[PATCH v2 2/3] KVM: x86: Virtualize HWCR.TscFreqSel[bit 24]

[Jim Mattson <jmattson@google.com>]

On certain CPUs, Linux guests expect HWCR.TscFreqSel[bit 24] to be
set. If it isn't set, they complain:
	[Firmware Bug]: TSC doesn't count with P0 frequency!

Allow userspace to set this bit in the virtual HWCR to eliminate the
above complaint.

Attempts to clear this bit from within the guest are ignored, to match
the behavior of modern AMD processors.

Signed-off-by: Jim Mattson <jmattson@google.com>
---
 arch/x86/kvm/x86.c | 20 +++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 1a323cae219c..202972f393b5 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3695,18 +3695,32 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		break;
 	case MSR_EFER:
 		return set_efer(vcpu, msr_info);
-	case MSR_K7_HWCR:
+	case MSR_K7_HWCR: {
+		u64 allowed = BIT_ULL(18);	/* McStatusWrEn */
+
 		data &= ~(u64)0x40;	/* ignore flush filter disable */
 		data &= ~(u64)0x100;	/* ignore ignne emulation enable */
 		data &= ~(u64)0x8;	/* ignore TLB cache disable */
 
-		/* Handle McStatusWrEn */
-		if (data & ~BIT_ULL(18)) {
+		if (!msr_info->host_initiated)
+			data &= ~BIT_ULL(24);	/* ignore TscFreqSel */
+		else
+			allowed |= BIT_ULL(24);	/* TscFreqSel */
+
+		if (data & ~allowed) {
 			kvm_pr_unimpl_wrmsr(vcpu, msr, data);
 			return 1;
 		}
+
+		/*
+		 * TscFreqSel is read-only from within the
+		 * guest. Attempts to clear it are ignored.
+		 */
+		if (!msr_info->host_initiated)
+			data |= vcpu->arch.msr_hwcr & BIT_ULL(24);
 		vcpu->arch.msr_hwcr = data;
 		break;
+	}
 	case MSR_FAM10H_MMIO_CONF_BASE:
 		if (data != 0) {
 			kvm_pr_unimpl_wrmsr(vcpu, msr, data);
-- 
2.42.0.515.g380fc7ccd1-goog