[PATCH v6 3/6] x86/sev: Add support to perform RMP optimizations asynchronously

[Ashish Kalra]

From: Ashish Kalra <ashish.kalra@xxxxxxx>

When SEV-SNP is enabled, all writes to memory are checked to ensure
integrity of SNP guest memory. This imposes performance overhead on the
whole system.

RMPOPT is a new instruction that minimizes the performance overhead of
RMP checks on the hypervisor and on non-SNP guests by allowing RMP
checks to be skipped for 1GB regions of memory that are known not to
contain any SEV-SNP guest memory.

Add support for performing RMP optimizations asynchronously using a
dedicated workqueue.

Enable RMPOPT optimizations for up to 2TB of system RAM starting from
the lowest physical memory address aligned down to a 1GB boundary at
RMP initialization time. RMP checks can initially be skipped for 1GB
memory ranges that do not contain SEV-SNP guest memory (excluding
preassigned pages such as the RMP table and firmware pages). As SNP
guests are launched, RMPUPDATE will disable the corresponding RMPOPT
optimizations.

Suggested-by: Thomas Lendacky <thomas.lendacky@xxxxxxx>
Suggested-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Reviewed-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
Signed-off-by: Ashish Kalra <ashish.kalra@xxxxxxx>
---
 arch/x86/virt/svm/sev.c | 196 +++++++++++++++++++++++++++++++++++++++-
 1 file changed, 193 insertions(+), 3 deletions(-)

diff --git a/arch/x86/virt/svm/sev.c b/arch/x86/virt/svm/sev.c
index 089c9a14edc7..d7e40a5fe5ca 100644
--- a/arch/x86/virt/svm/sev.c
+++ b/arch/x86/virt/svm/sev.c
@@ -19,6 +19,7 @@
 #include <linux/iommu.h>
 #include <linux/amd-iommu.h>
 #include <linux/nospec.h>
+#include <linux/workqueue.h>
 
 #include <asm/sev.h>
 #include <asm/processor.h>
@@ -125,7 +126,18 @@ static void *rmp_bookkeeping __ro_after_init;
 static u64 probed_rmp_base, probed_rmp_size;
 
 static cpumask_t rmpopt_cpumask;
-static phys_addr_t rmpopt_pa_start;
+static phys_addr_t rmpopt_pa_start, rmpopt_pa_end;
+
+enum rmpopt_function {
+	RMPOPT_FUNC_VERIFY_AND_REPORT_STATUS,
+	RMPOPT_FUNC_REPORT_STATUS
+};
+
+#define RMPOPT_WORK_TIMEOUT	10000
+
+static struct workqueue_struct *rmpopt_wq;
+static struct delayed_work rmpopt_delayed_work;
+static DEFINE_MUTEX(rmpopt_wq_mutex);
 
 static LIST_HEAD(snp_leaked_pages_list);
 static DEFINE_SPINLOCK(snp_leaked_pages_list_lock);
@@ -566,6 +578,14 @@ static void rmpopt_cleanup(void)
 {
 	int cpu;
 
+	guard(mutex)(&rmpopt_wq_mutex);
+
+	if (!rmpopt_wq)
+		return;
+
+	cancel_delayed_work_sync(&rmpopt_delayed_work);
+	destroy_workqueue(rmpopt_wq);
+
 	cpus_read_lock();
 
 	for_each_cpu(cpu, &rmpopt_cpumask)
@@ -574,7 +594,8 @@ static void rmpopt_cleanup(void)
 	cpus_read_unlock();
 
 	cpumask_clear(&rmpopt_cpumask);
-	rmpopt_pa_start = 0;
+	rmpopt_pa_start = rmpopt_pa_end = 0;
+	rmpopt_wq = NULL;
 }
 
 void snp_shutdown(void)
@@ -592,6 +613,134 @@ void snp_shutdown(void)
 }
 EXPORT_SYMBOL_FOR_MODULES(snp_shutdown, "ccp");
 
