FAILED: patch "[PATCH] KVM: SEV: Do cache maintenance on the source VM during" failed to apply to 6.6-stable tree

The patch below does not apply to the 6.6-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.6.y
git checkout FETCH_HEAD
git cherry-pick -x 93de2a6a4b91b72607136dd656edf03fb399d27f
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026092918-providing-domelike-5f91@gregkh' --subject-prefix 'PATCH 6.6.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 93de2a6a4b91b72607136dd656edf03fb399d27f Mon Sep 17 00:00:00 2001
From: Sean Christopherson <[email protected]>
Date: Wed, 23 Sep 2026 09:37:21 -0700
Subject: [PATCH] KVM: SEV: Do cache maintenance on the source VM during
intra-host migration

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.

Message-ID: <[email protected]>

index cdc1c04f60da..63eb2155a774 100644
@@ -2048,6 +2048,12 @@ static void sev_migrate_from(struct kvm *dst_kvm, struct kvm *src_kvm)
 	src->pages_locked = 0;
 	src->es_active = false;
 
+	/*
+	 * Do cache maintenance on the source VM as it is no longer an SEV VM,
+	 * i.e. memory reclaim flows won't trigger cache maintenance on the VM.
+	 */
+	sev_writeback_caches(src_kvm);
+
 	list_cut_before(&dst->regions_list, &src->regions_list, &src->regions_list);
 
 	mutex_lock(&sev_mirror_lock);

@@ -2187,6 +2193,10 @@ int sev_vm_move_enc_context_from(struct kvm *kvm, unsigned int source_fd)
 	 * the set of CPUs from the source.  If a CPU was used to run a vCPU in
 	 * the source VM but is never used for the destination VM, then the CPU
 	 * can only have cached memory that was accessible to the source VM.
+	 * Furthermore, KVM *must* perform cache maintenance on the source VM,
+	 * as the source VM may have access to memory that the destination VM
+	 * does not, i.e. KVM could skip flushes if memory is reclaimed from
+	 * the old VM but not the new VM.
 	 */
 	if (!zalloc_cpumask_var(&dst_sev->have_run_cpus, GFP_KERNEL_ACCOUNT)) {
 		ret = -ENOMEM;

From: Sean Christopherson <[email protected]>

[ Upstream commit 93de2a6a4b91b72607136dd656edf03fb399d27f ]

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.

Message-ID: <[email protected]>

---
 arch/x86/kvm/svm/sev.c | 6 ++++++
 1 file changed, 6 insertions(+)

index 0ba286bd50c27..2bc25a89f36b3 100644
@@ -1723,6 +1723,12 @@ static void sev_migrate_from(struct kvm *dst_kvm, struct kvm *src_kvm)
 	src->pages_locked = 0;
 	src->es_active = false;
 
+	/*
+	 * Do cache maintenance on the source VM as it is no longer an SEV VM,
+	 * i.e. memory reclaim flows won't trigger cache maintenance on the VM.
+	 */
+	wbinvd_on_all_cpus();
+
 	list_cut_before(&dst->regions_list, &src->regions_list, &src->regions_list);
 
 	mutex_lock(&sev_mirror_lock);

2.53.0