[PATCH v4 4/4] x86/kvm: use __decrypted attribute in shared variables

[Brijesh Singh]

Commit: 368a540e0232 (x86/kvmclock: Remove memblock dependency)
caused SEV guest regression. When SEV is active, we map the shared
variables (wall_clock and hv_clock_boot) with C=0 to ensure that both
the guest and the hypervisor are able to access the data. To map the
variables we use kernel_physical_mapping_init() to split the large pages,
but splitting large pages requires allocating a new PMD, which fails now
that kvmclock initialization is called early during boot.

Recently we added a special .data..decrypted section to hold the shared
variables. This section is mapped with C=0 early during boot. Use
__decrypted attribute to put the wall_clock and hv_clock_boot in
.data..decrypted section so that they are mapped with C=0.

Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
Reviewed-by: Tom Lendacky <thomas.lendacky@xxxxxxx>
Fixes: 368a540e0232 ("x86/kvmclock: Remove memblock dependency")
Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
Cc: kvm@xxxxxxxxxxxxxxx
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Cc: kvm@xxxxxxxxxxxxxxx
Cc: "Radim KrÄmÃÅ" <rkrcmar@xxxxxxxxxx>
---
 arch/x86/kernel/kvmclock.c | 30 +++++++++++++++++++++++++-----
 1 file changed, 25 insertions(+), 5 deletions(-)

diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
index 1e67646..08f5f8a 100644
--- a/arch/x86/kernel/kvmclock.c
+++ b/arch/x86/kernel/kvmclock.c
@@ -28,6 +28,7 @@
 #include <linux/sched/clock.h>
 #include <linux/mm.h>
 #include <linux/slab.h>
+#include <linux/set_memory.h>
 
 #include <asm/hypervisor.h>
 #include <asm/mem_encrypt.h>
@@ -61,8 +62,8 @@ early_param("no-kvmclock-vsyscall", parse_no_kvmclock_vsyscall);
 	(PAGE_SIZE / sizeof(struct pvclock_vsyscall_time_info))
 
 static struct pvclock_vsyscall_time_info
-			hv_clock_boot[HVC_BOOT_ARRAY_SIZE] __aligned(PAGE_SIZE);
-static struct pvclock_wall_clock wall_clock;
+			hv_clock_boot[HVC_BOOT_ARRAY_SIZE] __decrypted __aligned(PAGE_SIZE);
+static struct pvclock_wall_clock wall_clock __decrypted;
 static DEFINE_PER_CPU(struct pvclock_vsyscall_time_info *, hv_clock_per_cpu);
 
 static inline struct pvclock_vcpu_time_info *this_cpu_pvti(void)
@@ -267,10 +268,29 @@ static int kvmclock_setup_percpu(unsigned int cpu)
 		return 0;
 
 	/* Use the static page for the first CPUs, allocate otherwise */
-	if (cpu < HVC_BOOT_ARRAY_SIZE)
+	if (cpu < HVC_BOOT_ARRAY_SIZE) {
 		p = &hv_clock_boot[cpu];
-	else
-		p = kzalloc(sizeof(*p), GFP_KERNEL);
+	} else {
+		int rc;
+		unsigned int sz = sizeof(*p);
+
+		if (sev_active())
+			sz = PAGE_ALIGN(sz);
+
+		p = kzalloc(sz, GFP_KERNEL);
+
+		/*
+		 * The physical address of per-cpu variable will be shared with
+		 * the hypervisor. Let's clear the C-bit before we assign the
+		 * memory to per_cpu variable.
+		 */
+		if (p && sev_active()) {
+			rc = set_memory_decrypted((unsigned long)p, sz >> PAGE_SHIFT);
+			if (rc)
+				return rc;
+			memset(p, 0, sz);
+		}
+	}
 
 	per_cpu(hv_clock_per_cpu, cpu) = p;
 	return p ? 0 : -ENOMEM;
-- 
2.7.4