+static inline bool __rmpopt(u64 pa_start, u64 op_type)
+{
+	bool optimized;
+
+	asm volatile(".byte 0xf2, 0x0f, 0x01, 0xfc"
+		     : "=@ccc" (optimized)
+		     : "a" (pa_start), "c" (op_type)
+		     : "memory", "cc");
+
+	return optimized;
+}
+
+static void rmpopt(u64 pa)
+{
+	u64 pa_start = ALIGN_DOWN(pa, SZ_1G);
+	u64 op_type = RMPOPT_FUNC_VERIFY_AND_REPORT_STATUS;
+
+	__rmpopt(pa_start, op_type);
+}
+
+/*
+ * 'val' is a system physical address.
+ */
+static void rmpopt_smp(void *val)
+{
+	rmpopt((u64)val);
+}
+
+/*
+ * RMPOPT optimizations skip RMP checks at 1GB granularity if this
+ * range of memory does not contain any SNP guest memory.
+ */
+static void rmpopt_work_handler(struct work_struct *work)
+{
+	cpumask_var_t follower_mask;
+	phys_addr_t pa;
+	int this_cpu;
+
+	pr_info("Attempt RMP optimizations on physical address range @1GB alignment [0x%016llx - 0x%016llx]\n",
+		rmpopt_pa_start, rmpopt_pa_end);
+
+	if (!alloc_cpumask_var(&follower_mask, GFP_KERNEL))
+		return;
+
+	/*
+	 * RMPOPT scans the RMP table, stores the result of the scan in the
+	 * reserved processor memory. The RMP scan is the most expensive
+	 * part. If a second RMPOPT occurs, it can skip the expensive scan
+	 * if they can see a cached result in the reserved processor memory.
+	 *
+	 * Do RMPOPT on one CPU alone. Then, follow that up with RMPOPT
+	 * on every other primary thread. Followers are "designed to"
+	 * skip the scan if they see the "cached" scan results.
+	 */
+	cpumask_copy(follower_mask, &rmpopt_cpumask);
+
+	/*
+	 * Pin the worker to the current CPU for the leader loop so that
+	 * this_cpu remains valid and the RMPOPT instruction executes on
+	 * the correct CPU.
+	 *
+	 * Use migrate_disable() rather than get_cpu() to prevent
+	 * migration while still allowing preemption.
+	 */
+	migrate_disable();
+	this_cpu = smp_processor_id();
+
+	if (cpumask_test_cpu(this_cpu, follower_mask)) {
+		/*
+		 * Current CPU is a primary thread in rmpopt_cpumask.
+		 * Run leader locally and remove from follower mask.
+		 */
+		cpumask_clear_cpu(this_cpu, follower_mask);
+
+		for (pa = rmpopt_pa_start; pa < rmpopt_pa_end; pa += SZ_1G)
+			rmpopt(pa);
+	} else if (cpumask_intersects(topology_sibling_cpumask(this_cpu),
+				      follower_mask)) {
+		/*
+		 * Current CPU is a sibling thread whose primary is in
+		 * rmpopt_cpumask.  RMPOPT_BASE MSR is per-core, so it
+		 * is safe to run the leader locally.  Remove the sibling's
+		 * primary from the follower mask as this core is already
+		 * covered by the leader.
+		 */
+		cpumask_andnot(follower_mask, follower_mask,
+			       topology_sibling_cpumask(this_cpu));
+
+		for (pa = rmpopt_pa_start; pa < rmpopt_pa_end; pa += SZ_1G)
+			rmpopt(pa);
+	} else {
+		/*
+		 * Current CPU does not have RMPOPT_BASE MSR programmed.
+		 * Pick an explicit leader from the cpumask to avoid #UD.
+		 */
+		int leader_cpu = cpumask_first(follower_mask);
+
+		if (WARN_ON_ONCE(leader_cpu >= nr_cpu_ids)) {
+			migrate_enable();
+			goto out;
+		}
+
+		cpumask_clear_cpu(leader_cpu, follower_mask);
+
+		cpus_read_lock();
+		for (pa = rmpopt_pa_start; pa < rmpopt_pa_end; pa += SZ_1G)
+			smp_call_function_single(leader_cpu, rmpopt_smp,
+						 (void *)pa, true);
+		cpus_read_unlock();
+	}
+
+	migrate_enable();
+
+	/* Followers: run RMPOPT on remaining cores */
+	cpus_read_lock();
+	for (pa = rmpopt_pa_start; pa < rmpopt_pa_end; pa += SZ_1G) {
+		on_each_cpu_mask(follower_mask, rmpopt_smp,
+				 (void *)pa, true);
+
+		 /* Give a chance for other threads to run */
+		cond_resched();
+	}
+	cpus_read_unlock();
+
+out:
+	free_cpumask_var(follower_mask);
+}
+
 void snp_setup_rmpopt(void)
 {
 	u64 rmpopt_base;
@@ -600,11 +749,35 @@ void snp_setup_rmpopt(void)
 	if (!cpu_feature_enabled(X86_FEATURE_RMPOPT))
 		return;
 
