linux-kernel - Re: [RFC PATCH v2 2/6] x86/sev: add SVSM vTPM probe/send

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Message-ID: <20250310113006.GFZ87NPu-LgFVVtsEG@fat_crate.local>
Date: Mon, 10 Mar 2025 12:30:06 +0100
From: Borislav Petkov <bp@...en8.de>
To: Stefano Garzarella <sgarzare@...hat.com>
Cc: Jarkko Sakkinen <jarkko@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Claudio Carvalho <cclaudio@...ux.ibm.com>,
	Peter Huewe <peterhuewe@....de>, x86@...nel.org,
	Dov Murik <dovmurik@...ux.ibm.com>, linux-coco@...ts.linux.dev,
	Dionna Glaze <dionnaglaze@...gle.com>,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
	Jason Gunthorpe <jgg@...pe.ca>, linux-integrity@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Tom Lendacky <thomas.lendacky@....com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [RFC PATCH v2 2/6] x86/sev: add SVSM vTPM probe/send_command
 functions

On Fri, Feb 28, 2025 at 06:07:16PM +0100, Stefano Garzarella wrote:
> +bool snp_svsm_vtpm_probe(void)
> +{
> +	struct svsm_call call = {};
> +	u64 send_cmd_mask = 0;
> +	u64 platform_cmds;
> +	u64 features;
> +	int ret;
> +
> +	/* The vTPM device is available only if we have a SVSM */

s/if we have a SVSM/if an SVSM is present/

> +	if (!snp_vmpl)
> +		return false;
> +
> +	call.caa = svsm_get_caa();
> +	call.rax = SVSM_VTPM_CALL(SVSM_VTPM_QUERY);
> +
> +	ret = svsm_perform_call_protocol(&call);
> +


^ Superfluous newline.

> +	if (ret != SVSM_SUCCESS)
> +		return false;
> +
> +	features = call.rdx_out;
> +	platform_cmds = call.rcx_out;
> +
> +	/* No feature supported, it should be zero */
> +	if (features)
> +		pr_warn("SNP SVSM vTPM unsupported features: 0x%llx\n",
> +			features);

So

	return false;

here?

> +
> +	/* TPM_SEND_COMMAND - platform command 8 */
> +	send_cmd_mask = 1 << 8;

	BIT_ULL(8);

> +
> +	return (platform_cmds & send_cmd_mask) == send_cmd_mask;
> +}
> +EXPORT_SYMBOL_GPL(snp_svsm_vtpm_probe);
> +
> +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);
> +}

In any case, you can zap all those local vars, use comments instead and slim
down the function, diff ontop:

diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
index 3902af4b1385..6d7e97c1f567 100644
--- a/arch/x86/coco/sev/core.c
+++ b/arch/x86/coco/sev/core.c
@@ -2631,12 +2631,9 @@ static int snp_issue_guest_request(struct snp_guest_req *req, struct snp_req_dat
 bool snp_svsm_vtpm_probe(void)
 {
 	struct svsm_call call = {};
-	u64 send_cmd_mask = 0;
-	u64 platform_cmds;
-	u64 features;
 	int ret;
 
-	/* The vTPM device is available only if we have a SVSM */
+	/* The vTPM device is available only if a SVSM is present */
 	if (!snp_vmpl)
 		return false;
 
@@ -2644,22 +2641,17 @@ bool snp_svsm_vtpm_probe(void)
 	call.rax = SVSM_VTPM_CALL(SVSM_VTPM_QUERY);
 
 	ret = svsm_perform_call_protocol(&call);
-
 	if (ret != SVSM_SUCCESS)
 		return false;
 
-	features = call.rdx_out;
-	platform_cmds = call.rcx_out;
-
 	/* No feature supported, it should be zero */
-	if (features)
-		pr_warn("SNP SVSM vTPM unsupported features: 0x%llx\n",
-			features);
-
-	/* TPM_SEND_COMMAND - platform command 8 */
-	send_cmd_mask = 1 << 8;
+	if (call.rdx_out) {
+		pr_warn("SNP SVSM vTPM unsupported features: 0x%llx\n", call.rdx_out);
+		return false;
+	}
 
-	return (platform_cmds & send_cmd_mask) == send_cmd_mask;
+	/* Check platform commands is TPM_SEND_COMMAND - platform command 8 */
+	return (call.rcx_out & BIT_ULL(8)) == BIT_ULL(8);
 }
 EXPORT_SYMBOL_GPL(snp_svsm_vtpm_probe);
 

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette