Re: [PATCH v3 2/2] KVM: VMX: untangle VMXON revision_id setting when using eVMCS

Krish Sadhukhan
2020-03-06T22:21:31Z

On 3/6/20 5:02 AM, Vitaly Kuznetsov wrote:
> As stated in alloc_vmxon_regions(), VMXON region needs to be tagged with
> revision id from MSR_IA32_VMX_BASIC even in case of eVMCS. The logic to
> do so is not very straightforward: first, we set
> hdr.revision_id = KVM_EVMCS_VERSION in alloc_vmcs_cpu() just to reset it
> back to vmcs_config.revision_id in alloc_vmxon_regions(). Simplify this by
> introducing 'enum vmcs_type' parameter to alloc_vmcs_cpu()/alloc_vmcs().
>
> No functional change intended.
>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
> arch/x86/kvm/vmx/nested.c | 2 +-
> arch/x86/kvm/vmx/vmx.c | 34 +++++++++++++++-------------------
> arch/x86/kvm/vmx/vmx.h | 12 +++++++++---
> 3 files changed, 25 insertions(+), 23 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index e920d7834d73..8c0ed62b29be 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -4566,7 +4566,7 @@ static struct vmcs *alloc_shadow_vmcs(struct kvm_vcpu *vcpu)
> WARN_ON(loaded_vmcs == &vmx->vmcs01 && loaded_vmcs->shadow_vmcs);
>
> if (!loaded_vmcs->shadow_vmcs) {
> - loaded_vmcs->shadow_vmcs = alloc_vmcs(true);
> + loaded_vmcs->shadow_vmcs = alloc_vmcs(SHADOW_VMCS_REGION);
> if (loaded_vmcs->shadow_vmcs)
> vmcs_clear(loaded_vmcs->shadow_vmcs);
> }
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index dab19e4e5f2b..a45d3721e7d7 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -2554,7 +2554,7 @@ static __init int setup_vmcs_config(struct vmcs_config *vmcs_conf,
> return 0;
> }
>
> -struct vmcs *alloc_vmcs_cpu(bool shadow, int cpu, gfp_t flags)
> +struct vmcs *alloc_vmcs_cpu(enum vmcs_type type, int cpu, gfp_t flags)
> {
> int node = cpu_to_node(cpu);
> struct page *pages;
> @@ -2566,13 +2566,21 @@ struct vmcs *alloc_vmcs_cpu(bool shadow, int cpu, gfp_t flags)
> vmcs = page_address(pages);
> memset(vmcs, 0, vmcs_config.size);
>
> - /* KVM supports Enlightened VMCS v1 only */
> - if (static_branch_unlikely(&enable_evmcs))
> + /*
> + * When eVMCS is enabled, vmcs->revision_id needs to be set to the
> + * supported eVMCS version (KVM_EVMCS_VERSION) instead of revision_id
> + * reported by MSR_IA32_VMX_BASIC.
> + *
> + * However, even though not explicitly documented by TLFS, VMXArea
> + * passed as VMXON argument should still be marked with revision_id
> + * reported by physical CPU.
> + */
> + if (type != VMXON_REGION && static_branch_unlikely(&enable_evmcs))
> vmcs->hdr.revision_id = KVM_EVMCS_VERSION;
> else
> vmcs->hdr.revision_id = vmcs_config.revision_id;
>
> - if (shadow)
> + if (type == SHADOW_VMCS_REGION)
> vmcs->hdr.shadow_vmcs = 1;
> return vmcs;
> }
> @@ -2599,7 +2607,7 @@ void free_loaded_vmcs(struct loaded_vmcs *loaded_vmcs)
>
> int alloc_loaded_vmcs(struct loaded_vmcs *loaded_vmcs)
> {
> - loaded_vmcs->vmcs = alloc_vmcs(false);
> + loaded_vmcs->vmcs = alloc_vmcs(VMCS_REGION);
> if (!loaded_vmcs->vmcs)
> return -ENOMEM;
>
> @@ -2652,25 +2660,13 @@ static __init int alloc_vmxon_regions(void)
> for_each_possible_cpu(cpu) {
> struct vmcs *vmcs;
>
> - vmcs = alloc_vmcs_cpu(false, cpu, GFP_KERNEL);
> + /* The VMXON region is really just a special type of VMCS. */

Not sure if this is the right way to correlate the two.

AFAIU, the SDM calls VMXON region as a memory area that holds the VMCS
data structure and it calls VMCS the data structure that is used by
software to switch between VMX root-mode and not-root-mode. So VMXON is
a memory area whereas VMCS is the structure of the data that resides in
that memory area.

So if we follow this interpretation, your enum should rather look like,

enum vmcs_type {
+    VMCS,
+    EVMCS,
+    SHADOW_VMCS

> + vmcs = alloc_vmcs_cpu(VMXON_REGION, cpu, GFP_KERNEL);
> if (!vmcs) {
> free_vmxon_regions();
> return -ENOMEM;
> }
>
> - /*
> - * When eVMCS is enabled, alloc_vmcs_cpu() sets
> - * vmcs->revision_id to KVM_EVMCS_VERSION instead of
> - * revision_id reported by MSR_IA32_VMX_BASIC.
> - *
> - * However, even though not explicitly documented by
> - * TLFS, VMXArea passed as VMXON argument should
> - * still be marked with revision_id reported by
> - * physical CPU.
> - */
> - if (static_branch_unlikely(&enable_evmcs))
> - vmcs->hdr.revision_id = vmcs_config.revision_id;
> -
> per_cpu(vmxarea, cpu) = vmcs;
> }
> return 0;
> diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> index e64da06c7009..a5eb92638ac2 100644
> --- a/arch/x86/kvm/vmx/vmx.h
> +++ b/arch/x86/kvm/vmx/vmx.h
> @@ -489,16 +489,22 @@ static inline struct pi_desc *vcpu_to_pi_desc(struct kvm_vcpu *vcpu)
> return &(to_vmx(vcpu)->pi_desc);
> }
>
> -struct vmcs *alloc_vmcs_cpu(bool shadow, int cpu, gfp_t flags);
> +enum vmcs_type {
> + VMXON_REGION,
> + VMCS_REGION,
> + SHADOW_VMCS_REGION,
> +};
> +
> +struct vmcs *alloc_vmcs_cpu(enum vmcs_type type, int cpu, gfp_t flags);
> void free_vmcs(struct vmcs *vmcs);
> int alloc_loaded_vmcs(struct loaded_vmcs *loaded_vmcs);
> void free_loaded_vmcs(struct loaded_vmcs *loaded_vmcs);
> void loaded_vmcs_init(struct loaded_vmcs *loaded_vmcs);
> void loaded_vmcs_clear(struct loaded_vmcs *loaded_vmcs);
>
> -static inline struct vmcs *alloc_vmcs(bool shadow)
> +static inline struct vmcs *alloc_vmcs(enum vmcs_type type)
> {
> - return alloc_vmcs_cpu(shadow, raw_smp_processor_id(),
> + return alloc_vmcs_cpu(type, raw_smp_processor_id(),
> GFP_KERNEL_ACCOUNT);
> }
>