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Message-ID: <YQLa4UErfNbdjv3l@google.com>
Date: Thu, 29 Jul 2021 16:44:17 +0000
From: Sean Christopherson <seanjc@...gle.com>
To: Chenyi Qiang <chenyi.qiang@...el.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>,
Vitaly Kuznetsov <vkuznets@...hat.com>,
Wanpeng Li <wanpengli@...cent.com>,
Jim Mattson <jmattson@...gle.com>,
Joerg Roedel <joro@...tes.org>,
Xiaoyao Li <xiaoyao.li@...el.com>, kvm@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/5] KVM: X86: Expose PKS to guest
On Fri, Feb 05, 2021, Chenyi Qiang wrote:
> @@ -539,6 +540,7 @@ struct kvm_vcpu_arch {
> unsigned long cr8;
> u32 host_pkru;
> u32 pkru;
> + u32 pkrs;
> u32 hflags;
> u64 efer;
> u64 apic_base;
...
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 89af692deb7e..2266d98ace6f 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -250,6 +250,7 @@ static void vmx_sync_vmcs_host_state(struct vcpu_vmx *vmx,
> dest->ds_sel = src->ds_sel;
> dest->es_sel = src->es_sel;
> #endif
> + dest->pkrs = src->pkrs;
This is wrong. It also arguably belongs in patch 04.
The part that's missing is actually updating vmcs.HOST_IA32_PKRS. FS/GS are
handled by vmx_set_host_fs_gs(), while LDT/DS/ES are oddballs where the selector
is also the state that's restored.
In other words, this will cause nested VM-Exit to load the wrong PKRS.
> }
>
> static void vmx_switch_vmcs(struct kvm_vcpu *vcpu, struct loaded_vmcs *vmcs)
> diff --git a/arch/x86/kvm/vmx/vmcs.h b/arch/x86/kvm/vmx/vmcs.h
> index 1472c6c376f7..b5ba6407c5e1 100644
> --- a/arch/x86/kvm/vmx/vmcs.h
> +++ b/arch/x86/kvm/vmx/vmcs.h
> @@ -40,6 +40,7 @@ struct vmcs_host_state {
> #ifdef CONFIG_X86_64
> u16 ds_sel, es_sel;
> #endif
> + u32 pkrs;
> };
>
> struct vmcs_controls_shadow {
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 47b8357b9751..a3d95492e2b7 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -363,6 +363,8 @@ module_param_cb(vmentry_l1d_flush, &vmentry_l1d_flush_ops, NULL, 0644);
> static u32 vmx_segment_access_rights(struct kvm_segment *var);
> static __always_inline void vmx_disable_intercept_for_msr(struct kvm_vcpu *vcpu,
> u32 msr, int type);
> +static __always_inline void vmx_enable_intercept_for_msr(struct kvm_vcpu *vcpu,
> + u32 msr, int type);
>
> void vmx_vmexit(void);
>
> @@ -660,6 +662,8 @@ static bool is_valid_passthrough_msr(u32 msr)
> case MSR_IA32_RTIT_ADDR0_A ... MSR_IA32_RTIT_ADDR3_B:
> /* PT MSRs. These are handled in pt_update_intercept_for_msr() */
> return true;
> + case MSR_IA32_PKRS:
> + return true;
This is wrong with respect to MSR filtering, as KVM will fail to intercept the
MSR in response to a filter change. See vmx_msr_filter_changed().. I also think
that special casing PKRS is a bad idea in general. More later.
> }
>
> r = possible_passthrough_msr_slot(msr) != -ENOENT;
...
> + case MSR_IA32_PKRS:
> + if (!kvm_pkrs_valid(data))
> + return 1;
> + if (!kvm_cpu_cap_has(X86_FEATURE_PKS) ||
> + (!msr_info->host_initiated &&
> + !guest_cpuid_has(vcpu, X86_FEATURE_PKS)))
> + return 1;
> + if (vcpu->arch.pkrs != data) {
This will need to be modified if we go with my below proposal.
> + vcpu->arch.pkrs = data;
> + vmcs_write64(GUEST_IA32_PKRS, data);
> + }
> + break;
> case MSR_TSC_AUX:
> if (!msr_info->host_initiated &&
> !guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP))
> @@ -2479,7 +2518,8 @@ static __init int setup_vmcs_config(struct vmcs_config *vmcs_conf,
> VM_EXIT_LOAD_IA32_EFER |
> VM_EXIT_CLEAR_BNDCFGS |
> VM_EXIT_PT_CONCEAL_PIP |
> - VM_EXIT_CLEAR_IA32_RTIT_CTL;
> + VM_EXIT_CLEAR_IA32_RTIT_CTL |
> + VM_EXIT_LOAD_IA32_PKRS;
> if (adjust_vmx_controls(min, opt, MSR_IA32_VMX_EXIT_CTLS,
> &_vmexit_control) < 0)
> return -EIO;
> @@ -2503,7 +2543,8 @@ static __init int setup_vmcs_config(struct vmcs_config *vmcs_conf,
> VM_ENTRY_LOAD_IA32_EFER |
> VM_ENTRY_LOAD_BNDCFGS |
> VM_ENTRY_PT_CONCEAL_PIP |
> - VM_ENTRY_LOAD_IA32_RTIT_CTL;
> + VM_ENTRY_LOAD_IA32_RTIT_CTL |
> + VM_ENTRY_LOAD_IA32_PKRS;
> if (adjust_vmx_controls(min, opt, MSR_IA32_VMX_ENTRY_CTLS,
> &_vmentry_control) < 0)
> return -EIO;
> @@ -3103,8 +3144,9 @@ int vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
> * is in force while we are in guest mode. Do not let guests control
> * this bit, even if host CR4.MCE == 0.
