commit 5df2c49263338da7221f24b3ad67ffd21d88047c

Author: Daniel P. Berrangé <berrange@redhat.com>

Date:   Wed Aug 4 18:05:59 2021 +0100

​

    util: directly query KVM for TSC scaling support

    We currently query the host MSRs to determine if TSC scaling is

    supported. This works OK when running privileged and can open

    the /dev/cpu/0/msr. When unprivileged we fallback to querying

    MSRs from /dev/kvm. This is incorrect because /dev/kvm only

    reports accurate info for MSRs that are valid to use from inside

    a guest.  The TSC scaling support MSR is not, thus we always end

    up reporting lack of TSC scaling when unprivileged.

    The solution to this is easy, because KVM can directly report

    whether TSC scaling is available, which matches what QEMU will

    do at startup.

    Closes: https://gitlab.com/libvirt/libvirt/-/issues/188

    Reported-by: Roman Mohr <rmohr@redhat.com>

    Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

    Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

​

Index: libvirt-7.1.0/src/util/virhostcpu.c

===================================================================

--- libvirt-7.1.0.orig/src/util/virhostcpu.c

+++ libvirt-7.1.0/src/util/virhostcpu.c

@@ -1336,9 +1336,6 @@ virHostCPUGetMSR(unsigned long index,

 }

-# define VMX_PROCBASED_CTLS2_MSR 0x48b

-# define VMX_USE_TSC_SCALING (1 << 25)

-

 /*

  * This function should only be called when the host CPU supports invariant TSC

  * (invtsc CPUID feature).

@@ -1349,11 +1346,10 @@ virHostCPUGetMSR(unsigned long index,

 virHostCPUTscInfoPtr

 virHostCPUGetTscInfo(void)

 {

-    virHostCPUTscInfoPtr info;

+    g_autofree virHostCPUTscInfoPtr info = g_new0(virHostCPUTscInfo, 1);

     VIR_AUTOCLOSE kvmFd = -1;

     VIR_AUTOCLOSE vmFd = -1;

     VIR_AUTOCLOSE vcpuFd = -1;

-    uint64_t msr = 0;

     int rc;

     if ((kvmFd = open(KVM_DEVICE, O_RDONLY)) < 0) {

@@ -1378,23 +1374,19 @@ virHostCPUGetTscInfo(void)

                              _("Unable to probe TSC frequency"));

         return NULL;

     }

-

-    info = g_new0(virHostCPUTscInfo, 1);

-

     info->frequency = rc * 1000ULL;

-    if (virHostCPUGetMSR(VMX_PROCBASED_CTLS2_MSR, &msr) == 0) {

-        /* High 32 bits of the MSR value indicate whether specific control

-         * can be set to 1. */

-        msr >>= 32;

-

-        info->scaling = virTristateBoolFromBool(!!(msr & VMX_USE_TSC_SCALING));

+    if ((rc = ioctl(kvmFd, KVM_CHECK_EXTENSION, KVM_CAP_TSC_CONTROL)) < 0) {

+        virReportSystemError(errno, "%s",

+                             _("Unable to query TSC scaling support"));

+        return NULL;

     }

+    info->scaling = rc ? VIR_TRISTATE_BOOL_YES : VIR_TRISTATE_BOOL_NO;

     VIR_DEBUG("Detected TSC frequency %llu Hz, scaling %s",

               info->frequency, virTristateBoolTypeToString(info->scaling));

-    return info;

+    return g_steal_pointer(&info);

 }

 #else

​