[PATCH v2 1/2] x86/sev: Do not initialize SNP if missing CPUs

[Tycho Andersen]

From: "Tycho Andersen (AMD)" <tycho@xxxxxxxxxx>

The SEV firmware checks that the SNP enable bit is set on each CPU during
SNP initialization, and will fail if it is not. If there are some CPUs
offline, they will not run the setup functions, so SNP initialization will
always fail.

Skip the IPIs in this case and return an error so that the CCP driver can
skip the SNP_INIT that will fail. Also print the CPU masks as a breadcrumb
so people can figure out what happened.

Suggested-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
Signed-off-by: Tycho Andersen (AMD) <tycho@xxxxxxxxxx>
---
 arch/x86/include/asm/sev.h |  4 ++--
 arch/x86/virt/svm/sev.c    | 15 +++++++++++++--
 2 files changed, 15 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
index 09e605c85de4..594cfa19cbd4 100644
--- a/arch/x86/include/asm/sev.h
+++ b/arch/x86/include/asm/sev.h
@@ -661,7 +661,7 @@ static inline void snp_leak_pages(u64 pfn, unsigned int pages)
 {
 	__snp_leak_pages(pfn, pages, true);
 }
-void snp_prepare(void);
+int snp_prepare(void);
 void snp_shutdown(void);
 #else
 static inline bool snp_probe_rmptable_info(void) { return false; }
@@ -679,7 +679,7 @@ static inline void __snp_leak_pages(u64 pfn, unsigned int npages, bool dump_rmp)
 static inline void snp_leak_pages(u64 pfn, unsigned int npages) {}
 static inline void kdump_sev_callback(void) { }
 static inline void snp_fixup_e820_tables(void) {}
-static inline void snp_prepare(void) {}
+static inline int snp_prepare(void) { return -ENODEV; }
 static inline void snp_shutdown(void) {}
 #endif
 
diff --git a/arch/x86/virt/svm/sev.c b/arch/x86/virt/svm/sev.c
index 41f76f15caa1..160e60f5f3fb 100644
--- a/arch/x86/virt/svm/sev.c
+++ b/arch/x86/virt/svm/sev.c
@@ -511,8 +511,9 @@ static void clear_hsave_pa(void *arg)
 	wrmsrq(MSR_VM_HSAVE_PA, 0);
 }
 
-void snp_prepare(void)
+int snp_prepare(void)
 {
+	int ret = -EOPNOTSUPP;
 	u64 val;
 
 	/*
@@ -521,12 +522,19 @@ void snp_prepare(void)
 	 */
 	rdmsrq(MSR_AMD64_SYSCFG, val);
 	if (val & MSR_AMD64_SYSCFG_SNP_EN)
-		return;
+		return 0;
 
 	clear_rmp();
 
 	cpus_read_lock();
 
+	if (!cpumask_equal(cpu_online_mask, cpu_present_mask)) {
+		pr_warn("Skipping SNP initialization. CPUs online %*pbl, present %*pbl\n",
+			cpumask_pr_args(cpu_online_mask),
+			cpumask_pr_args(cpu_present_mask));
+		goto unlock;
+	}
+
 	/*
 	 * MtrrFixDramModEn is not shared between threads on a core,
 	 * therefore it must be set on all CPUs prior to enabling SNP.
@@ -537,7 +545,10 @@ void snp_prepare(void)
 	/* SNP_INIT requires MSR_VM_HSAVE_PA to be cleared on all CPUs. */
 	on_each_cpu(clear_hsave_pa, NULL, 1);
 
+	ret = 0;
+unlock:
 	cpus_read_unlock();
+	return ret;
 }
 EXPORT_SYMBOL_FOR_MODULES(snp_prepare, "ccp");
 
-- 
2.53.0