[pve-devel] [PATCH qemu-server 3/7] Add QEMU CPU flag querying helpers

[Stefan Reiter]

* query_understood_cpu_flags returns all flags that QEMU/KVM knows about
* query_supported_cpu_flags returns all flags that QEMU/KVM can use on
  this particular host.

To get supported flags, a temporary VM is started with QEMU, so we can
issue the "query-cpu-model-expansion" QMP command. This is how libvirt
queries supported flags for its "host-passthrough" CPU type.

query_supported_cpu_flags is thus rather slow and shouldn't be called
unnecessarily.

Signed-off-by: Stefan Reiter <s.reiter at proxmox.com>
---

Changes from RFC:

* Clearer regexes
* Use existing QMP infrastructure
* Add locking for temporary VM start
* Add comments


 PVE/QemuServer.pm | 84 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 84 insertions(+)

diff --git a/PVE/QemuServer.pm b/PVE/QemuServer.pm
index 8c519b5..97fa955 100644
--- a/PVE/QemuServer.pm
+++ b/PVE/QemuServer.pm
@@ -3500,6 +3500,90 @@ sub get_command_for_arch($) {
     return $cmd;
 }
 
+# The format of the flags returned here uses dashes '-' as seperators,
+# this is neither the default for 'query_supported_cpu_flags' below, nor for
+# /proc/cpuinfo.
+#
+# To compare (or array_intersect) flags, it's a good idea to convert them all
+# to a common format first (e.g. map s/\.|-/_/g).
+sub query_understood_cpu_flags {
+    my $flags = [];
+    my $flag_section = 0;
+
+    run_command(
+	[get_command_for_arch('x86_64'), '-cpu', 'help'],
+	noerr => 1,
+	quiet => 1,
+	outfunc => sub {
+	    my ($line) = @_;
+
+	    if ($flag_section) {
+		return if $line =~ m/^\s*$/;
+		$line =~ s/^\s*|\s*$//g;
+		push @$flags, split(/\s+/, $line);
+	    } elsif ($line =~ m/^\s*Recognized CPUID flags:\s*$/) {
+		$flag_section = 1;
+	    }
+	}
+    );
+
+    return $flags;
+}
+
+# This function needs to start a temporary VM and is therefore rather expensive.
+# If you need the value returned from this, you can get it much cheaper from
+# pvestatd using PVE::Cluster::get_node_kv('cpuflags').
+#
+# See also note to query_understood_cpu_flags above.
+sub query_supported_cpu_flags {
+    my $flags = [];
+
+    my $vmid = -1;
+    my $pidfile = pidfile_name($vmid);
+
+    PVE::QemuConfig->lock_config($vmid, sub {
+	# We start a temporary (frozen) VM with vmid -1 to allow us to send a QMP command
+	my $rc = run_command([
+	    get_command_for_arch('x86_64'),
+	    '-cpu', 'kvm64',
+	    '-display', 'none',
+	    '-chardev', "socket,id=qmp,path=/var/run/qemu-server/$vmid.qmp,server,nowait",
+	    '-mon', 'chardev=qmp,mode=control',
+	    '-pidfile', $pidfile,
+	    '-S', '-daemonize'
+	], noerr => 1, quiet => 1);
+	return if $rc;
+
+	my $cmd_result = vm_mon_cmd_nocheck(
+	    $vmid,
+	    'query-cpu-model-expansion',
+	    type => 'full',
+	    model => { name => 'host' }
+	);
+
+	my $props = $cmd_result->{model}->{props};
+	if (%$props) {
+	    foreach my $prop (keys %$props) {
+		push @$flags, $prop if "$props->{$prop}" eq '1';
+	    }
+	}
+
+	# force stop with 10 sec timeout and 'nocheck'
+	vm_stop(undef, $vmid, 1, 1, 10, 0, 1);
+    });
+
+    # QEMU returns some flags multiple times, with '_', '.' or '-' as seperator
+    # (e.g. lahf_lm and lahf-lm; sse4.2, sse4-2 and sse4_2; ...).
+    # We only keep those with underscores, since they match the ones from
+    # /proc/cpuinfo (they do the same thing, but we get rid of duplicates).
+    @$flags = grep {
+	my $replaced = (my $underscore = $_) =~ s/\.|-/_/g;
+	!($replaced && grep { $_ eq $underscore } @$flags)
+    } @$flags;
+
+    return $flags;
+}
+
 sub get_cpu_options {
     my ($conf, $arch, $kvm, $machine_type, $kvm_off, $kvmver, $winversion, $gpu_passthrough) = @_;
 
-- 
2.20.1