> */
> - unsigned long hw_cr4;
> + unsigned long hw_cr4, old_cr4;
>
> + old_cr4 = kvm_read_cr4(vcpu);
> hw_cr4 = (cr4_read_shadow() & X86_CR4_MCE) | (cr4 & ~X86_CR4_MCE);
> if (is_unrestricted_guest(vcpu))
> hw_cr4 |= KVM_VM_CR4_ALWAYS_ON_UNRESTRICTED_GUEST;
> @@ -3152,7 +3194,7 @@ int vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
> }
>
> /*
> - * SMEP/SMAP/PKU is disabled if CPU is in non-paging mode in
> + * SMEP/SMAP/PKU/PKS is disabled if CPU is in non-paging mode in
> * hardware. To emulate this behavior, SMEP/SMAP/PKU needs
> * to be manually disabled when guest switches to non-paging
> * mode.
> @@ -3160,10 +3202,29 @@ int vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
> * If !enable_unrestricted_guest, the CPU is always running
> * with CR0.PG=1 and CR4 needs to be modified.
> * If enable_unrestricted_guest, the CPU automatically
> - * disables SMEP/SMAP/PKU when the guest sets CR0.PG=0.
> + * disables SMEP/SMAP/PKU/PKS when the guest sets CR0.PG=0.
> */
> if (!is_paging(vcpu))
> - hw_cr4 &= ~(X86_CR4_SMEP | X86_CR4_SMAP | X86_CR4_PKE);
> + hw_cr4 &= ~(X86_CR4_SMEP | X86_CR4_SMAP | X86_CR4_PKE |
> + X86_CR4_PKS);
> + }
> +
> + if ((hw_cr4 ^ old_cr4) & X86_CR4_PKS) {
Comparing hw_cr4 and old_cr4 is wrong, they are representative of two different
contexts. old_cr4 is the guest's previous CR4 irrespective of KVM maniuplations,
whereas hw_cr4 does include KVM's modification, e.g. the is_paging() logic above
may clear CR4.PKS and may lead to incorrect behavior.
E.g.:
guest.CR4.PKS = 1
guest.CR0.PG = 0
guest_hw.CR4.PKS = 0 // KVM
vmcs.LOAD_PKRS = 0 // KVM
guest.CR0.PG = 1
guest_hw.CR4.PKS = 1 // KVM
vmcs.LOAD_PKRS not modified because guest.CR4.PKS == guest_hw.CR4.PKS == 1
This logic also fails to handle the case where L1 doesn't intercept CR4.PKS
modifications for L2.
The VM-Exit path that does:
> + if (static_cpu_has(X86_FEATURE_PKS) &&
> + kvm_read_cr4_bits(vcpu, X86_CR4_PKS))
> + vcpu->arch.pkrs = vmcs_read64(GUEST_IA32_PKRS)
is also flawed in that, per this scheme, it may unnecessarily read vmcs.GUEST_PKRS,
though I don't think it can get the wrong value, unless of course it's running L2...
In short, IMO toggling PKRS interception/load on CR4 changes is a really, really
bad idea. UMIP emulation attempted do fancy toggling and got it wrong multiple
times, and this is more complicated than what UMIP was trying to do.
The only motiviation for toggling PKRS interception/load is to avoid the VMREAD in
the VM-Exit path. Given that vcpu->arch.pkrs is rarely accessed by KVM, e.g. only
on host userspace MSR reads and and GVA->GPA translation via permission_fault(),
keeping vcpu->arch.pkrs up-to-date at all times is unnecessary, i.e. it can be
synchronized on-demand. And regarding permission_fault(), that's indirectly gated
by CR4.PKS=1, thus deferring the VMREAD to permission_fault() is guaranteed to be
more performant than reading on every VM-Exit (with CR4.PKS=1).
So:
- Disable PKRS MSR interception if it's exposed to the guest.
- Load PKRS on entry/exit if it's exposed to the guest.
- Add VCPU_EXREG_PKRS and clear its bits in vmx_register_cache_reset().
