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, 210 insertions, 0 deletions

diff --git a/include/asm-generic/div64.h b/include/asm-generic/div64.h
new file mode 100644
index 000000000..25e7b4b58
--- /dev/null
+++ b/include/asm-generic/div64.h
@@ -0,0 +1,210 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_GENERIC_DIV64_H
+#define _ASM_GENERIC_DIV64_H
+/*
+ * Copyright (C) 2003 Bernardo Innocenti <bernie@develer.com>
+ * Based on former asm-ppc/div64.h and asm-m68knommu/div64.h
+ *
+ * Optimization for constant divisors on 32-bit machines:
+ * Copyright (C) 2006-2015 Nicolas Pitre
+ *
+ * The semantics of do_div() is, in C++ notation, observing that the name
+ * is a function-like macro and the n parameter has the semantics of a C++
+ * reference:
+ *
+ * uint32_t do_div(uint64_t &n, uint32_t base)
+ * {
+ * 	uint32_t remainder = n % base;
+ * 	n = n / base;
+ * 	return remainder;
+ * }
+ *
+ * NOTE: macro parameter n is evaluated multiple times,
+ *       beware of side effects!
+ */
+
+#include <linux/types.h>
+#include <linux/compiler.h>
+
+#if BITS_PER_LONG == 64
+
+/**
+ * do_div - returns 2 values: calculate remainder and update new dividend
+ * @n: uint64_t dividend (will be updated)
+ * @base: uint32_t divisor
+ *
+ * Summary:
+ * ``uint32_t remainder = n % base;``
+ * ``n = n / base;``
+ *
+ * Return: (uint32_t)remainder
+ *
+ * NOTE: macro parameter @n is evaluated multiple times,
+ * beware of side effects!
+ */
+# define do_div(n,base) ({					\
+	uint32_t __base = (base);				\
+	uint32_t __rem;						\
+	__rem = ((uint64_t)(n)) % __base;			\
+	(n) = ((uint64_t)(n)) / __base;				\
+	__rem;							\
+ })
+
+#elif BITS_PER_LONG == 32
+
+#include <linux/log2.h>
+
+/*
+ * If the divisor happens to be constant, we determine the appropriate
+ * inverse at compile time to turn the division into a few inline
+ * multiplications which ought to be much faster.
+ *
+ * (It is unfortunate that gcc doesn't perform all this internally.)
+ */
+
+#define __div64_const32(n, ___b)					\
+({									\
+	/*								\
+	 * Multiplication by reciprocal of b: n / b = n * (p / b) / p	\
+	 *								\
+	 * We rely on the fact that most of this code gets optimized	\
+	 * away at compile time due to constant propagation and only	\
+	 * a few multiplication instructions should remain.		\
+	 * Hence this monstrous macro (static inline doesn't always	\
+	 * do the trick here).						\
+	 */								\
+	uint64_t ___res, ___x, ___t, ___m, ___n = (n);			\
+	uint32_t ___p;							\
+	bool ___bias = false;						\
+									\
+	/* determine MSB of b */					\
+	___p = 1 << ilog2(___b);					\
+									\
+	/* compute m = ((p << 64) + b - 1) / b */			\
+	___m = (~0ULL / ___b) * ___p;					\
+	___m += (((~0ULL % ___b + 1) * ___p) + ___b - 1) / ___b;	\
+									\
+	/* one less than the dividend with highest result */		\
+	___x = ~0ULL / ___b * ___b - 1;					\
+									\
+	/* test our ___m with res = m * x / (p << 64) */		\
+	___res = (___m & 0xffffffff) * (___x & 0xffffffff);		\
+	___t = (___m & 0xffffffff) * (___x >> 32) + (___res >> 32);	\
+	___res = (___m >> 32) * (___x >> 32) + (___t >> 32);		\
+	___t = (___m >> 32) * (___x & 0xffffffff) + (___t & 0xffffffff);\
+	___res = (___res + (___t >> 32)) / ___p;			\
+									\
+	/* Now validate what we've got. */				\
+	if (___res != ___x / ___b) {					\
+		/*							\
+		 * We can't get away without a bias to compensate	\
+		 * for bit truncation errors.  To avoid it we'd need an	\
+		 * additional bit to represent m which would overflow	\
+		 * a 64-bit variable.					\
+		 *							\
+		 * Instead we do m = p / b and n / b = (n * m + m) / p.	\
+		 */							\
+		___bias = true;						\
+		/* Compute m = (p << 64) / b */				\
+		___m = (~0ULL / ___b) * ___p;				\
+		___m += ((~0ULL % ___b + 1) * ___p) / ___b;		\
+	}								\
+									\
+	/* Reduce m / p to help avoid overflow handling later. */	\
+	___p /= (___m & -___m);						\
+	___m /= (___m & -___m);						\
+									\
+	/*								\
+	 * Perform (m_bias + m * n) / (1 << 64).			\
+	 * From now on there will be actual runtime code generated.	\
+	 */								\
+	___res = __arch_xprod_64(___m, ___n, ___bias);			\
+									\
+	___res /= ___p;							\
+})
+
+#ifndef __arch_xprod_64
+/*
+ * Default C implementation for __arch_xprod_64()
+ *
+ * Prototype: uint64_t __arch_xprod_64(const uint64_t m, uint64_t n, bool bias)
+ * Semantic:  retval = ((bias ? m : 0) + m * n) >> 64
+ *
+ * The product is a 128-bit value, scaled down to 64 bits.
