On Thu, 5 Jun 2025 10:24:34 +0100 Daniel P. Berrangé <berra...@redhat.com> wrote:
> On Tue, Jun 03, 2025 at 05:02:38PM +0200, Igor Mammedov wrote: > > On Wed, 28 May 2025 13:23:49 +0800 > > Zhao Liu <zhao1....@intel.com> wrote: > > > > > On Wed, May 28, 2025 at 10:09:56AM +0800, Xiaoyao Li wrote: > > > > Date: Wed, 28 May 2025 10:09:56 +0800 > > > > From: Xiaoyao Li <xiaoyao...@intel.com> > > > > Subject: Re: [PATCH v4 04/19] target/i386/cpu: Remove > > > > X86CPU::check_cpuid > > > > field > > > > > > > > On 5/12/2025 4:39 PM, Philippe Mathieu-Daudé wrote: > > > > > The X86CPU::check_cpuid boolean was only set in the > > > > > pc_compat_2_4[] array, via the 'check=off' property. > > > > > We removed all machines using that array, lets remove > > > > > that CPU property and simplify x86_cpu_realizefn(). > > > > > > > > No. > > > > > > > > We cannot do this. Because it changes the behavior of QEMU. > > > > > > > > 'check_cpuid' is true by default while 'enforce_cpuid' is false. So that > > > > QEMU emits warnings in x86_cpu_filter_features() by default when user > > > > requests unsupported CPU features. If remove "check" property and the > > > > internal 'check_cpuid', QEMU will not do it unless user sets > > > > enforce_cpuid > > > > explicitly. > > > > > > One option would be to have x86_cpu_filter_features() unconditionally > > > turn on verbose and print warnings, but some people might want to turn > > > off these warning prints, I don't know if anyone would, but it would be > > > possible. > > > > > > The other option is still to keep the “check” property. > > > > > > IMO, the latter option is the better way to reduce Philippe's burden. > > > > we essentially loose warnings by default when some features aren't > > available, > > qemu still continues to run though. > > > > Given that Daniel acked it from libvirt side, libvirt doesn't care about > > warnings > > (it does its has its own cpu model calculation). Likely other mgmt do not > > care > > about it either, and if they do they probably doing something wrong and > > should use QMP to get that data. > > Acking it was a mistake on my part - I mis-interpreted the patch and so > didn't notice we were loosing the verbose printing of missing features > by default. > > I'm actually curious why we made the 'check' feature tied to machine > types at all. If it doesn't affect guest ABI, just causes verbose > info on stderr, it feels like something we could have just had on > all machine types new & old. Git history brings us back to > > commit 3e68482224129c3ddc061af7c9d438b882ecfdd1 > Author: Eduardo Habkost <ehabk...@redhat.com> > Date: Tue Nov 3 17:18:50 2015 -0200 > > target-i386: Set "check=off" by default on pc-*-2.4 and older > > The default CPU model (qemu64) have some issues today: it enables some > features (ABM and SSE4a) that are not present in many host CPUs. That > means many hosts (but not all of them) had those features silently > disabled in the default configuration in QEMU 2.4 and older. > > With the new "check=on" default, this causes warnings to be printed in > the default configuration, because of the lack of SSE4A on all Intel > hosts, and the lack of ABM on Sandy Bridge and older hosts: > > $ qemu-system-x86_64 -machine pc,accel=kvm > warning: host doesn't support requested feature: > CPUID.80000001H:ECX.abm [bit 5] > warning: host doesn't support requested feature: > CPUID.80000001H:ECX.sse4a [bit 6] > > Those issues will be fixed in pc-*-2.5 and newer. But as we can't change > the guest ABI in pc-*-2.4, disable "check" mode by default in pc-*-2.4 > and older so we don't print spurious warnings. > > IOW, we wanted to have 'check' unconditionally on by default, but > had to do a temp hack to avoid spamming all configurations with > the broken 'qemu64' CPU model design. > > > That leaves us with human users, for that case I'd say one should use > > enforce_cpuid if feature availability matters. > > IMHO even with mgmt apps, it is worth having 'check=on' by default > as the log message has value in debugging scenarios. It could have > the potential to highlight situations where an mgmt app has > unwittingly done something wrong with CPU config. At the very least > though its a warning to humans debugging that they should not trust > the QEMU command line as a expressing the full CPU featureset. in that line of thought, maybe hardcode 'check=on' and drop conditional/property? > > With regards, > Daniel
