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, 106 insertions, 0 deletions

diff --git a/Documentation/trace/ring-buffer-map.rst b/Documentation/trace/ring-buffer-map.rst
new file mode 100644
index 000000000..8e296bcc0
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+++ b/Documentation/trace/ring-buffer-map.rst
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+.. SPDX-License-Identifier: GPL-2.0
+
+==================================
+Tracefs ring-buffer memory mapping
+==================================
+
+:Author: Vincent Donnefort <vdonnefort@google.com>
+
+Overview
+========
+Tracefs ring-buffer memory map provides an efficient method to stream data
+as no memory copy is necessary. The application mapping the ring-buffer becomes
+then a consumer for that ring-buffer, in a similar fashion to trace_pipe.
+
+Memory mapping setup
+====================
+The mapping works with a mmap() of the trace_pipe_raw interface.
+
+The first system page of the mapping contains ring-buffer statistics and
+description. It is referred to as the meta-page. One of the most important
+fields of the meta-page is the reader. It contains the sub-buffer ID which can
+be safely read by the mapper (see ring-buffer-design.rst).
+
+The meta-page is followed by all the sub-buffers, ordered by ascending ID. It is
+therefore effortless to know where the reader starts in the mapping:
+
+.. code-block:: c
+
+        reader_id = meta->reader->id;
+        reader_offset = meta->meta_page_size + reader_id * meta->subbuf_size;
+
+When the application is done with the current reader, it can get a new one using
+the trace_pipe_raw ioctl() TRACE_MMAP_IOCTL_GET_READER. This ioctl also updates
+the meta-page fields.
+
+Limitations
+===========
+When a mapping is in place on a Tracefs ring-buffer, it is not possible to
+either resize it (either by increasing the entire size of the ring-buffer or
+each subbuf). It is also not possible to use snapshot and causes splice to copy
+the ring buffer data instead of using the copyless swap from the ring buffer.
+
+Concurrent readers (either another application mapping that ring-buffer or the
+kernel with trace_pipe) are allowed but not recommended. They will compete for
+the ring-buffer and the output is unpredictable, just like concurrent readers on
+trace_pipe would be.
+
+Example
+=======
+
+.. code-block:: c
+
+        #include <fcntl.h>
+        #include <stdio.h>
+        #include <stdlib.h>
+        #include <unistd.h>
+
+        #include <linux/trace_mmap.h>
+
+        #include <sys/mman.h>
+        #include <sys/ioctl.h>
+
+        #define TRACE_PIPE_RAW "/sys/kernel/tracing/per_cpu/cpu0/trace_pipe_raw"
+
+        int main(void)
+        {
+                int page_size = getpagesize(), fd, reader_id;
+                unsigned long meta_len, data_len;
+                struct trace_buffer_meta *meta;
+                void *map, *reader, *data;
+
+                fd = open(TRACE_PIPE_RAW, O_RDONLY | O_NONBLOCK);
+                if (fd < 0)
+                        exit(EXIT_FAILURE);
+
+                map = mmap(NULL, page_size, PROT_READ, MAP_SHARED, fd, 0);
+                if (map == MAP_FAILED)
+                        exit(EXIT_FAILURE);
+
+                meta = (struct trace_buffer_meta *)map;
+                meta_len = meta->meta_page_size;
+
+                printf("entries:        %llu\n", meta->entries);
+                printf("overrun:        %llu\n", meta->overrun);
+                printf("read:           %llu\n", meta->read);
+                printf("nr_subbufs:     %u\n", meta->nr_subbufs);
+
+                data_len = meta->subbuf_size * meta->nr_subbufs;
+                data = mmap(NULL, data_len, PROT_READ, MAP_SHARED, fd, meta_len);
+                if (data == MAP_FAILED)
+                        exit(EXIT_FAILURE);
+
+                if (ioctl(fd, TRACE_MMAP_IOCTL_GET_READER) < 0)
+                        exit(EXIT_FAILURE);
+
+                reader_id = meta->reader.id;
+                reader = data + meta->subbuf_size * reader_id;
+
+                printf("Current reader address: %p\n", reader);
+
+                munmap(data, data_len);
+                munmap(meta, meta_len);
+                close (fd);
+
+                return 0;
+        }