Re: [PATCH] i386/cpu: ARCH_CAPABILITIES should not be advertised on AMD

[Sean Christopherson <seanjc@google.com>]

On Tue, Jul 01, 2025, Konrad Rzeszutek Wilk wrote:
> On Tue, Jul 01, 2025 at 03:05:00PM +0200, Igor Mammedov wrote:
> > On Tue, 1 Jul 2025 20:36:43 +0800
> > Zhao Liu <zhao1.liu@intel.com> wrote:
> > 
> > > On Tue, Jul 01, 2025 at 07:12:44PM +0800, Xiaoyao Li wrote:
> > > > Date: Tue, 1 Jul 2025 19:12:44 +0800
> > > > From: Xiaoyao Li <xiaoyao.li@intel.com>
> > > > Subject: Re: [PATCH] i386/cpu: ARCH_CAPABILITIES should not be advertised
> > > >  on AMD
> > > > 
> > > > On 7/1/2025 6:26 PM, Zhao Liu wrote:  
> > > > > > unless it was explicitly requested by the user.  
> > > > > But this could still break Windows, just like issue #3001, which enables
> > > > > arch-capabilities for EPYC-Genoa. This fact shows that even explicitly
> > > > > turning on arch-capabilities in AMD Guest and utilizing KVM's emulated
> > > > > value would even break something.
> > > > > 
> > > > > So even for named CPUs, arch-capabilities=on doesn't reflect the fact
> > > > > that it is purely emulated, and is (maybe?) harmful.  
> > > > 
> > > > It is because Windows adds wrong code. So it breaks itself and it's just the
> > > > regression of Windows.  
> > > 
> > > Could you please tell me what the Windows's wrong code is? And what's
> > > wrong when someone is following the hardware spec?
> > 
> > the reason is that it's reserved on AMD hence software shouldn't even try
> > to use it or make any decisions based on that.
> > 
> > 
> > PS:
> > on contrary, doing such ad-hoc 'cleanups' for the sake of misbehaving
> > guest would actually complicate QEMU for no big reason.
> 
> The guest is not misbehaving. It is following the spec.
> > 
> > Also
> > KVM does do have plenty of such code, and it's not actively preventing guests from using it.
> > Given that KVM is not welcoming such change, I think QEMU shouldn't do that either.
> 
> Because KVM maintainer does not want to touch the guest ABI. He agrees
> this is a bug.

No, I agreed that KVM's behavior is pointless, annoying, and odd[*].  I do not
agree that KVM's behavior is an outright bug.

[*] https://lore.kernel.org/all/aF1S2EIJWN47zLDG@google.com