RE: [PATCH] KVM: nVMX: Dynamically compute max VMCS index for vmcs12

[Hu, Robert]

> -----Original Message-----
> From: Sean Christopherson <seanjc@xxxxxxxxxx>
> Sent: Tuesday, June 22, 2021 01:08
> To: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>; Wanpeng Li
> <wanpengli@xxxxxxxxxxx>; Jim Mattson <jmattson@xxxxxxxxxx>; Joerg
> Roedel <joro@xxxxxxxxxx>; kvm@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH] KVM: nVMX: Dynamically compute max VMCS index for
> vmcs12
> 
> On Mon, Jun 21, 2021, Paolo Bonzini wrote:
> > On 18/06/21 23:46, Sean Christopherson wrote:
> > > Calculate the max VMCS index for vmcs12 by walking the array to find
> > > the actual max index.  Hardcoding the index is prone to bitrot, and
> > > the calculation is only done on KVM bringup (albeit on every CPU,
> > > but there aren't _that_ many null entries in the array).
> > >
> > > Fixes: 3c0f99366e34 ("KVM: nVMX: Add a TSC multiplier field in
> > > VMCS12")
> > > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > > ---
> > >
> > > Note, the vmx test in kvm-unit-tests will still fail using stock
> > > QEMU, as QEMU also hardcodes and overwrites the MSR.  The test
> > > passes if I hack KVM to ignore userspace (it was easier than rebuilding
> QEMU).
> >
> > Queued, thanks.  Without having checked the kvm-unit-tests sources
> > very thoroughly, this might be a configuration issue in
> > kvm-unit-tests; in theory "-cpu host" (unlike "-cpu
> > host,migratable=no") should not enable TSC scaling.
> 
> As noted in the code comments, KVM allows VMREAD/VMWRITE to all defined
> fields, whether or not the field should actually exist for the vCPU model doesn't
> enter into the equation.  That's technically wrong as there are a number of
> fields that the SDM explicitly states exist iff a certain feature is supported.  To
> fix that we'd need to add a "feature flag" to vmcs_field_to_offset_table that is
> checked against the vCPU model, though updating the MSR would probably fall
> onto userspace's shoulders?
[Hu, Robert] 
Perhaps more easier and proper to do this in KVM side.
QEMU sets actual feature set down to KVM, and KVM updates IA32_VMX_VMCS_ENUM
MSR accordingly. We don't see a channel that QEMU constructs a VMCS and sets a whole
to KVM.

> 
> And FWIW, this is the QEMU code:
> 
>   #define VMCS12_MAX_FIELD_INDEX (0x17)
> 
>   static void kvm_msr_entry_add_vmx(X86CPU *cpu, FeatureWordArray f)
>   {
>       ...
> 
>       /*
>        * Just to be safe, write these with constant values.  The CRn_FIXED1
>        * MSRs are generated by KVM based on the vCPU's CPUID.
>        */
>       kvm_msr_entry_add(cpu, MSR_IA32_VMX_CR0_FIXED0,
>                         CR0_PE_MASK | CR0_PG_MASK | CR0_NE_MASK);
>       kvm_msr_entry_add(cpu, MSR_IA32_VMX_CR4_FIXED0,
>                         CR4_VMXE_MASK);
>       kvm_msr_entry_add(cpu, MSR_IA32_VMX_VMCS_ENUM,
>                         VMCS12_MAX_FIELD_INDEX << 1);
>   }