linux-kernel - Re: [PATCH RFC 3/3] x86/kvm/hyper-v: don't allow to turn on unsupported VMX controls for nested guests

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Date:   Thu, 16 Jan 2020 08:21:02 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Liran Alon <liran.alon@...cle.com>, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        linux-kernel@...r.kernel.org, Roman Kagan <rkagan@...tuozzo.com>
Subject: Re: [PATCH RFC 3/3] x86/kvm/hyper-v: don't allow to turn on
 unsupported VMX controls for nested guests

On Thu, Jan 16, 2020 at 09:55:57AM +0100, Vitaly Kuznetsov wrote:
> Liran Alon <liran.alon@...cle.com> writes:
> 
> >> On 15 Jan 2020, at 19:10, Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
> >> 
> >> Sane L1 hypervisors are not supposed to turn any of the unsupported VMX
> >> controls on for its guests and nested_vmx_check_controls() checks for
> >> that. This is, however, not the case for the controls which are supported
> >> on the host but are missing in enlightened VMCS and when eVMCS is in use.
> >> 
> >> It would certainly be possible to add these missing checks to
> >> nested_check_vm_execution_controls()/_vm_exit_controls()/.. but it seems
> >> preferable to keep eVMCS-specific stuff in eVMCS and reduce the impact on
> >> non-eVMCS guests by doing less unrelated checks. Create a separate
> >> nested_evmcs_check_controls() for this purpose.
> >> 
> >> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> >> ---
> >> arch/x86/kvm/vmx/evmcs.c  | 56 ++++++++++++++++++++++++++++++++++++++-
> >> arch/x86/kvm/vmx/evmcs.h  |  1 +
> >> arch/x86/kvm/vmx/nested.c |  3 +++
> >> 3 files changed, 59 insertions(+), 1 deletion(-)
> >> 
> >> diff --git a/arch/x86/kvm/vmx/evmcs.c b/arch/x86/kvm/vmx/evmcs.c
> >> index b5d6582ba589..88f462866396 100644
> >> --- a/arch/x86/kvm/vmx/evmcs.c
> >> +++ b/arch/x86/kvm/vmx/evmcs.c
> >> @@ -4,9 +4,11 @@
> >> #include <linux/smp.h>
> >> 
> >> #include "../hyperv.h"
> >> -#include "evmcs.h"
> >> #include "vmcs.h"
> >> +#include "vmcs12.h"
> >> +#include "evmcs.h"
> >> #include "vmx.h"
> >> +#include "trace.h"
> >> 
> >> DEFINE_STATIC_KEY_FALSE(enable_evmcs);
> >> 
> >> @@ -378,6 +380,58 @@ void nested_evmcs_filter_control_msr(u32 msr_index, u64 *pdata)
> >> 	*pdata = ctl_low | ((u64)ctl_high << 32);
> >> }
> >> 
> >> +int nested_evmcs_check_controls(struct vmcs12 *vmcs12)
> >> +{
> >> +	int ret = 0;
> >> +	u32 unsupp_ctl;
> >> +
> >> +	unsupp_ctl = vmcs12->pin_based_vm_exec_control &
> >> +		EVMCS1_UNSUPPORTED_PINCTRL;
> >> +	if (unsupp_ctl) {
> >> +		trace_kvm_nested_vmenter_failed(
> >> +			"eVMCS: unsupported pin-based VM-execution controls",
> >> +			unsupp_ctl);
> >
> > Why not move "CC” macro from nested.c to nested.h and use it here as-well instead of replicating it’s logic?
> >
> 
> Because error messages I add are both human readable and conform to SDM!
> :-)
> 
> On a more serious not yes, we should probably do that. We may even want
> to use it in non-nesting (and non VMX) code in KVM.

No, the CC() macro is short for Consistency Check, i.e. specific to nVMX.
Even if KVM ends up adding nested_evmcs_check_controls(), which I'm hoping
can be avoided, I'd still be hesitant to expose CC() in nested.h.