KVM: SEV: Do cache maintenance on the source VM during intra-host migrationgrafted

Manually perform cache maintenance on the source VM during intra-host
migration to ensure no stale data is left in CPU caches after the VM is
destroyed.  Because the source VM is "converted" to a non-SEV VM, KVM's
memory reclaim flows won't trigger cache maintenance, e.g. when all guest
memory is reclaimed in response to detaching from the mmu_notifier.

Note, relying on the destination VM to do cache maintenance isn't an option
as KVM doesn't require identical guest memory configurations, i.e. the
source VM may have access to memory that the destination VM does not.
Enforcing equivalent memory configurations is infeasible, as it would
require a *deep* comparison of memslots, e.g. to verify that not only are
the memslot identical, but what the memslots point at is also identical.

Fixes: b56639318bb2 ("KVM: SEV: Add support for SEV intra host migration")
Cc: stable@vger.kernel.org
Reported-by: Stefan Teodorescu <fane@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20260923163721.1584779-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 338 insertions, 0 deletions

diff --git a/arch/riscv/mm/context.c b/arch/riscv/mm/context.c
new file mode 100644
index 000000000..55c20ad1f
--- /dev/null
+++ b/arch/riscv/mm/context.c
@@ -0,0 +1,338 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2012 Regents of the University of California
+ * Copyright (C) 2017 SiFive
+ * Copyright (C) 2021 Western Digital Corporation or its affiliates.
+ */
+
+#include <linux/bitops.h>
+#include <linux/cpumask.h>
+#include <linux/mm.h>
+#include <linux/percpu.h>
+#include <linux/slab.h>
+#include <linux/spinlock.h>
+#include <linux/static_key.h>
+#include <asm/tlbflush.h>
+#include <asm/cacheflush.h>
+#include <asm/mmu_context.h>
+#include <asm/switch_to.h>
+
+#ifdef CONFIG_MMU
+
+DEFINE_STATIC_KEY_FALSE(use_asid_allocator);
+
+static unsigned long num_asids;
+
+static atomic_long_t current_version;
+
+static DEFINE_RAW_SPINLOCK(context_lock);
+static cpumask_t context_tlb_flush_pending;
+static unsigned long *context_asid_map;
+
+static DEFINE_PER_CPU(atomic_long_t, active_context);
+static DEFINE_PER_CPU(unsigned long, reserved_context);
+
+static bool check_update_reserved_context(unsigned long cntx,
+					  unsigned long newcntx)
+{
+	int cpu;
+	bool hit = false;
+
+	/*
+	 * Iterate over the set of reserved CONTEXT looking for a match.
+	 * If we find one, then we can update our mm to use new CONTEXT
+	 * (i.e. the same CONTEXT in the current_version) but we can't
+	 * exit the loop early, since we need to ensure that all copies
+	 * of the old CONTEXT are updated to reflect the mm. Failure to do
+	 * so could result in us missing the reserved CONTEXT in a future
+	 * version.
+	 */
+	for_each_possible_cpu(cpu) {
+		if (per_cpu(reserved_context, cpu) == cntx) {
+			hit = true;
+			per_cpu(reserved_context, cpu) = newcntx;
+		}
+	}
+
+	return hit;
+}
+
+static void __flush_context(void)
+{
+	int i;
+	unsigned long cntx;
+
+	/* Must be called with context_lock held */
+	lockdep_assert_held(&context_lock);
+
+	/* Update the list of reserved ASIDs and the ASID bitmap. */
+	bitmap_zero(context_asid_map, num_asids);
+
+	/* Mark already active ASIDs as used */
+	for_each_possible_cpu(i) {
+		cntx = atomic_long_xchg_relaxed(&per_cpu(active_context, i), 0);
+		/*
+		 * If this CPU has already been through a rollover, but
+		 * hasn't run another task in the meantime, we must preserve
+		 * its reserved CONTEXT, as this is the only trace we have of
+		 * the process it is still running.
+		 */
+		if (cntx == 0)
+			cntx = per_cpu(reserved_context, i);
+
+		__set_bit(cntx2asid(cntx), context_asid_map);
+		per_cpu(reserved_context, i) = cntx;
+	}
+
+	/* Mark ASID #0 as used because it is used at boot-time */
+	__set_bit(0, context_asid_map);
+
+	/* Queue a TLB invalidation for each CPU on next context-switch */
+	cpumask_setall(&context_tlb_flush_pending);
+}
+
+static unsigned long __new_context(struct mm_struct *mm)
+{
+	static u32 cur_idx = 1;
+	unsigned long cntx = atomic_long_read(&mm->context.id);
+	unsigned long asid, ver = atomic_long_read(&current_version);
+
+	/* Must be called with context_lock held */
+	lockdep_assert_held(&context_lock);
+
+	if (cntx != 0) {
+		unsigned long newcntx = ver | cntx2asid(cntx);
+
+		/*
+		 * If our current CONTEXT was active during a rollover, we
+		 * can continue to use it and this was just a false alarm.
