Re: [PATCH v3 1/4] x86/sev: add SVSM vTPM probe/send_command functions

[Jarkko Sakkinen <jarkko@kernel.org>]

On Fri, Mar 14, 2025 at 10:27:07AM -0500, Tom Lendacky wrote:
> On 3/11/25 04:42, Stefano Garzarella wrote:
> > 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@HansenPartnership.com>
> > Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
> > Co-developed-by: Claudio Carvalho <cclaudio@linux.ibm.com>
> > Signed-off-by: Claudio Carvalho <cclaudio@linux.ibm.com>
> > Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> 
> One minor nit below, otherwise:
> 
> Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
> 
> > ---
> > 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   | 31 +++++++++++++++++++++++++++++++
> >  2 files changed, 38 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);
> > +
> >  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..2166bdff88b7 100644
> > --- a/arch/x86/coco/sev/core.c
> > +++ b/arch/x86/coco/sev/core.c
> > @@ -2628,6 +2628,37 @@ static int snp_issue_guest_request(struct snp_guest_req *req, struct snp_req_dat
> >  	return ret;
> >  }
> >  
> > +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)) == BIT_ULL(8);
> 
> It's a bool function, so this could simplified to just:
> 
> 	return call.rcx_out & BIT_ULL(8);

Or perhaps even just "call.rcx_out & 0x100". I don't think BIT_ULL()
here brings much additional clarity or anything useful...


> 
> Thanks,
> Tom

BR, Jarkko