & HYPERV_VS_PROPERTIES_EAX_CONFIDENTIAL_VMBUS_AVAILABLE;
> +}
This same code to read the VIRT_STACK_PROPERTIES CPUID is in
ms_hyperv_msi_ext_dest_id(). Now that we have two things based
off that CPUID value, I'd suggest the following:
1) Add a virt_stack field to struct ms_hyperv_info. That struct already
stores all the little flags fields that Hyper-V provides.
2) Populate this field in ms_hyperv_init_platform() along with all the
existing code that reads CPUID values and populates the other fields
in ms_hyperv.
3) Update ms_hyperv_msi_ext_dest_id() to check against the
virt_stack field instead of reading the CPUID.
4) Have hv_confidential_vmbus_available() also check against the
virt_stack field.
Arguably, you could drop hv_confidential_vmbus_available() entirely
(along with the stub in hv_common.c) and just have vmbus_bus_init()
directly test the flag in ms_hyperv.virt_stack. We decided long ago not
to add a plethora of accessor functions for flags in ms_hyperv, so
accessing ms_hyperv.virt_stack directly would be consistent with current
practice.
The virt_stack field would not be populated on ARM64, so it will be 0,
which produces the desired behavior.
> +
> u64 hv_get_msr(unsigned int reg)
> {
> if (hv_nested)
> diff --git a/drivers/hv/hv_common.c b/drivers/hv/hv_common.c
> index 8836cf9fad40..fae63c54e531 100644
> --- a/drivers/hv/hv_common.c
> +++ b/drivers/hv/hv_common.c
> @@ -716,6 +716,12 @@ u64 __weak hv_tdx_hypercall(u64 control, u64 param1, u64
> param2)
> }
> EXPORT_SYMBOL_GPL(hv_tdx_hypercall);
>
> +bool __weak hv_confidential_vmbus_available(void)
> +{
> + return false;
> +}
> +EXPORT_SYMBOL_GPL(hv_confidential_vmbus_available);
> +
> void hv_identify_partition_type(void)
> {
> /* Assume guest role */
> diff --git a/drivers/hv/hyperv_vmbus.h b/drivers/hv/hyperv_vmbus.h
> index b61f01fc1960..6e4c3acc1169 100644
> --- a/drivers/hv/hyperv_vmbus.h
> +++ b/drivers/hv/hyperv_vmbus.h
> @@ -334,6 +334,8 @@ extern const struct vmbus_channel_message_table_entry
>
> /* General vmbus interface */
>
> +bool vmbus_is_confidential(void);
> +
> struct hv_device *vmbus_device_create(const guid_t *type,
> const guid_t *instance,
> struct vmbus_channel *channel);
> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
> index a366365f2c49..8d9488c3174d 100644
> --- a/drivers/hv/vmbus_drv.c
> +++ b/drivers/hv/vmbus_drv.c
> @@ -57,6 +57,18 @@ static long __percpu *vmbus_evt;
> int vmbus_irq;
> int vmbus_interrupt;
>
> +/*
> + * If the Confidential VMBus is used, the data on the "wire" is not
> + * visible to either the host or the hypervisor.
> + */
> +static bool is_confidential;
> +
> +bool vmbus_is_confidential(void)
> +{
> + return is_confidential;
> +}
> +EXPORT_SYMBOL_GPL(vmbus_is_confidential);
> +
> /*
> * The panic notifier below is responsible solely for unloading the
> * vmbus connection, which is necessary in a panic event.
