linux-kernel - Re: [PATCH] KVM: x86: Only advertise KVM_CAP_READONLY

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Message-ID: <ZttxDQ6v-1OAVja9@google.com>
Date: Fri, 6 Sep 2024 14:15:57 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Tom Dohrmann <erbse.13@....de>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Paolo Bonzini <pbonzini@...hat.com>, Michael Roth <michael.roth@....com>
Subject: Re: [PATCH] KVM: x86: Only advertise KVM_CAP_READONLY_MEM when
 supported by VM

On Fri, Sep 06, 2024, Shakeel Butt wrote:
> On Mon, Sep 02, 2024 at 02:42:19PM GMT, Tom Dohrmann wrote:
> > Until recently, KVM_CAP_READONLY_MEM was unconditionally supported on
> > x86, but this is no longer the case for SEV-ES and SEV-SNP VMs.
> > 
> > When KVM_CHECK_EXTENSION is invoked on a VM, only advertise
> > KVM_CAP_READONLY_MEM when it's actually supported.
> > 
> > Fixes: 66155de93bcf ("KVM: x86: Disallow read-only memslots for SEV-ES and SEV-SNP (and TDX)")
> > Cc: Sean Christopherson <seanjc@...gle.com>
> > Cc: Paolo Bonzini <pbonzini@...hat.com>
> > Cc: Michael Roth <michael.roth@....com>
> > Signed-off-by: Tom Dohrmann <erbse.13@....de>
> > ---
> >  arch/x86/kvm/x86.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 70219e406987..9ad7fe279e72 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -4656,7 +4656,6 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
> >  	case KVM_CAP_ASYNC_PF_INT:
> >  	case KVM_CAP_GET_TSC_KHZ:
> >  	case KVM_CAP_KVMCLOCK_CTRL:
> > -	case KVM_CAP_READONLY_MEM:
> >  	case KVM_CAP_IOAPIC_POLARITY_IGNORED:
> >  	case KVM_CAP_TSC_DEADLINE_TIMER:
> >  	case KVM_CAP_DISABLE_QUIRKS:
> > @@ -4815,6 +4814,8 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
> >  	case KVM_CAP_VM_TYPES:
> >  		r = kvm_caps.supported_vm_types;
> >  		break;
> > +	case KVM_CAP_READONLY_MEM:
> > +		r = kvm ? kvm_arch_has_readonly_mem(kvm) : 1;
> 
> Need a break here otherwise -Wimplicit-fallthrough option will warn.

Heh, Nathan beat you to it:

https://lore.kernel.org/all/20240905-kvm-x86-avoid-clang-implicit-fallthrough-v1-1-f2e785f1aa45@kernel.org