+		 */
+		if (check_update_reserved_context(cntx, newcntx))
+			return newcntx;
+
+		/*
+		 * We had a valid CONTEXT in a previous life, so try to
+		 * re-use it if possible.
+		 */
+		if (!__test_and_set_bit(cntx2asid(cntx), context_asid_map))
+			return newcntx;
+	}
+
+	/*
+	 * Allocate a free ASID. If we can't find one then increment
+	 * current_version and flush all ASIDs.
+	 */
+	asid = find_next_zero_bit(context_asid_map, num_asids, cur_idx);
+	if (asid != num_asids)
+		goto set_asid;
+
+	/* We're out of ASIDs, so increment current_version */
+	ver = atomic_long_add_return_relaxed(BIT(SATP_ASID_BITS), &current_version);
+
+	/* Flush everything  */
+	__flush_context();
+
+	/* We have more ASIDs than CPUs, so this will always succeed */
+	asid = find_next_zero_bit(context_asid_map, num_asids, 1);
+
+set_asid:
+	__set_bit(asid, context_asid_map);
+	cur_idx = asid;
+	return asid | ver;
+}
+
+static void set_mm_asid(struct mm_struct *mm, unsigned int cpu)
+{
+	unsigned long flags;
+	bool need_flush_tlb = false;
+	unsigned long cntx, old_active_cntx;
+
+	cntx = atomic_long_read(&mm->context.id);
+
+	/*
+	 * If our active_context is non-zero and the context matches the
+	 * current_version, then we update the active_context entry with a
+	 * relaxed cmpxchg.
+	 *
+	 * Following is how we handle racing with a concurrent rollover:
+	 *
+	 * - We get a zero back from the cmpxchg and end up waiting on the
+	 *   lock. Taking the lock synchronises with the rollover and so
+	 *   we are forced to see the updated version.
+	 *
+	 * - We get a valid context back from the cmpxchg then we continue
+	 *   using old ASID because __flush_context() would have marked ASID
+	 *   of active_context as used and next context switch we will
+	 *   allocate new context.
+	 */
+	old_active_cntx = atomic_long_read(&per_cpu(active_context, cpu));
+	if (old_active_cntx &&
+	    (cntx2version(cntx) == atomic_long_read(&current_version)) &&
+	    atomic_long_cmpxchg_relaxed(&per_cpu(active_context, cpu),
+					old_active_cntx, cntx))
+		goto switch_mm_fast;
+
+	raw_spin_lock_irqsave(&context_lock, flags);
+
+	/* Check that our ASID belongs to the current_version. */
+	cntx = atomic_long_read(&mm->context.id);
+	if (cntx2version(cntx) != atomic_long_read(&current_version)) {
+		cntx = __new_context(mm);
+		atomic_long_set(&mm->context.id, cntx);
+	}
+
+	if (cpumask_test_and_clear_cpu(cpu, &context_tlb_flush_pending))
+		need_flush_tlb = true;
+
+	atomic_long_set(&per_cpu(active_context, cpu), cntx);
+
+	raw_spin_unlock_irqrestore(&context_lock, flags);
+
+switch_mm_fast:
+	csr_write(CSR_SATP, virt_to_pfn(mm->pgd) |
+		  (cntx2asid(cntx) << SATP_ASID_SHIFT) |
+		  satp_mode);
+
+	if (need_flush_tlb)
+		local_flush_tlb_all();
+}
+
+static void set_mm_noasid(struct mm_struct *mm)
+{
+	/* Switch the page table and blindly nuke entire local TLB */
+	csr_write(CSR_SATP, virt_to_pfn(mm->pgd) | satp_mode);
+	local_flush_tlb_all_asid(0);
+}
+
+static inline void set_mm(struct mm_struct *prev,
+			  struct mm_struct *next, unsigned int cpu)
+{
+	/*
+	 * The mm_cpumask indicates which harts' TLBs contain the virtual
+	 * address mapping of the mm. Compared to noasid, using asid
+	 * can't guarantee that stale TLB entries are invalidated because
+	 * the asid mechanism wouldn't flush TLB for every switch_mm for
+	 * performance. So when using asid, keep all CPUs footmarks in
+	 * cpumask() until mm reset.
