[PATCH 4/6] KVM: x86/pmu: Don't tell userspace to save PMU MSRs if PMU is disabled

From: Sean Christopherson <seanjc@google.com>
To: Sean Christopherson <seanjc@google.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	Aaron Lewis <aaronlewis@google.com>,
	Weijiang Yang <weijiang.yang@intel.com>,
	Like Xu <likexu@tencent.com>
Subject: [PATCH 4/6] KVM: x86/pmu: Don't tell userspace to save PMU MSRs if PMU is disabled
Date: Tue, 24 Jan 2023 23:49:03 +0000	[thread overview]
Message-ID: <20230124234905.3774678-5-seanjc@google.com> (raw)
In-Reply-To: <20230124234905.3774678-1-seanjc@google.com>

Omit all PMU MSRs from the "MSRs to save" list if the PMU is disabled so
that userspace doesn't waste time saving and restoring dummy values.  KVM
provides "error" semantics (read zeros, drop writes) for such known-but-
unsupported MSRs, i.e. has fudged around this issue for quite some time.
Keep the "error" semantics as-is for now, the logic will be cleaned up in
a separate patch.

Cc: Aaron Lewis <aaronlewis@google.com>
Cc: Weijiang Yang <weijiang.yang@intel.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/x86.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 9b6e1af63531..25da2cc09e55 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7081,8 +7081,10 @@ static void kvm_init_msr_list(void)
 	for (i = 0; i < ARRAY_SIZE(msrs_to_save_base); i++)
 		kvm_probe_msr_to_save(msrs_to_save_base[i]);
 
-	for (i = 0; i < ARRAY_SIZE(msrs_to_save_pmu); i++)
-		kvm_probe_msr_to_save(msrs_to_save_pmu[i]);
+	if (enable_pmu) {
+		for (i = 0; i < ARRAY_SIZE(msrs_to_save_pmu); i++)
+			kvm_probe_msr_to_save(msrs_to_save_pmu[i]);
+	}
 
 	for (i = 0; i < ARRAY_SIZE(emulated_msrs_all); i++) {
 		if (!static_call(kvm_x86_has_emulated_msr)(NULL, emulated_msrs_all[i]))
-- 
2.39.1.456.gfc5497dd1b-goog

