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Date: Wed, 23 Nov 2016 11:16:52 -0800
From: David Matlack <dmatlack@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm list <kvm@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Jim Mattson <jmattson@...gle.com>,
Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH 3/4] KVM: nVMX: accurate emulation of MSR_IA32_CR{0,4}_FIXED1
On Wed, Nov 23, 2016 at 1:06 AM, Paolo Bonzini <pbonzini@...hat.com> wrote:
>
>> Set MSR_IA32_CR{0,4}_FIXED1 to match the CPU's MSRs.
>>
>> In addition, MSR_IA32_CR4_FIXED1 should reflect the available CR4 bits
>> according to CPUID. Whenever guest CPUID is updated by userspace,
>> regenerate MSR_IA32_CR4_FIXED1 to match it.
>>
>> Signed-off-by: David Matlack <dmatlack@...gle.com>
>
> Oh, I thought userspace would do that! Doing it in KVM is fine as well,
> but then do we need to give userspace access to CR{0,4}_FIXED{0,1} at all?
I think it should be safe for userspace to skip restoring CR4_FIXED1,
since it is 100% generated based on CPUID. But I'd prefer to keep it
accessible from userspace, for consistency with the other VMX MSRs and
for flexibility. The auditing should ensure userspace doesn't restore
a CR4_FIXED1 that is inconsistent with CPUID.
Userspace should restore CR0_FIXED1 in case future CPUs change which
bits of CR0 are valid in VMX operation. Userspace should also restore
CR{0,4}_FIXED0 so we have the flexibility to change the defaults in
KVM. Both of these situations seem unlikely but we might as well play
it safe, the cost is small.
>
> Paolo
>
>> ---
>> Note: "x86/cpufeature: Add User-Mode Instruction Prevention definitions" has
>> not hit kvm/master yet so the macros for X86_CR4_UMIP and X86_FEATURE_UMIP
>> are not available.
>>
>> arch/x86/kvm/vmx.c | 54
>> +++++++++++++++++++++++++++++++++++++++++++++++++++---
>> 1 file changed, 51 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index a2a5ad8..ac5d9c0 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -2852,16 +2852,18 @@ static void nested_vmx_setup_ctls_msrs(struct
>> vcpu_vmx *vmx)
>> vmx->nested.nested_vmx_basic |= VMX_BASIC_INOUT;
>>
>> /*
>> - * These MSRs specify bits which the guest must keep fixed (on or off)
>> + * These MSRs specify bits which the guest must keep fixed on
>> * while L1 is in VMXON mode (in L1's root mode, or running an L2).
>> * We picked the standard core2 setting.
>> */
>> #define VMXON_CR0_ALWAYSON (X86_CR0_PE | X86_CR0_PG | X86_CR0_NE)
>> #define VMXON_CR4_ALWAYSON X86_CR4_VMXE
>> vmx->nested.nested_vmx_cr0_fixed0 = VMXON_CR0_ALWAYSON;
>> - vmx->nested.nested_vmx_cr0_fixed1 = -1ULL;
>> vmx->nested.nested_vmx_cr4_fixed0 = VMXON_CR4_ALWAYSON;
>> - vmx->nested.nested_vmx_cr4_fixed1 = -1ULL;
>> +
>> + /* These MSRs specify bits which the guest must keep fixed off. */
>> + rdmsrl(MSR_IA32_VMX_CR0_FIXED1, vmx->nested.nested_vmx_cr0_fixed1);
>> + rdmsrl(MSR_IA32_VMX_CR4_FIXED1, vmx->nested.nested_vmx_cr4_fixed1);
>>
>> /* highest index: VMX_PREEMPTION_TIMER_VALUE */
>> vmx->nested.nested_vmx_vmcs_enum = 0x2e;
>> @@ -9580,6 +9582,49 @@ static void vmcs_set_secondary_exec_control(u32
>> new_ctl)
>> (new_ctl & ~mask) | (cur_ctl & mask));
>> }
>>
>> +/*
>> + * Generate MSR_IA32_VMX_CR4_FIXED1 according to CPUID. Only set bits
>> + * (indicating "allowed-1") if they are supported in the guest's CPUID.