+	 */
+	cpumask_set_cpu(cpu, mm_cpumask(next));
+	if (static_branch_unlikely(&use_asid_allocator)) {
+		set_mm_asid(next, cpu);
+	} else {
+		cpumask_clear_cpu(cpu, mm_cpumask(prev));
+		set_mm_noasid(next);
+	}
+}
+
+static int __init asids_init(void)
+{
+	unsigned long asid_bits, old;
+
+	/* Figure-out number of ASID bits in HW */
+	old = csr_read(CSR_SATP);
+	asid_bits = old | (SATP_ASID_MASK << SATP_ASID_SHIFT);
+	csr_write(CSR_SATP, asid_bits);
+	asid_bits = (csr_read(CSR_SATP) >> SATP_ASID_SHIFT)  & SATP_ASID_MASK;
+	asid_bits = fls_long(asid_bits);
+	csr_write(CSR_SATP, old);
+
+	/*
+	 * In the process of determining number of ASID bits (above)
+	 * we polluted the TLB of current HART so let's do TLB flushed
+	 * to remove unwanted TLB enteries.
+	 */
+	local_flush_tlb_all();
+
+	/* Pre-compute ASID details */
+	if (asid_bits) {
+		num_asids = 1 << asid_bits;
+	}
+
+	/*
+	 * Use ASID allocator only if number of HW ASIDs are
+	 * at-least twice more than CPUs
+	 */
+	if (num_asids > (2 * num_possible_cpus())) {
+		atomic_long_set(&current_version, BIT(SATP_ASID_BITS));
+
+		context_asid_map = bitmap_zalloc(num_asids, GFP_KERNEL);
+		if (!context_asid_map)
+			panic("Failed to allocate bitmap for %lu ASIDs\n",
+			      num_asids);
+
+		__set_bit(0, context_asid_map);
+
+		static_branch_enable(&use_asid_allocator);
+
+		pr_info("ASID allocator using %lu bits (%lu entries)\n",
+			asid_bits, num_asids);
+	} else {
+		pr_info("ASID allocator disabled (%lu bits)\n", asid_bits);
+	}
+
+	return 0;
+}
+early_initcall(asids_init);
+#else
+static inline void set_mm(struct mm_struct *prev,
+			  struct mm_struct *next, unsigned int cpu)
+{
+	/* Nothing to do here when there is no MMU */
+}
+#endif
+
+/*
+ * When necessary, performs a deferred icache flush for the given MM context,
+ * on the local CPU.  RISC-V has no direct mechanism for instruction cache
+ * shoot downs, so instead we send an IPI that informs the remote harts they
+ * need to flush their local instruction caches.  To avoid pathologically slow
+ * behavior in a common case (a bunch of single-hart processes on a many-hart
+ * machine, ie 'make -j') we avoid the IPIs for harts that are not currently
+ * executing a MM context and instead schedule a deferred local instruction
+ * cache flush to be performed before execution resumes on each hart.  This
+ * actually performs that local instruction cache flush, which implicitly only
+ * refers to the current hart.
+ *
+ * The "cpu" argument must be the current local CPU number.
+ */
+static inline void flush_icache_deferred(struct mm_struct *mm, unsigned int cpu,
+					 struct task_struct *task)
+{
+#ifdef CONFIG_SMP
+	if (cpumask_test_and_clear_cpu(cpu, &mm->context.icache_stale_mask)) {
+		/*
+		 * Ensure the remote hart's writes are visible to this hart.
+		 * This pairs with a barrier in flush_icache_mm.
+		 */
+		smp_mb();
+
+		/*
+		 * If cache will be flushed in switch_to, no need to flush here.
+		 */
+		if (!(task && switch_to_should_flush_icache(task)))
+			local_flush_icache_all();
+	}
+#endif
+}
+
+void switch_mm(struct mm_struct *prev, struct mm_struct *next,
+	struct task_struct *task)
+{
+	unsigned int cpu;
+
+	if (unlikely(prev == next))
+		return;
+
+	membarrier_arch_switch_mm(prev, next, task);
+
+	/*
+	 * Mark the current MM context as inactive, and the next as
+	 * active.  This is at least used by the icache flushing
+	 * routines in order to determine who should be flushed.
+	 */
+	cpu = smp_processor_id();
+
+	set_mm(prev, next, cpu);
+
+	flush_icache_deferred(next, cpu, task);
+}