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, 86 insertions, 0 deletions

diff --git a/arch/alpha/lib/strncat.S b/arch/alpha/lib/strncat.S
new file mode 100644
index 000000000..a913a7c84
--- /dev/null
+++ b/arch/alpha/lib/strncat.S
@@ -0,0 +1,86 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * arch/alpha/lib/strncat.S
+ * Contributed by Richard Henderson (rth@tamu.edu)
+ *
+ * Append no more than COUNT characters from the null-terminated string SRC
+ * to the null-terminated string DST.  Always null-terminate the new DST.
+ *
+ * This differs slightly from the semantics in libc in that we never write
+ * past count, whereas libc may write to count+1.  This follows the generic
+ * implementation in lib/string.c and is, IMHO, more sensible.
+ */
+#include <linux/export.h>
+	.text
+
+	.align 3
+	.globl strncat
+	.ent strncat
+strncat:
+	.frame $30, 0, $26
+	.prologue 0
+
+	mov	$16, $0		# set up return value
+	beq	$18, $zerocount
+
+	/* Find the end of the string.  */
+
+	ldq_u   $1, 0($16)	# load first quadword ($16 may be misaligned)
+	lda     $2, -1($31)
+	insqh   $2, $16, $2
+	andnot  $16, 7, $16
+	or      $2, $1, $1
+	cmpbge  $31, $1, $2	# bits set iff byte == 0
+	bne     $2, $found
+
+$loop:	ldq     $1, 8($16)
+	addq    $16, 8, $16
+	cmpbge  $31, $1, $2
+	beq     $2, $loop
+
+$found:	negq    $2, $3		# clear all but least set bit
+	and     $2, $3, $2
+
+	and     $2, 0xf0, $3	# binary search for that set bit
+	and	$2, 0xcc, $4
+	and	$2, 0xaa, $5
+	cmovne	$3, 4, $3
+	cmovne	$4, 2, $4
+	cmovne	$5, 1, $5
+	addq	$3, $4, $3
+	addq	$16, $5, $16
+	addq	$16, $3, $16
+
+	/* Now do the append.  */
+
+	bsr	$23, __stxncpy
+
+	/* Worry about the null termination.  */
+
+	zapnot	$1, $27, $2	# was last byte a null?
+	bne	$2, 0f
+	ret
+
+0:	cmplt	$27, $24, $2	# did we fill the buffer completely?
+	or	$2, $18, $2
+	bne	$2, 2f
+
+	and	$24, 0x80, $2	# no zero next byte
+	bne	$2, 1f
+
+	/* Here there are bytes left in the current word.  Clear one.  */
+	addq	$24, $24, $24	# end-of-count bit <<= 1
+2:	zap	$1, $24, $1
+	stq_u	$1, 0($16)
+	ret
+
+1:	/* Here we must read the next DST word and clear the first byte.  */
+	ldq_u	$1, 8($16)
+	zap	$1, 1, $1
+	stq_u	$1, 8($16)
+
+$zerocount:
+	ret
+
+	.end strncat
+	EXPORT_SYMBOL(strncat)