Re: [PATCH v4 1/4] x86/sev: add SVSM vTPM probe/send_command functions

[Dionna Amalie Glaze]

On Mon, Mar 24, 2025 at 3:47 AM Stefano Garzarella <sgarzare@xxxxxxxxxx> wrote:
>
> From: Stefano Garzarella <sgarzare@xxxxxxxxxx>
>
> Add two new functions to probe and send commands to the SVSM vTPM.
> They leverage the two calls defined by the AMD SVSM specification [1]
> for the vTPM protocol: SVSM_VTPM_QUERY and SVSM_VTPM_CMD.
>
> Expose these functions to be used by other modules such as a tpm
> driver.
>
> [1] "Secure VM Service Module for SEV-SNP Guests"
>     Publication # 58019 Revision: 1.00
>
> Co-developed-by: James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>
> Co-developed-by: Claudio Carvalho <cclaudio@xxxxxxxxxxxxx>
> Signed-off-by: Claudio Carvalho <cclaudio@xxxxxxxxxxxxx>
> Reviewed-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
> Signed-off-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>
> ---
> v4:
> - added Tom's R-b
> - added functions documentation [Jarkko]
> - simplified TPM_SEND_COMMAND check [Tom/Jarkko]
> v3:
> - removed link to the spec because those URLs are unstable [Borislav]
> - squashed "x86/sev: add SVSM call macros for the vTPM protocol" patch
>   in this one [Borislav]
> - slimmed down snp_svsm_vtpm_probe() [Borislav]
> - removed features check and any print related [Tom]
> ---
>  arch/x86/include/asm/sev.h |  7 +++++
>  arch/x86/coco/sev/core.c   | 59 ++++++++++++++++++++++++++++++++++++++
>  2 files changed, 66 insertions(+)
>
> diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
> index ba7999f66abe..09471d058ce5 100644
> --- a/arch/x86/include/asm/sev.h
> +++ b/arch/x86/include/asm/sev.h
> @@ -384,6 +384,10 @@ struct svsm_call {
>  #define SVSM_ATTEST_SERVICES           0
>  #define SVSM_ATTEST_SINGLE_SERVICE     1
>
> +#define SVSM_VTPM_CALL(x)              ((2ULL << 32) | (x))
> +#define SVSM_VTPM_QUERY                        0
> +#define SVSM_VTPM_CMD                  1
> +
>  #ifdef CONFIG_AMD_MEM_ENCRYPT
>
>  extern u8 snp_vmpl;
> @@ -481,6 +485,9 @@ void snp_msg_free(struct snp_msg_desc *mdesc);
>  int snp_send_guest_request(struct snp_msg_desc *mdesc, struct snp_guest_req *req,
>                            struct snp_guest_request_ioctl *rio);
>
> +bool snp_svsm_vtpm_probe(void);
> +int snp_svsm_vtpm_send_command(u8 *buffer);
> +

These should be defined static inline with trivial definitions in the
#else case, yes?

>  void __init snp_secure_tsc_prepare(void);
>  void __init snp_secure_tsc_init(void);
>
> diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
> index 96c7bc698e6b..034aab7e76d2 100644
> --- a/arch/x86/coco/sev/core.c
> +++ b/arch/x86/coco/sev/core.c
> @@ -2628,6 +2628,65 @@ static int snp_issue_guest_request(struct snp_guest_req *req, struct snp_req_dat
>         return ret;
>  }
>
> +/**
> + * snp_svsm_vtpm_probe() - Probe if SVSM provides a vTPM device
> + *
> + * This function checks that there is SVSM and that it supports at least
> + * TPM_SEND_COMMAND which is the only request we use so far.
> + *
> + * Return: true if the platform provides a vTPM SVSM device, false otherwise.
> + */
> +bool snp_svsm_vtpm_probe(void)
> +{
> +       struct svsm_call call = {};
> +
> +       /* The vTPM device is available only if a SVSM is present */
> +       if (!snp_vmpl)
> +               return false;
> +
> +       call.caa = svsm_get_caa();
> +       call.rax = SVSM_VTPM_CALL(SVSM_VTPM_QUERY);
> +
> +       if (svsm_perform_call_protocol(&call))
> +               return false;
> +
> +       /* Check platform commands contains TPM_SEND_COMMAND - platform command 8 */
> +       return call.rcx_out & BIT_ULL(8);
> +}
> +EXPORT_SYMBOL_GPL(snp_svsm_vtpm_probe);
> +
> +/**
> + * snp_svsm_vtpm_send_command() - execute a vTPM operation on SVSM
> + * @buffer: A buffer used to both send the command and receive the response.
> + *
> + * This function executes a SVSM_VTPM_CMD call as defined by
> + * "Secure VM Service Module for SEV-SNP Guests" Publication # 58019 Revision: 1.00
> + *
> + * All command request/response buffers have a common structure as specified by
> + * the following table:
> + *     Byte      Size       In/Out    Description
> + *     Offset    (Bytes)
> + *     0x000     4          In        Platform command
> + *                          Out       Platform command response size
> + *
> + * Each command can build upon this common request/response structure to create
> + * a structure specific to the command.
> + * See include/linux/tpm_svsm.h for more details.
> + *
> + * Return: 0 on success, -errno on failure
> + */
> +int snp_svsm_vtpm_send_command(u8 *buffer)
> +{
> +       struct svsm_call call = {};
> +
> +       call.caa = svsm_get_caa();
> +       call.rax = SVSM_VTPM_CALL(SVSM_VTPM_CMD);
> +       call.rcx = __pa(buffer);
> +
> +       return svsm_perform_call_protocol(&call);
> +}
> +EXPORT_SYMBOL_GPL(snp_svsm_vtpm_send_command);
> +
>  static struct platform_device sev_guest_device = {
>         .name           = "sev-guest",
>         .id             = -1,
> --
> 2.49.0
>


-- 
-Dionna Glaze, PhD, CISSP, CCSP (she/her)