Re: [PATCH docs/manager/qemu-server v4 00/17] Add API and UI for custom CPU models

[Arthur Bied-Charreton <a.bied-charreton@proxmox.com>]

On Mon, May 11, 2026 at 12:42:02PM +0200, Fiona Ebner wrote:
> I found a pre-existing bug in query_supported_cpu_flags() that we should
> fix. Namely the following replacement is wrong in general:
> 
> >             foreach my $prop (keys %$props) {
> >                 next if $props->{$prop} ne '1';
> >                 # QEMU returns some flags multiple times, with '_', '.' or '-'
> >                 # (e.g. lahf_lm and lahf-lm; sse4.2, sse4-2 and sse4_2; ...).
> >                 # We only keep those with underscores, to match /proc/cpuinfo
> >                 $prop =~ s/\.|-/_/g;
> >                 $flags->{$prop} = 1;
> >             }
> 
> In particular, flags like 'virt-ssbd' or 'amd-ssbd' or 'amd-no-ssb' are
> not present multiple times, but only in their exact form. Because of the
> replacement, we only consider e.g. 'virt_ssbd' and then mistakenly
> detect the flags as not supported on any node.
> 
> Let's just make an explicit map of what is actually duplicate and not do
> such general replacement here.

After looking a little more into it I think we should also apply the
same logic to the recognized CPU flags, or normalize the flags in 
query_supported_cpu_flags() to match the `qemu-system-$arch -cpu help`
output rather than /proc/cpuinfo, since that is the source we use?

For example, /proc/cpuinfo writes the sse4.2 flag as sse4_2, while
`qemu-system-$arch -cpu help` writes it as sse4.2, so normalizing it to
sse4_2 in query_supported_cpu_flags() means it does not match anything
in the output of query_understood_cpu_flags(). QEMU accepts both
versions.

Is there a reason why we need to match /proc/cpuinfo specifically that
I am overlooking, or is it just about having a consistent output for CPU
flags in general?