>> + */
>> +static void nested_vmx_cr4_fixed1_update(struct kvm_vcpu *vcpu)
>> +{
>> + struct vcpu_vmx *vmx = to_vmx(vcpu);
>> + struct kvm_cpuid_entry2 *entry;
>> +
>> +#define update(_cr4_mask, _reg, _cpuid_mask) do { \
>> + if (entry && (entry->_reg & (_cpuid_mask))) \
>> + vmx->nested.nested_vmx_cr4_fixed1 |= (_cr4_mask); \
>> +} while (0)
>> +
>> + vmx->nested.nested_vmx_cr4_fixed1 = X86_CR4_PCE;
>> +
>> + entry = kvm_find_cpuid_entry(vcpu, 0x1, 0);
>> + update(X86_CR4_VME, edx, bit(X86_FEATURE_VME));
>> + update(X86_CR4_PVI, edx, bit(X86_FEATURE_VME));
>> + update(X86_CR4_TSD, edx, bit(X86_FEATURE_TSC));
>> + update(X86_CR4_DE, edx, bit(X86_FEATURE_DE));
>> + update(X86_CR4_PSE, edx, bit(X86_FEATURE_PSE));
>> + update(X86_CR4_PAE, edx, bit(X86_FEATURE_PAE));
>> + update(X86_CR4_MCE, edx, bit(X86_FEATURE_MCE));
>> + update(X86_CR4_PGE, edx, bit(X86_FEATURE_PGE));
>> + update(X86_CR4_OSFXSR, edx, bit(X86_FEATURE_FXSR));
>> + update(X86_CR4_OSXMMEXCPT, edx, bit(X86_FEATURE_XMM));
>> + update(X86_CR4_VMXE, ecx, bit(X86_FEATURE_VMX));
>> + update(X86_CR4_SMXE, ecx, bit(X86_FEATURE_SMX));
>> + update(X86_CR4_PCIDE, ecx, bit(X86_FEATURE_PCID));
>> + update(X86_CR4_OSXSAVE, ecx, bit(X86_FEATURE_XSAVE));
>> +
>> + entry = kvm_find_cpuid_entry(vcpu, 0x7, 0);
>> + update(X86_CR4_FSGSBASE, ebx, bit(X86_FEATURE_FSGSBASE));
>> + update(X86_CR4_SMEP, ebx, bit(X86_FEATURE_SMEP));
>> + update(X86_CR4_SMAP, ebx, bit(X86_FEATURE_SMAP));
>> + update(X86_CR4_PKE, ecx, bit(X86_FEATURE_PKU));
>> + /* TODO: Use X86_CR4_UMIP and X86_FEATURE_UMIP macros */
>> + update(bit(11), ecx, bit(2));
>> +
>> +#undef update
>> +}
>> +
>> static void vmx_cpuid_update(struct kvm_vcpu *vcpu)
>> {
>> struct kvm_cpuid_entry2 *best;
>> @@ -9621,6 +9666,9 @@ static void vmx_cpuid_update(struct kvm_vcpu *vcpu)
>> else
>> to_vmx(vcpu)->msr_ia32_feature_control_valid_bits &=
>> ~FEATURE_CONTROL_VMXON_ENABLED_OUTSIDE_SMX;
>> +
>> + if (nested_vmx_allowed(vcpu))
>> + nested_vmx_cr4_fixed1_update(vcpu);
>> }
>>
>> static void vmx_set_supported_cpuid(u32 func, struct kvm_cpuid_entry2
>> *entry)
>> --
>> 2.8.0.rc3.226.g39d4020
>>
>>
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