- Add helpers to read/write/cache PKRS and use accordingly.
> + /* pass through PKRS to guest when CR4.PKS=1 */
> + if (guest_cpuid_has(vcpu, X86_FEATURE_PKS) && hw_cr4 & X86_CR4_PKS) {
> + vmx_disable_intercept_for_msr(vcpu, MSR_IA32_PKRS, MSR_TYPE_RW);
> + vm_entry_controls_setbit(vmx, VM_ENTRY_LOAD_IA32_PKRS);
> + vm_exit_controls_setbit(vmx, VM_EXIT_LOAD_IA32_PKRS);
> + /*
> + * Every vm exit saves guest pkrs automatically, sync vcpu->arch.pkrs
> + * to VMCS.GUEST_PKRS to avoid the pollution by host pkrs.
> + */
> + vmcs_write64(GUEST_IA32_PKRS, vcpu->arch.pkrs);
> + } else {
> + vmx_enable_intercept_for_msr(vcpu, MSR_IA32_PKRS, MSR_TYPE_RW);
> + vm_entry_controls_clearbit(vmx, VM_ENTRY_LOAD_IA32_PKRS);
> + vm_exit_controls_clearbit(vmx, VM_EXIT_LOAD_IA32_PKRS);
> + }
> }
>
> vmcs_writel(CR4_READ_SHADOW, cr4);
...
> @@ -6776,6 +6841,10 @@ static fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu)
>
> pt_guest_exit(vmx);
>
> + if (static_cpu_has(X86_FEATURE_PKS) &&
> + kvm_read_cr4_bits(vcpu, X86_CR4_PKS))
> + vcpu->arch.pkrs = vmcs_read64(GUEST_IA32_PKRS);
> +
> kvm_load_host_xsave_state(vcpu);
>
> vmx->nested.nested_run_pending = 0;
> @@ -7280,6 +7349,14 @@ static __init void vmx_set_cpu_caps(void)
> if (vmx_pt_mode_is_host_guest())
> kvm_cpu_cap_check_and_set(X86_FEATURE_INTEL_PT);
>
> + /*
> + * PKS is not yet implemented for shadow paging.
> + * If not support VM_{ENTRY, EXIT}_LOAD_IA32_PKRS,
> + * don't expose the PKS as well.
> + */
> + if (enable_ept && cpu_has_load_ia32_pkrs())
> + kvm_cpu_cap_check_and_set(X86_FEATURE_PKS);
> +
> if (vmx_umip_emulated())
> kvm_cpu_cap_set(X86_FEATURE_UMIP);
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index f5ede41bf9e6..684ef760481c 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1213,7 +1213,7 @@ static const u32 msrs_to_save_all[] = {
> MSR_IA32_RTIT_ADDR1_A, MSR_IA32_RTIT_ADDR1_B,
> MSR_IA32_RTIT_ADDR2_A, MSR_IA32_RTIT_ADDR2_B,
> MSR_IA32_RTIT_ADDR3_A, MSR_IA32_RTIT_ADDR3_B,
> - MSR_IA32_UMWAIT_CONTROL,
> + MSR_IA32_UMWAIT_CONTROL, MSR_IA32_PKRS,
>
> MSR_ARCH_PERFMON_FIXED_CTR0, MSR_ARCH_PERFMON_FIXED_CTR1,
> MSR_ARCH_PERFMON_FIXED_CTR0 + 2, MSR_ARCH_PERFMON_FIXED_CTR0 + 3,
> @@ -5718,6 +5718,10 @@ static void kvm_init_msr_list(void)
> intel_pt_validate_hw_cap(PT_CAP_num_address_ranges) * 2)
> continue;
> break;
> + case MSR_IA32_PKRS:
> + if (!kvm_cpu_cap_has(X86_FEATURE_PKS))
> + continue;
> + break;
> case MSR_ARCH_PERFMON_PERFCTR0 ... MSR_ARCH_PERFMON_PERFCTR0 + 17:
> if (msrs_to_save_all[i] - MSR_ARCH_PERFMON_PERFCTR0 >=
> min(INTEL_PMC_MAX_GENERIC, x86_pmu.num_counters_gp))
> @@ -10026,6 +10030,8 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>
> vcpu->arch.ia32_xss = 0;
>
> + vcpu->arch.pkrs = 0;
This is wrong and also unreviewable. It's wrong because the write isn't propagted
to vmcs.GUEST_PKRS, e.g. if the guest enables CR0.PG and CR4.PKS without writing
PKRS, KVM will run the guest with a stale value.
It's unreviewable because the SDM doesn't specify whether PKRS is cleared or
preserved on INIT. The SDM needs an entry for PRKS in Table 9-1. "IA-32 and Intel
64 Processor States Following Power-up, Reset, or INIT" before this can be merged.
> +
> kvm_x86_ops.vcpu_reset(vcpu, init_event);
> }
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