linux-kernel - Re: [PATCH Part1 v5 07/38] x86/sev: Add support for hypervisor feature VMGEXIT

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Message-ID: <YSNutt/E0bm0kKsl@zn.tnic>
Date:   Mon, 23 Aug 2021 11:47:34 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-efi@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>, Peter Gonda <pgonda@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>, tony.luck@...el.com,
        marcorr@...gle.com, sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH Part1 v5 07/38] x86/sev: Add support for hypervisor
 feature VMGEXIT

On Fri, Aug 20, 2021 at 10:19:02AM -0500, Brijesh Singh wrote:
> Version 2 of GHCB specification introduced advertisement of a features
> that are supported by the hypervisor. Add support to query the HV
> features on boot.
> 
> Version 2 of GHCB specification adds several new NAEs, most of them are
> optional except the hypervisor feature. Now that hypervisor feature NAE
> is implemented, so bump the GHCB maximum support protocol version.
> 
> Signed-off-by: Brijesh Singh <brijesh.singh@....com>
> ---
>  arch/x86/include/asm/mem_encrypt.h |  2 ++
>  arch/x86/include/asm/sev-common.h  |  3 +++
>  arch/x86/include/asm/sev.h         |  2 +-
>  arch/x86/include/uapi/asm/svm.h    |  2 ++
>  arch/x86/kernel/sev-shared.c       | 23 +++++++++++++++++++++++
>  5 files changed, 31 insertions(+), 1 deletion(-)

I think you can simplify more.

The HV features are read twice - once in the decompressor stub and again
in kernel proper - but I guess that's not such a big deal.

Also, sev_hv_features can be static.

Diff ontop:

---
diff --git a/arch/x86/include/asm/mem_encrypt.h b/arch/x86/include/asm/mem_encrypt.h
index fb857f2e72cb..df14291d65de 100644
--- a/arch/x86/include/asm/mem_encrypt.h
+++ b/arch/x86/include/asm/mem_encrypt.h
@@ -26,7 +26,6 @@ enum sev_feature_type {
 
 extern u64 sme_me_mask;
 extern u64 sev_status;
-extern u64 sev_hv_features;
 
 void sme_encrypt_execute(unsigned long encrypted_kernel_vaddr,
 			 unsigned long decrypted_kernel_vaddr,
@@ -67,7 +66,6 @@ bool sev_feature_enabled(unsigned int feature_type);
 #else	/* !CONFIG_AMD_MEM_ENCRYPT */
 
 #define sme_me_mask	0ULL
-#define sev_hv_features	0ULL
 
 static inline void __init sme_early_encrypt(resource_size_t paddr,
 					    unsigned long size) { }
diff --git a/arch/x86/kernel/sev-shared.c b/arch/x86/kernel/sev-shared.c
index 8bd67087d79e..d657c2c5a1ee 100644
--- a/arch/x86/kernel/sev-shared.c
+++ b/arch/x86/kernel/sev-shared.c
@@ -24,7 +24,7 @@
 static u16 __ro_after_init ghcb_version;
 
 /* Bitmap of SEV features supported by the hypervisor */
-u64 __ro_after_init sev_hv_features = 0;
+static u64 __ro_after_init sev_hv_features;
 
 static bool __init sev_es_check_cpu_features(void)
 {
@@ -51,10 +51,18 @@ static void __noreturn sev_es_terminate(unsigned int set, unsigned int reason)
 		asm volatile("hlt\n" : : : "memory");
 }
 
+/*
+ * The hypervisor features are available from GHCB version 2 onward.
+ */
 static bool get_hv_features(void)
 {
 	u64 val;
 
+	sev_hv_features = 0;
+
+	if (ghcb_version < 2)
+		return false;
+
 	sev_es_wr_ghcb_msr(GHCB_MSR_HV_FT_REQ);
 	VMGEXIT();
 
@@ -85,8 +93,7 @@ static bool sev_es_negotiate_protocol(void)
 
 	ghcb_version = min_t(size_t, GHCB_MSR_PROTO_MAX(val), GHCB_PROTOCOL_MAX);
 
-	/* The hypervisor features are available from version 2 onward. */
-	if (ghcb_version >= 2 && !get_hv_features())
+	if (!get_hv_features())
 		return false;
 
 	return true;


-- 
Regards/Gruss,
    Boris.

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