> diff --git a/include/asm-generic/mshyperv.h b/include/asm-generic/mshyperv.h
> index dbd4c2f3aee3..acb017f6c423 100644
> --- a/include/asm-generic/mshyperv.h
> +++ b/include/asm-generic/mshyperv.h
> @@ -332,6 +332,7 @@ bool hv_is_hibernation_supported(void);
> enum hv_isolation_type hv_get_isolation_type(void);
> bool hv_is_isolation_supported(void);
> bool hv_isolation_type_snp(void);
> +bool hv_confidential_vmbus_available(void);
> u64 hv_ghcb_hypercall(u64 control, void *input, void *output, u32 input_size);
> u64 hv_tdx_hypercall(u64 control, u64 param1, u64 param2);
> void hyperv_cleanup(void);
> diff --git a/include/hyperv/hvgdk_mini.h b/include/hyperv/hvgdk_mini.h
> index 79b7324e4ef5..4b189f9d39cb 100644
> --- a/include/hyperv/hvgdk_mini.h
> +++ b/include/hyperv/hvgdk_mini.h
> @@ -260,6 +260,7 @@ union hv_hypervisor_version_info {
> #define HYPERV_CPUID_VIRT_STACK_PROPERTIES 0x40000082
> /* Support for the extended IOAPIC RTE format */
> #define HYPERV_VS_PROPERTIES_EAX_EXTENDED_IOAPIC_RTE BIT(2)
> +#define HYPERV_VS_PROPERTIES_EAX_CONFIDENTIAL_VMBUS_AVAILABLE BIT(3)
>
> #define HYPERV_HYPERVISOR_PRESENT_BIT 0x80000000
> #define HYPERV_CPUID_MIN 0x40000005
> diff --git a/include/linux/hyperv.h b/include/linux/hyperv.h
> index a59c5c3e95fb..a1820fabbfc0 100644
> --- a/include/linux/hyperv.h
> +++ b/include/linux/hyperv.h
> @@ -265,16 +265,18 @@ static inline u32 hv_get_avail_to_write_percent(
> * Linux kernel.
> */
>
> -#define VERSION_WS2008 ((0 << 16) | (13))
> -#define VERSION_WIN7 ((1 << 16) | (1))
> -#define VERSION_WIN8 ((2 << 16) | (4))
> -#define VERSION_WIN8_1 ((3 << 16) | (0))
> -#define VERSION_WIN10 ((4 << 16) | (0))
> -#define VERSION_WIN10_V4_1 ((4 << 16) | (1))
> -#define VERSION_WIN10_V5 ((5 << 16) | (0))
> -#define VERSION_WIN10_V5_1 ((5 << 16) | (1))
> -#define VERSION_WIN10_V5_2 ((5 << 16) | (2))
> -#define VERSION_WIN10_V5_3 ((5 << 16) | (3))
> +#define VMBUS_MAKE_VERSION(MAJ, MIN) ((((u32)MAJ) << 16) | (MIN))
> +#define VERSION_WS2008 VMBUS_MAKE_VERSION(0, 13)
> +#define VERSION_WIN7 VMBUS_MAKE_VERSION(1, 1)
> +#define VERSION_WIN8 VMBUS_MAKE_VERSION(2, 4)
> +#define VERSION_WIN8_1 VMBUS_MAKE_VERSION(3, 0)
> +#define VERSION_WIN10 VMBUS_MAKE_VERSION(4, 0)
> +#define VERSION_WIN10_V4_1 VMBUS_MAKE_VERSION(4, 1)
> +#define VERSION_WIN10_V5 VMBUS_MAKE_VERSION(5, 0)
> +#define VERSION_WIN10_V5_1 VMBUS_MAKE_VERSION(5, 1)
> +#define VERSION_WIN10_V5_2 VMBUS_MAKE_VERSION(5, 2)
> +#define VERSION_WIN10_V5_3 VMBUS_MAKE_VERSION(5, 3)
> +#define VERSION_WIN10_V6_0 VMBUS_MAKE_VERSION(6, 0)
>
> /* Make maximum size of pipe payload of 16K */
> #define MAX_PIPE_DATA_PAYLOAD (sizeof(u8) * 16384)
> @@ -335,14 +337,22 @@ struct vmbus_channel_offer {
> } __packed;
>
> /* Server Flags */
> -#define VMBUS_CHANNEL_ENUMERATE_DEVICE_INTERFACE 1
> -#define VMBUS_CHANNEL_SERVER_SUPPORTS_TRANSFER_PAGES 2
> -#define VMBUS_CHANNEL_SERVER_SUPPORTS_GPADLS 4
> -#define VMBUS_CHANNEL_NAMED_PIPE_MODE 0x10
> -#define VMBUS_CHANNEL_LOOPBACK_OFFER 0x100
> -#define VMBUS_CHANNEL_PARENT_OFFER 0x200
> -#define VMBUS_CHANNEL_REQUEST_MONITORED_NOTIFICATION 0x400
> -#define VMBUS_CHANNEL_TLNPI_PROVIDER_OFFER 0x2000
> +#define VMBUS_CHANNEL_ENUMERATE_DEVICE_INTERFACE 0x0001
> +/*
> + * This flag indicates that the channel is offered by the paravisor, and must
> + * use encrypted memory for the channel ring buffer.
> + */
> +#define VMBUS_CHANNEL_CONFIDENTIAL_RING_BUFFER 0x0002
> +/*
> + * This flag indicates that the channel is offered by the paravisor, and must
> + * use encrypted memory for GPA direct packets and additional GPADLs.
> + */
> +#define VMBUS_CHANNEL_CONFIDENTIAL_EXTERNAL_MEMORY 0x0004
> +#define VMBUS_CHANNEL_NAMED_PIPE_MODE 0x0010
> +#define VMBUS_CHANNEL_LOOPBACK_OFFER 0x0100
> +#define VMBUS_CHANNEL_PARENT_OFFER 0x0200
> +#define VMBUS_CHANNEL_REQUEST_MONITORED_NOTIFICATION 0x0400
> +#define VMBUS_CHANNEL_TLNPI_PROVIDER_OFFER 0x2000
>
> struct vmpacket_descriptor {
> u16 type;
> @@ -621,6 +631,12 @@ struct vmbus_channel_relid_released {
> u32 child_relid;
> } __packed;
>
> +/*
> + * Used by the paravisor only, means that the encrypted ring buffers and
> + * the encrypted external memory are supported
> + */
> +#define VMBUS_FEATURE_FLAG_CONFIDENTIAL_CHANNELS 0x10
> +
> struct vmbus_channel_initiate_contact {
> struct vmbus_channel_message_header header;
> u32 vmbus_version_requested;
> @@ -630,7 +646,8 @@ struct vmbus_channel_initiate_contact {
> struct {
> u8 msg_sint;
> u8 msg_vtl;
> - u8 reserved[6];
> + u8 reserved[2];
> + u32 feature_flags; /* VMBus version 6.0 */
> };
> };
> u64 monitor_page1;
> @@ -1008,6 +1025,10 @@ struct vmbus_channel {
>
> /* boolean to control visibility of sysfs for ring buffer */
> bool ring_sysfs_visible;
> + /* The ring buffer is encrypted */
> + bool co_ring_buffer;
> + /* The external memory is encrypted */
> + bool co_external_memory;
> };
>
> #define lock_requestor(channel, flags) \
> @@ -1032,6 +1053,16 @@ u64 vmbus_request_addr_match(struct vmbus_channel *channel, u64 trans_id,
> u64 rqst_addr);
> u64 vmbus_request_addr(struct vmbus_channel *channel, u64 trans_id);
>
> +static inline bool is_co_ring_buffer(const struct vmbus_channel_offer_channel *o)
> +{
> + return !!(o->offer.chn_flags & VMBUS_CHANNEL_CONFIDENTIAL_RING_BUFFER);
> +}
> +
> +static inline bool is_co_external_memory(const struct vmbus_channel_offer_channel *o)
> +{
> + return !!(o->offer.chn_flags & VMBUS_CHANNEL_CONFIDENTIAL_EXTERNAL_MEMORY);
> +}
> +
> static inline bool is_hvsock_offer(const struct vmbus_channel_offer_channel *o)
> {
> return !!(o->offer.chn_flags & VMBUS_CHANNEL_TLNPI_PROVIDER_OFFER);
> --
> 2.43.0
>
