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, 111 insertions, 0 deletions

diff --git a/drivers/firmware/efi/ovmf-debug-log.c b/drivers/firmware/efi/ovmf-debug-log.c
new file mode 100644
index 000000000..5b2471ffa
--- /dev/null
+++ b/drivers/firmware/efi/ovmf-debug-log.c
@@ -0,0 +1,111 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/efi.h>
+#include <linux/init.h>
+#include <linux/io.h>
+#include <linux/kernel.h>
+#include <linux/kobject.h>
+#include <linux/module.h>
+#include <linux/platform_device.h>
+#include <linux/sysfs.h>
+
+#define OVMF_DEBUG_LOG_MAGIC1  0x3167646d666d766f  // "ovmfmdg1"
+#define OVMF_DEBUG_LOG_MAGIC2  0x3267646d666d766f  // "ovmfmdg2"
+
+struct ovmf_debug_log_header {
+	u64    magic1;
+	u64    magic2;
+	u64    hdr_size;
+	u64    log_size;
+	u64    lock; // edk2 spinlock
+	u64    head_off;
+	u64    tail_off;
+	u64    truncated;
+	u8     fw_version[128];
+};
+
+static struct ovmf_debug_log_header *hdr;
+static u8 *logbuf;
+static u64 logbufsize;
+
+static ssize_t ovmf_log_read(struct file *filp, struct kobject *kobj,
+			     const struct bin_attribute *attr, char *buf,
+			     loff_t offset, size_t count)
+{
+	u64 start, end;
+
+	start = hdr->head_off + offset;
+	if (hdr->head_off > hdr->tail_off && start >= hdr->log_size)
+		start -= hdr->log_size;
+
+	end = start + count;
+	if (start > hdr->tail_off) {
+		if (end > hdr->log_size)
+			end = hdr->log_size;
+	} else {
+		if (end > hdr->tail_off)
+			end = hdr->tail_off;
+	}
+
+	if (start > logbufsize || end > logbufsize)
+		return 0;
+	if (start >= end)
+		return 0;
+
+	memcpy(buf, logbuf + start, end - start);
+	return end - start;
+}
+
+static struct bin_attribute ovmf_log_bin_attr = {
+	.attr = {
+		.name = "ovmf_debug_log",
+		.mode = 0444,
+	},
+	.read = ovmf_log_read,
+};
+
+int __init ovmf_log_probe(unsigned long ovmf_debug_log_table)
+{
+	int ret = -EINVAL;
+	u64 size;
+
+	/* map + verify header */
+	hdr = memremap(ovmf_debug_log_table, sizeof(*hdr), MEMREMAP_WB);
+	if (!hdr) {
+		pr_err("OVMF debug log: header map failed\n");
+		return -EINVAL;
+	}
+
+	if (hdr->magic1 != OVMF_DEBUG_LOG_MAGIC1 ||
+	    hdr->magic2 != OVMF_DEBUG_LOG_MAGIC2) {
+		printk(KERN_ERR "OVMF debug log: magic mismatch\n");
+		goto err_unmap;
+	}
+
+	size = hdr->hdr_size + hdr->log_size;
+	pr_info("OVMF debug log: firmware version: \"%s\"\n", hdr->fw_version);
+	pr_info("OVMF debug log: buffer size: %lluk\n", size / 1024);
+
+	/* map complete log buffer */
+	memunmap(hdr);
+	hdr = memremap(ovmf_debug_log_table, size, MEMREMAP_WB);
+	if (!hdr) {
+		pr_err("OVMF debug log: buffer map failed\n");
+		return -EINVAL;
+	}
+	logbuf = (void *)hdr + hdr->hdr_size;
+	logbufsize = hdr->log_size;
+
+	ovmf_log_bin_attr.size = size;
+	ret = sysfs_create_bin_file(efi_kobj, &ovmf_log_bin_attr);
+	if (ret != 0) {
+		pr_err("OVMF debug log: sysfs register failed\n");
+		goto err_unmap;
+	}
+
+	return 0;
+
+err_unmap:
+	memunmap(hdr);
+	return ret;
+}