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, 165 insertions, 0 deletions

diff --git a/arch/alpha/lib/memchr.S b/arch/alpha/lib/memchr.S
new file mode 100644
index 000000000..45366e32f
--- /dev/null
+++ b/arch/alpha/lib/memchr.S
@@ -0,0 +1,165 @@
+/* Copyright (C) 1996 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+   Contributed by David Mosberger (davidm@cs.arizona.edu).
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Library General Public License as
+   published by the Free Software Foundation; either version 2 of the
+   License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Library General Public License for more details.
+
+   You should have received a copy of the GNU Library General Public
+   License along with the GNU C Library; see the file COPYING.LIB.  If not,
+   write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+   Boston, MA 02111-1307, USA.  */
+
+/* Finds characters in a memory area.  Optimized for the Alpha:
+
+      - memory accessed as aligned quadwords only
+      - uses cmpbge to compare 8 bytes in parallel
+      - does binary search to find 0 byte in last
+        quadword (HAKMEM needed 12 instructions to
+        do this instead of the 9 instructions that
+        binary search needs).
+
+For correctness consider that:
+
+      - only minimum number of quadwords may be accessed
+      - the third argument is an unsigned long
+*/
+#include <linux/export.h>
+        .set noreorder
+        .set noat
+
+	.globl memchr
+	.ent memchr
+memchr:
+	.frame $30,0,$26,0
+	.prologue 0
+
+	# Hack -- if someone passes in (size_t)-1, hoping to just
+	# search til the end of the address space, we will overflow
+	# below when we find the address of the last byte.  Given
+	# that we will never have a 56-bit address space, cropping
+	# the length is the easiest way to avoid trouble.
+	zap	$18, 0x80, $5	#-e0	:
+
+	beq	$18, $not_found	# .. e1 :
+        ldq_u   $1, 0($16)	# e1	: load first quadword
+	insbl	$17, 1, $2	# .. e0 : $2 = 000000000000ch00
+	and	$17, 0xff, $17	#-e0    : $17 = 00000000000000ch
+	cmpult	$18, 9, $4	# .. e1 :
+	or	$2, $17, $17	# e0    : $17 = 000000000000chch
+        lda     $3, -1($31)	# .. e1 :
+	sll	$17, 16, $2	#-e0    : $2 = 00000000chch0000
+	addq	$16, $5, $5	# .. e1 :
+	or	$2, $17, $17	# e1    : $17 = 00000000chchchch
+	unop			#	:
+	sll	$17, 32, $2	#-e0    : $2 = chchchch00000000
+	or	$2, $17, $17	# e1	: $17 = chchchchchchchch
+	extql	$1, $16, $7	# e0    : 
+	beq	$4, $first_quad	# .. e1 :
+
+	ldq_u	$6, -1($5)	#-e1	: eight or less bytes to search
+	extqh	$6, $16, $6	# .. e0 :
+	mov	$16, $0		# e0	:
+	or	$7, $6, $1	# .. e1 : $1 = quadword starting at $16
+
+	# Deal with the case where at most 8 bytes remain to be searched
+	# in $1.  E.g.:
+	#	$18 = 6
+	#	$1 = ????c6c5c4c3c2c1
+$last_quad:
+	negq	$18, $6		#-e0	:
+        xor	$17, $1, $1	# .. e1 :
+	srl	$3, $6, $6	# e0    : $6 = mask of $18 bits set
+        cmpbge  $31, $1, $2	# .. e1 :
+	and	$2, $6, $2	#-e0	:
+        beq     $2, $not_found	# .. e1 :
+
+$found_it:
+	# Now, determine which byte matched:
+        negq    $2, $3		# e0	:
+        and     $2, $3, $2	# e1	:
+
+        and     $2, 0x0f, $1	#-e0	:
+        addq    $0, 4, $3	# .. e1 :
+        cmoveq  $1, $3, $0	# e0	:
+
+        addq    $0, 2, $3	# .. e1 :
+        and     $2, 0x33, $1	#-e0	:
+        cmoveq  $1, $3, $0	# .. e1 :
+
+        and     $2, 0x55, $1	# e0	:
+        addq    $0, 1, $3	# .. e1 :
+        cmoveq  $1, $3, $0	#-e0	:
+
+$done:	ret			# .. e1 :
+
+	# Deal with the case where $18 > 8 bytes remain to be
+	# searched.  $16 may not be aligned.
+	.align 4
+$first_quad:
+	andnot	$16, 0x7, $0	#-e1	:
+        insqh   $3, $16, $2	# .. e0	: $2 = 0000ffffffffffff ($16<0:2> ff)
+        xor	$1, $17, $1	# e0	:
+	or	$1, $2, $1	# e1	: $1 = ====ffffffffffff
+        cmpbge  $31, $1, $2	#-e0	:
+        bne     $2, $found_it	# .. e1 :
+
+	# At least one byte left to process.
+
+	ldq	$1, 8($0)	# e0	:
+	subq	$5, 1, $18	# .. e1 :
+	addq	$0, 8, $0	#-e0	:
+
+	# Make $18 point to last quad to be accessed (the
+	# last quad may or may not be partial).
+
+	andnot	$18, 0x7, $18	# .. e1 :
+	cmpult	$0, $18, $2	# e0	:
+	beq	$2, $final	# .. e1 :
+
+	# At least two quads remain to be accessed.
+
+	subq	$18, $0, $4	#-e0	: $4 <- nr quads to be processed
+	and	$4, 8, $4	# e1	: odd number of quads?
+	bne	$4, $odd_quad_count # e1 :
+
+	# At least three quads remain to be accessed
+
+	mov	$1, $4		# e0	: move prefetched value to correct reg
+
+	.align	4
+$unrolled_loop:
+	ldq	$1, 8($0)	#-e0	: prefetch $1
+	xor	$17, $4, $2	# .. e1 :
+	cmpbge	$31, $2, $2	# e0	:
+	bne	$2, $found_it	# .. e1 :
+
+	addq	$0, 8, $0	#-e0	:
+$odd_quad_count:
+	xor	$17, $1, $2	# .. e1 :
+	ldq	$4, 8($0)	# e0	: prefetch $4
+	cmpbge	$31, $2, $2	# .. e1 :
+	addq	$0, 8, $6	#-e0	:
+	bne	$2, $found_it	# .. e1	:
+
+	cmpult	$6, $18, $6	# e0	:
+	addq	$0, 8, $0	# .. e1 :
+	bne	$6, $unrolled_loop #-e1 :
+
+	mov	$4, $1		# e0	: move prefetched value into $1
+$final:	subq	$5, $0, $18	# .. e1	: $18 <- number of bytes left to do
+	bne	$18, $last_quad	# e1	:
+
+$not_found:
+	mov	$31, $0		#-e0	:
+	ret			# .. e1 :
+
+        .end memchr
+	EXPORT_SYMBOL(memchr)