linux-kernel - Re: [PATCH 0/3] KVM: x86: Fix supported VM

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Message-ID: <ZivMkK5PJbCQXnw2@google.com>
Date: Fri, 26 Apr 2024 08:47:28 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Kai Huang <kai.huang@...el.com>
Cc: Xiaoyao Li <xiaoyao.li@...el.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, 
	"pbonzini@...hat.com" <pbonzini@...hat.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/3] KVM: x86: Fix supported VM_TYPES caps

On Fri, Apr 26, 2024, Kai Huang wrote:
> On Thu, 2024-04-25 at 07:30 -0700, Sean Christopherson wrote:
> > On Thu, Apr 25, 2024, Xiaoyao Li wrote:
> > > On 4/24/2024 12:53 AM, Sean Christopherson wrote:
> > > > Fix a goof where KVM fails to re-initialize the set of supported VM types,
> > > > resulting in KVM overreporting the set of supported types when a vendor
> > > > module is reloaded with incompatible settings.  E.g. unload kvm-intel.ko,
> > > > reload with ept=0, and KVM will incorrectly treat SW_PROTECTED_VM as
> > > > supported.
> > > 
> > > Hah, this reminds me of the bug of msrs_to_save[] and etc.
> > > 
> > >    7a5ee6edb42e ("KVM: X86: Fix initialization of MSR lists")
> > 
> > Yeah, and we had the same bug with allow_smaller_maxphyaddr
> > 
> >   88213da23514 ("kvm: x86: disable the narrow guest module parameter on unload")
> > 
> > If the side effects of linking kvm.ko into kvm-{amd,intel}.ko weren't so painful
> > for userspace, 
> > 
> 
> Do we have any real side effects for _userspace_ here?

kvm.ko ceasing to exist, and "everything" being tied to the vendor module is the
big problem.  E.g. params from the kernel command line for kvm.??? will become
ineffective, etc.  Some of that can be handled in the kernel, e.g. KVM can create
a sysfs symlink so that the accesses through sysfs continue to work, but AFAIK
params don't supporting such aliasing/links.

I don't think there are any deal breakers, but I don't expect it to Just Work either.