+ * Hoping for compile-time optimization of  conditional code.
+ * Architectures may provide their own optimized assembly implementation.
+ */
+#ifdef CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE
+static __always_inline
+#else
+static inline
+#endif
+uint64_t __arch_xprod_64(const uint64_t m, uint64_t n, bool bias)
+{
+	uint32_t m_lo = m;
+	uint32_t m_hi = m >> 32;
+	uint32_t n_lo = n;
+	uint32_t n_hi = n >> 32;
+	uint64_t x, y;
+
+	/* Determine if overflow handling can be dispensed with. */
+	bool no_ovf = __builtin_constant_p(m) &&
+		      ((m >> 32) + (m & 0xffffffff) < 0x100000000);
+
+	if (no_ovf) {
+		x = (uint64_t)m_lo * n_lo + (bias ? m : 0);
+		x >>= 32;
+		x += (uint64_t)m_lo * n_hi;
+		x += (uint64_t)m_hi * n_lo;
+		x >>= 32;
+		x += (uint64_t)m_hi * n_hi;
+	} else {
+		x = (uint64_t)m_lo * n_lo + (bias ? m_lo : 0);
+		y = (uint64_t)m_lo * n_hi + (uint32_t)(x >> 32) + (bias ? m_hi : 0);
+		x = (uint64_t)m_hi * n_hi + (uint32_t)(y >> 32);
+		y = (uint64_t)m_hi * n_lo + (uint32_t)y;
+		x += (uint32_t)(y >> 32);
+	}
+
+	return x;
+}
+#endif
+
+#ifndef __div64_32
+extern uint32_t __div64_32(uint64_t *dividend, uint32_t divisor);
+#endif
+
+/* The unnecessary pointer compare is there
+ * to check for type safety (n must be 64bit)
+ */
+# define do_div(n,base) ({				\
+	uint32_t __base = (base);			\
+	uint32_t __rem;					\
+	(void)(((typeof((n)) *)0) == ((uint64_t *)0));	\
+	if (__builtin_constant_p(__base) &&		\
+	    is_power_of_2(__base)) {			\
+		__rem = (n) & (__base - 1);		\
+		(n) >>= ilog2(__base);			\
+	} else if (__builtin_constant_p(__base) &&	\
+		   __base != 0) {			\
+		uint32_t __res_lo, __n_lo = (n);	\
+		(n) = __div64_const32(n, __base);	\
+		/* the remainder can be computed with 32-bit regs */ \
+		__res_lo = (n);				\
+		__rem = __n_lo - __res_lo * __base;	\
+	} else if (likely(((n) >> 32) == 0)) {		\
+		__rem = (uint32_t)(n) % __base;		\
+		(n) = (uint32_t)(n) / __base;		\
+	} else {					\
+		__rem = __div64_32(&(n), __base);	\
+	}						\
+	__rem;						\
+ })
+
+#else /* BITS_PER_LONG == ?? */
+
+# error do_div() does not yet support the C64
+
+#endif /* BITS_PER_LONG */
+
+#endif /* _ASM_GENERIC_DIV64_H */