FAILED: patch "[PATCH] KVM: SVM: Add missing save/restore handling of LBR MSRs" failed to apply to 5.10-stable tree

The patch below does not apply to the 5.10-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-5.10.y
git checkout FETCH_HEAD
git cherry-pick -x 3700f0788da6acf73b2df56690f4b201aa4aefd2
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026050315-worrisome-thieving-d489@gregkh' --subject-prefix 'PATCH 5.10.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

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

From 3700f0788da6acf73b2df56690f4b201aa4aefd2 Mon Sep 17 00:00:00 2001
From: Yosry Ahmed <[email protected]>
Date: Tue, 3 Mar 2026 00:33:57 +0000
Subject: [PATCH] KVM: SVM: Add missing save/restore handling of LBR MSRs

MSR_IA32_DEBUGCTLMSR and LBR MSRs are currently not enumerated by KVM_GET_MSR_INDEX_LIST, and LBR MSRs cannot be set with KVM_SET_MSRS. So save/restore is completely broken.

Fix it by adding the MSRs to msrs_to_save_base, and allowing writes to LBR MSRs from userspace only (as they are read-only MSRs) if LBR virtualization is enabled. Additionally, to correctly restore L1's LBRs while L2 is running, make sure the LBRs are copied from the captured VMCB01 save area in svm_copy_vmrun_state().

Note, for VMX, this also fixes a flaw where MSR_IA32_DEBUGCTLMSR isn't reported as an MSR to save/restore.

Note #2, over-reporting MSR_IA32_LASTxxx on Intel is ok, as KVM already handles unsupported reads and writes thanks to commit b5e2fec0ebc3 ("KVM:
Ignore DEBUGCTL MSRs with no effect") (kvm_do_msr_access() will morph the unsupported userspace write into a nop).

index 9c64d036e30b..2b1066ce23f5 100644
@@ -1099,6 +1099,11 @@ void svm_copy_vmrun_state(struct vmcb_save_area *to_save,
 		to_save->isst_addr = from_save->isst_addr;
 		to_save->ssp = from_save->ssp;
 	}
+
+	if (kvm_cpu_cap_has(X86_FEATURE_LBRV)) {
+		svm_copy_lbrs(to_save, from_save);
+		to_save->dbgctl &= ~DEBUGCTL_RESERVED_BITS;
+	}
 }
 
 void svm_copy_vmloadsave_state(struct vmcb *to_vmcb, struct vmcb *from_vmcb)

index 6e87ec52fa06..64da02d1ee00 100644
@@ -351,6 +351,9 @@ static const u32 msrs_to_save_base[] = {
 	MSR_IA32_U_CET, MSR_IA32_S_CET,
 	MSR_IA32_PL0_SSP, MSR_IA32_PL1_SSP, MSR_IA32_PL2_SSP,
 	MSR_IA32_PL3_SSP, MSR_IA32_INT_SSP_TAB,
+	MSR_IA32_DEBUGCTLMSR,
+	MSR_IA32_LASTBRANCHFROMIP, MSR_IA32_LASTBRANCHTOIP,
+	MSR_IA32_LASTINTFROMIP, MSR_IA32_LASTINTTOIP,
 };
 
 static const u32 msrs_to_save_pmu[] = {