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, 183 insertions, 0 deletions

diff --git a/drivers/block/n64cart.c b/drivers/block/n64cart.c
new file mode 100644
index 000000000..328da73b6
--- /dev/null
+++ b/drivers/block/n64cart.c
@@ -0,0 +1,183 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Support for the N64 cart.
+ *
+ * Copyright (c) 2021 Lauri Kasanen
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+#include <linux/bitops.h>
+#include <linux/blkdev.h>
+#include <linux/dma-mapping.h>
+#include <linux/init.h>
+#include <linux/module.h>
+#include <linux/platform_device.h>
+#include <linux/string.h>
+
+enum {
+	PI_DRAM_REG = 0,
+	PI_CART_REG,
+	PI_READ_REG,
+	PI_WRITE_REG,
+	PI_STATUS_REG,
+};
+
+#define PI_STATUS_DMA_BUSY	(1 << 0)
+#define PI_STATUS_IO_BUSY	(1 << 1)
+
+#define CART_DOMAIN		0x10000000
+#define CART_MAX		0x1FFFFFFF
+
+#define MIN_ALIGNMENT		8
+
+static u32 __iomem *reg_base;
+
+static unsigned int start;
+module_param(start, uint, 0);
+MODULE_PARM_DESC(start, "Start address of the cart block data");
+
+static unsigned int size;
+module_param(size, uint, 0);
+MODULE_PARM_DESC(size, "Size of the cart block data, in bytes");
+
+static void n64cart_write_reg(const u8 reg, const u32 value)
+{
+	writel(value, reg_base + reg);
+}
+
+static u32 n64cart_read_reg(const u8 reg)
+{
+	return readl(reg_base + reg);
+}
+
+static void n64cart_wait_dma(void)
+{
+	while (n64cart_read_reg(PI_STATUS_REG) &
+		(PI_STATUS_DMA_BUSY | PI_STATUS_IO_BUSY))
+		cpu_relax();
+}
+
+/*
+ * Process a single bvec of a bio.
+ */
+static bool n64cart_do_bvec(struct device *dev, struct bio_vec *bv, u32 pos)
+{
+	dma_addr_t dma_addr;
+	const u32 bstart = pos + start;
+
+	/* Alignment check */
+	WARN_ON_ONCE((bv->bv_offset & (MIN_ALIGNMENT - 1)) ||
+		     (bv->bv_len & (MIN_ALIGNMENT - 1)));
+
+	dma_addr = dma_map_bvec(dev, bv, DMA_FROM_DEVICE, 0);
+	if (dma_mapping_error(dev, dma_addr))
+		return false;
+
+	n64cart_wait_dma();
+
+	n64cart_write_reg(PI_DRAM_REG, dma_addr);
+	n64cart_write_reg(PI_CART_REG, (bstart | CART_DOMAIN) & CART_MAX);
+	n64cart_write_reg(PI_WRITE_REG, bv->bv_len - 1);
+
+	n64cart_wait_dma();
+
+	dma_unmap_page(dev, dma_addr, bv->bv_len, DMA_FROM_DEVICE);
+	return true;
+}
+
+static void n64cart_submit_bio(struct bio *bio)
+{
+	struct bio_vec bvec;
+	struct bvec_iter iter;
+	struct device *dev = bio->bi_bdev->bd_disk->private_data;
+	u32 pos = bio->bi_iter.bi_sector << SECTOR_SHIFT;
+
+	bio_for_each_segment(bvec, bio, iter) {
+		if (!n64cart_do_bvec(dev, &bvec, pos)) {
+			bio_io_error(bio);
+			return;
+		}
+		pos += bvec.bv_len;
+	}
+
+	bio_endio(bio);
+}
+
+static const struct block_device_operations n64cart_fops = {
+	.owner		= THIS_MODULE,
+	.submit_bio	= n64cart_submit_bio,
+};
+
+/*
+ * The target device is embedded and RAM-constrained. We save RAM
+ * by initializing in __init code that gets dropped late in boot.
+ * For the same reason there is no module or unloading support.
+ */
+static int __init n64cart_probe(struct platform_device *pdev)
+{
+	struct queue_limits lim = {
+		.physical_block_size	= 4096,
+		.logical_block_size	= 4096,
+	};
+	struct gendisk *disk;
+	int err = -ENOMEM;
+
+	if (!start || !size) {
+		pr_err("start or size not specified\n");
+		return -ENODEV;
+	}
+
+	if (size & 4095) {
+		pr_err("size must be a multiple of 4K\n");
+		return -ENODEV;
+	}
+
+	reg_base = devm_platform_ioremap_resource(pdev, 0);
+	if (IS_ERR(reg_base))
+		return PTR_ERR(reg_base);
+
+	disk = blk_alloc_disk(&lim, NUMA_NO_NODE);
+	if (IS_ERR(disk)) {
+		err = PTR_ERR(disk);
+		goto out;
+	}
+
+	disk->first_minor = 0;
+	disk->flags = GENHD_FL_NO_PART;
+	disk->fops = &n64cart_fops;
+	disk->private_data = &pdev->dev;
+	strscpy(disk->disk_name, "n64cart");
+
+	set_capacity(disk, size >> SECTOR_SHIFT);
+	set_disk_ro(disk, 1);
+
+	err = add_disk(disk);
+	if (err)
+		goto out_cleanup_disk;
+
+	pr_info("n64cart: %u kb disk\n", size / 1024);
+
+	return 0;
+
+out_cleanup_disk:
+	put_disk(disk);
+out:
+	return err;
+}
+
+static struct platform_driver n64cart_driver = {
+	.driver = {
+		.name = "n64cart",
+	},
+};
+
+static int __init n64cart_init(void)
+{
+	return platform_driver_probe(&n64cart_driver, n64cart_probe);
+}
+
+module_init(n64cart_init);
+
+MODULE_AUTHOR("Lauri Kasanen <cand@gmx.com>");
+MODULE_DESCRIPTION("Driver for the N64 cart");
+MODULE_LICENSE("GPL");