Re: [PATCH 3/3] KVM: x86: Explicitly zero kvm_caps during vendor module load

["Huang, Kai" <kai.huang@intel.com>]

On Wed, 2024-04-24 at 08:33 -0700, Sean Christopherson wrote:
> On Wed, Apr 24, 2024, Kai Huang wrote:
> > On Tue, 2024-04-23 at 09:53 -0700, Sean Christopherson wrote:
> > > Zero out all of kvm_caps when loading a new vendor module to ensure that
> > > KVM can't inadvertently rely on global initialization of a field, and add
> > > a comment above the definition of kvm_caps to call out that all fields
> > > needs to be explicitly computed during vendor module load.
> > > 
> > > Signed-off-by: Sean Christopherson <seanjc@google.com>
> > > ---
> > >  arch/x86/kvm/x86.c | 7 +++++++
> > >  1 file changed, 7 insertions(+)
> > > 
> > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > index 44ce187bad89..8f3979d5fc80 100644
> > > --- a/arch/x86/kvm/x86.c
> > > +++ b/arch/x86/kvm/x86.c
> > > @@ -92,6 +92,11 @@
> > >  #define MAX_IO_MSRS 256
> > >  #define KVM_MAX_MCE_BANKS 32
> > >  
> > > +/*
> > > + * Note, kvm_caps fields should *never* have default values, all fields must be
> > > + * recomputed from scratch during vendor module load, e.g. to account for a
> > > + * vendor module being reloaded with different module parameters.
> > > + */
> > >  struct kvm_caps kvm_caps __read_mostly;
> > >  EXPORT_SYMBOL_GPL(kvm_caps);
> > >  
> > > @@ -9755,6 +9760,8 @@ int kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
> > >  		return -EIO;
> > >  	}
> > >  
> > > +	memset(&kvm_caps, 0, sizeof(kvm_caps));
> > > +
> > >  	x86_emulator_cache = kvm_alloc_emulator_cache();
> > >  	if (!x86_emulator_cache) {
> > >  		pr_err("failed to allocate cache for x86 emulator\n");
> > 
> > Why do the memset() here particularly?
> 
> So that it happens as early as possible, e.g. in case kvm_mmu_vendor_module_init()
> or some other function comes along and modifies kvm_caps.
> 
> > Isn't it better to put ...
> > 
> > 	memset(&kvm_caps, 0, sizeof(kvm_caps));
> > 	kvm_caps.supported_vm_types = BIT(KVM_X86_DEFAULT_VM);
> > 	kvm_caps.supported_mce_cap = MCG_CTL_P | MCG_SER_P;
> > 
> > ... together so it can be easily seen?
> > 
> > We can even have a helper to do above to "reset kvm_caps to default
> > values" I think.
> 
> Hmm, I don't think a helper is necessary, but I do agree that having all of the
> explicit initialization in one place would be better.  The alternative would be
> to use |= for BIT(KVM_X86_DEFAULT_VM), and MCG_CTL_P | MCG_SER_P, but I don't
> think that would be an improvement.  I'll tweak the first two patches to set the
> hardcoded caps earlier.
> 
> The main reason I don't want to add a helper is that coming up with a name would
> be tricky.  E.g. "kvm_reset_caps()" isn't a great fit because the caps are "reset"
> throughout module loading.  "kvm_set_default_caps()" kinda fits, but they aren't
> so much that they are KVM's defaults, rather they are the caps that KVM can always
> support regardless of hardware support, e.g. supported_xcr0 isn't optional, it
> just depends on hardware.

Personally I am fine with kvm_set_default_caps(), but obviously no strong
opinion here. :-)