+	guard(mutex)(&rmpopt_wq_mutex);
+
+	/*
+	 * Guard against re-initialization.  When SNP_SHUTDOWN_EX is issued
+	 * with x86_snp_shutdown=0, snp_shutdown() is not called and
+	 * rmpopt_cleanup() is skipped, but snp_initialized is still cleared.
+	 * A subsequent __sev_snp_init_locked() would call snp_setup_rmpopt()
+	 * again, leaking the existing workqueue, delayed work, debugfs
+	 * entries, and cpumask state.
+	 */
+	if (rmpopt_wq)
+		return;
+
+	/*
+	 * Create an RMPOPT-specific workqueue to avoid scheduling
+	 * RMPOPT workitem on the global system workqueue.
+	 */
+	rmpopt_wq = alloc_workqueue("rmpopt_wq", WQ_UNBOUND, 1);
+	if (!rmpopt_wq) {
+		pr_err("Failed to allocate RMPOPT workqueue\n");
+		return;
+	}
+
 	cpus_read_lock();
 
 	/*
 	 * The RMPOPT_BASE MSR is per-core, so only one thread per core needs
-	 * to set up the RMPOPT_BASE MSR.
+	 * to set up the RMPOPT_BASE MSR. Likewise, only one thread per core
+	 * needs to issue the RMPOPT instruction.
 	 *
 	 * Note: only online primary threads are included.  If a core's
 	 * primary thread is offline, that core is not covered.  CPU hotplug
@@ -628,6 +801,23 @@ void snp_setup_rmpopt(void)
 		wrmsrq_on_cpu(cpu, MSR_AMD64_RMPOPT_BASE, rmpopt_base);
 
 	cpus_read_unlock();
+
+	INIT_DELAYED_WORK(&rmpopt_delayed_work, rmpopt_work_handler);
+
+	rmpopt_pa_end = ALIGN(PFN_PHYS(max_pfn), SZ_1G);
+
+	/* Limit memory scanning to 2TB of RAM */
+	if ((rmpopt_pa_end - rmpopt_pa_start) > SZ_2T) {
+		pr_info("RMPOPT coverage limited to 2TB; memory above 0x%llx not optimized\n",
+			rmpopt_pa_start + SZ_2T);
+		rmpopt_pa_end = rmpopt_pa_start + SZ_2T;
+	}
+
+	/*
+	 * Once all per-CPU RMPOPT tables have been configured, enable RMPOPT
+	 * optimizations on all physical memory.
+	 */
+	queue_delayed_work(rmpopt_wq, &rmpopt_delayed_work, 0);
 }
 EXPORT_SYMBOL_FOR_MODULES(snp_setup_rmpopt, "ccp");
 
-- 
2.43.0