qemu-kvm/kvm-physmem-replace-assertion-with-error.patch

From 40d97d335471a77b1491c124d2c109db68bf8ca6 Mon Sep 17 00:00:00 2001
From: Paolo Bonzini <pbonzini@redhat.com>
Date: Mon, 17 Feb 2025 13:08:12 +0100
Subject: [PATCH 020/115] physmem: replace assertion with error
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RH-Author: Paolo Bonzini <pbonzini@redhat.com>
RH-MergeRequest: 391: TDX support, including attestation and device assignment
RH-Jira: RHEL-15710 RHEL-20798 RHEL-49728
RH-Acked-by: Yash Mankad <None>
RH-Acked-by: Peter Xu <peterx@redhat.com>
RH-Acked-by: David Hildenbrand <david@redhat.com>
RH-Commit: [20/115] 5c17949c668857760ee80279e5776e8dcf4c7c11 (bonzini/rhel-qemu-kvm)

It is possible to start QEMU with a confidential-guest-support object
even in TCG mode.  While there is already a check in qemu_machine_creation_done:

    if (machine->cgs && !machine->cgs->ready) {
        error_setg(errp, "accelerator does not support confidential guest %s",
                   object_get_typename(OBJECT(machine->cgs)));
        exit(1);
    }

the creation of RAMBlocks happens earlier, in qemu_init_board(), if
the command line does not override the default memory backend with
-M memdev.  Then the RAMBlock will try to use guest_memfd (because
machine_require_guest_memfd correctly returns true; at least correctly
according to the current implementation) and trigger the assertion
failure for kvm_enabled().  This happend with a command line as
simple as the following:

    qemu-system-x86_64 -m 512 -nographic -object sev-snp-guest,reduced-phys-bits=48,id=sev0 \
       -M q35,kernel-irqchip=split,confidential-guest-support=sev0
    qemu-system-x86_64: ../system/physmem.c:1871: ram_block_add: Assertion `kvm_enabled()' failed.

Cc: Xiaoyao Li <xiaoyao.li@intel.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Pankaj Gupta <pankaj.gupta@amd.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Link: https://lore.kernel.org/r/20250217120812.396522-1-pbonzini@redhat.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 6debfb2cb1795427d2dc6a741c7430a233c76695)
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 system/physmem.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/system/physmem.c b/system/physmem.c
index 94600a33ec..dccc95030b 100644
--- a/system/physmem.c
+++ b/system/physmem.c
@@ -1882,7 +1882,11 @@ static void ram_block_add(RAMBlock *new_block, Error **errp)
     if (new_block->flags & RAM_GUEST_MEMFD) {
         int ret;
 
-        assert(kvm_enabled());
+        if (!kvm_enabled()) {
+            error_setg(errp, "cannot set up private guest memory for %s: KVM required",
+                       object_get_typename(OBJECT(current_machine->cgs)));
+            goto out_free;
+        }
         assert(new_block->guest_memfd < 0);
 
         ret = ram_block_discard_require(true);
-- 
2.50.1